Summer Project Report

Si PIN X-Ray Detector and CSPA

Saptarshi Bandyopadhyay

Aerospace Engineering Department,

Indian Institute of Technology, Bombay

Tapaswi Makarand Murari

Electronics and Communication

Engineering Department,

National Institute of Technology Karnataka, Surathkal

Under the guidance of

Dr. P. Sreekumar

Dr. Seetha S.

V. Chandra Babu

Duration: 20th May 2007 – 30th June 2007 (6 weeks)

Space Astronomy and Instrumentation Division (SAID)

ISRO Satellite Center (ISAC)

Indian Space Research Organisation, Bangalore - 17.

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Acknowledgements

We would like to thank our guides Dr. P. Sreekumar, Dr. Seetha S. andSir V.Chandra Babu for all the help given by them in terms of their precioustime and counsel. It was indeed a great pleasure to be working in such anesteemed institution of our country, the ISRO Satellite Centre.

This would be an opportunity for us to thank all the Junior Research Fel-low’s for their co-operation in all contexts. For helping with books, providingthe requisite software, and computer facility.

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Contents

1 Aim of the Project 11.1 Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Photon Interaction Processes 32.1 Photoelectric Effect . . . . . . . . . . . . . . . . . . . . . . . . 42.2 Compton Scattering . . . . . . . . . . . . . . . . . . . . . . . 52.3 Pair Production . . . . . . . . . . . . . . . . . . . . . . . . . . 6

3 General Characteristics of X-Ray Detectors 73.1 Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73.2 Detector Response . . . . . . . . . . . . . . . . . . . . . . . . 83.3 Energy Resolution. The Fano Factor . . . . . . . . . . . . . . 9

3.3.1 Energy Resolution . . . . . . . . . . . . . . . . . . . . 93.3.2 Fano Factor . . . . . . . . . . . . . . . . . . . . . . . . 103.3.3 Electronic Noise . . . . . . . . . . . . . . . . . . . . . . 11

3.4 The Response Function . . . . . . . . . . . . . . . . . . . . . . 113.5 Response Time . . . . . . . . . . . . . . . . . . . . . . . . . . 123.6 Detector Efficiency . . . . . . . . . . . . . . . . . . . . . . . . 123.7 Dead Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.7.1 Extendable Dead Time . . . . . . . . . . . . . . . . . . 143.7.2 Non-extendable Dead Time . . . . . . . . . . . . . . . 15

3.8 Types of X-ray Detectors . . . . . . . . . . . . . . . . . . . . . 153.8.1 Proportional Counters . . . . . . . . . . . . . . . . . . 153.8.2 Microchannel Plates . . . . . . . . . . . . . . . . . . . 153.8.3 Scintillation Detectors . . . . . . . . . . . . . . . . . . 153.8.4 Solid State Detectors . . . . . . . . . . . . . . . . . . . 16

4 Study of PIN Detector Diodes and Processing Electronics 174.1 The PIN Photodiode . . . . . . . . . . . . . . . . . . . . . . . 174.2 The BEL PIN Diode . . . . . . . . . . . . . . . . . . . . . . . 19

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4.3 Introduction to XR100CR . . . . . . . . . . . . . . . . . . . . 204.4 Internals in Brief . . . . . . . . . . . . . . . . . . . . . . . . . 204.5 Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.5.1 Lemo Connector . . . . . . . . . . . . . . . . . . . . . 214.5.2 BNC . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.6 Efficiency Curves . . . . . . . . . . . . . . . . . . . . . . . . . 224.7 Study of the Amptek PX4 . . . . . . . . . . . . . . . . . . . . 23

4.7.1 Major Blocks . . . . . . . . . . . . . . . . . . . . . . . 234.7.2 Connections . . . . . . . . . . . . . . . . . . . . . . . . 234.7.3 Software Interface . . . . . . . . . . . . . . . . . . . . . 244.7.4 Usage for the Experiments . . . . . . . . . . . . . . . . 254.7.5 Alternatives . . . . . . . . . . . . . . . . . . . . . . . . 25

5 Count Rate observed with varying Time and Distance 275.1 Variation of Counts with Time . . . . . . . . . . . . . . . . . . 27

5.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 285.1.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 285.1.3 Observations . . . . . . . . . . . . . . . . . . . . . . . 285.1.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 29

5.2 Variation of Count Rate with Distance . . . . . . . . . . . . . 315.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . 315.2.2 Procedure . . . . . . . . . . . . . . . . . . . . . . . . . 325.2.3 Observation . . . . . . . . . . . . . . . . . . . . . . . . 335.2.4 Calculation . . . . . . . . . . . . . . . . . . . . . . . . 345.2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 35

6 BARC Charge Sensitive Pre-Amplifier 376.1 Block Diagram . . . . . . . . . . . . . . . . . . . . . . . . . . 386.2 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 396.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

7 Equivalent Circuit of a Proportional Detector using a Trans-former 437.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437.2 A203 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437.3 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 447.4 Observations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

7.4.1 Noise Analysis with Switching On of Instruments . . . 457.4.2 Output of CSPA with Loaded Pulse . . . . . . . . . . . 467.4.3 Transformer Characteristics and Loading . . . . . . . . 477.4.4 Noise Overriding Signal due to HV . . . . . . . . . . . 48

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7.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

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Chapter 1

Aim of the Project

The aim of this project was to familiarise ourselves with Si PIN X-ray de-tectors, and X-ray detectors in general and the related back end electronics,mainly the Charge Sensitive Preamplifier. A small study was made on theequivalent circuit of a proportional detector at the end of this project.

1.1 Studies

The initial part of the report and the project was to study the devices we wereto use and also to read up a little about the basic physics principles. Thus,the first part of the report contains the basic processes which are involvedduring photon interactions with atoms.

After this small introduction, we move on to the General Characteristicsof X-ray Detectors. In this part a brief description about terminology likeResolution, Dead Time, Efficiency, and others is explained.

This initial introduction, helps us to actually start studying the devicewhich we actually used, the XR100-CR Si PIN Diode Detector and the PX4.Also a small study on the working of a general Si PIN diode detector iscarried out.

1.2 Experiments

Followed by that are two experiments which we carried out during our tenureof six weeks. The first being to study the relation in the number of countsversus the time and another versus distance. The results were plotted andfitted. The expected variations were obtained, as documented further.

After knowing the few characteristics, we went on to testing of the BARCCharge Sensitive Preamplifier. The testing was carried out using a pulser

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instead of the detector itself, for calculating the gain of the amplifier.Apart from this, the Si PIN diode detector, we tried to work out some-

thing on the Proportional Counter X-ray Detector as well. Thus for testingof the electronics, we thought of using something like an equivalent circuitfor this type of a detector. This was accomplished with the help of Babu Sir,(the idea being his brainchild) using a transformer. Studies were carried outon the linearity, the loading effect and the noise levels of output.

In brief, this is what we fairly succeeded in doing in these six weeks ofthe Summer Project.

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Chapter 2

Photon Interaction Processes

Figure 2.1: Photon Interaction Dominance

To detect x-ray or gamma ray photons, they require their interaction withmatter (in gaseous, liquid, or solid form) in the detector absorber. Photonsinteract with matter in many different ways depending on their energy andtype of material used in the detector. Different interaction processes areutilised to design detectors which have optimum performance for a partic-ular energy region and for the type of measurements intended. Among thevarious interaction processes, three are of importance for detector design: thephotoelectric effect, Compton scattering and pair production.

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The relative probabilities of the three photon interaction processes as afunction of photon energy and atomic number Z of the detector materialare indicated in Figure 2.1. The left curve represents equal probability forthe photoelectric and Compton effect; the right curve corresponds to equalprobability for Compton scattering and pair production interactions.

2.1 Photoelectric Effect

Figure 2.2: Photoelectric and Compton Interactions

The photoelectric effect dominates at lower photon energies, i.e. around100eV to 100keV. In photoelectric interactions the incoming photon is ab-sorbed in an atom and an electron (called photo-electron) is ejected with akinetic energy corresponding to the photon energy minus the electron’s bind-ing energy, as shown in Figure 2.2 and in the Equation 2.1. The left handside is the energy of the photon, wherein ν is the frequency and the righthand side consists of two parts, the work function w0 and the excess energy

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in the form of kinetic energy.

hν = w0 +1

2mev

2 (2.1)

The photo-electron loses energy by ionisation and excitation of other atoms.The resulting charges are collected in the detector to generate a signal. Theinitial photon interaction generates an ionised atom with an electron vacancywhich is filled by an electron from an outer shell or by a free electron fromthe absorber. This process generates additional electrons and photons whichare either absorbed in the detector material or escape from the system. Thephotoelectric cross section (probability of occurrence) increases strongly withthe atomic number (Z) of the material (it is approximately proportional toZ5) which means, for example, photon absorption is much larger in lead(Z = 82) than in a lower Z material such as aluminium (Z = 13). Thephotoelectric cross section decreases with increasing energy E of the incidentphoton (it is proportional to E−7/2).

2.2 Compton Scattering

Compton scattering is the dominant interaction for medium photon energies(around 100keV to 10MeV). In Compton interactions the incident photoncollides with an electron usually in the outer shell of an atom where the elec-tron binding energy is very small and can be neglected, refer to Fig 2.2. Thephoton is scattered at an angle relative to the original direction of motion andit loses energy which is transferred to the electron. The scattered photon mayexperience further interactions (photoelectric or Compton scattering) and iseither stopped in or escapes from the detector absorber material. The elec-tron involved in the collision (recoil electron) is scattered at a different anglerelative to the photon and loses its energy through ionisation and excitationof other atoms until it is eventually stopped in the material. The amount ofenergy which can be transferred to the recoil electron depends on the scatter-ing angle and the incident photon energy and may vary from zero to a largefraction of the incident photon energy. Maximum energy transfer occurs ina head-on collision but the photon still keeps some of its energy. The crosssection for the Compton effect is proportional to the density of the materialand inversely proportional to the energy of the incident photon.

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Figure 2.3: Pair Production Interactions

2.3 Pair Production

At higher gamma ray energies (above several MeV) the pair production pro-cess becomes the dominant interaction. The incident gamma ray photon,which disappears in the process, is transformed into an electron-positronpair, refer to Fig 2.3. This interaction requires the presence of the Coulombfield of an atomic nucleus. The incident photon must have an energy of atleast 1.02 MeV to generate an electron-positron pair. (The energy equivalentof the rest mass of an electron or positron is 0.511 MeV each.) The incidentphoton energy in excess of the 1.02 MeV appears as kinetic energy of the elec-tron and positron. Both particles will lose their energy through interactionswith the detector absorber material. The positron is an unstable particleand after losing most of its kinetic energy it will combine with an electron.Both the electron and the positron disappear in this process which generatestwo 0.511 MeV photons (annihilation radiation). The probability of the pairproduction process increases with the atomic number Z of the absorber (it isproportional to Z2) and with increasing energy of the incoming gamma rayphoton.

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Chapter 3

General Characteristics ofX-Ray Detectors

As an introduction to the detectors, we will define and describe here somegeneral characteristics common to x-ray detectors as a class of devices.

Modern detectors are essentially electrical today i.e. at some point alongthe way the information from the detector is transformed into electrical im-pulses which can be treated by electronic means. This, of course, takesadvantage of the great progress that has been made in electronics and com-puters to provide for faster and more accurate treatment of the information.Indeed most modern detectors cannot be exploited otherwise.

Our discussion in the following sections, will be mainly concerned withsuch kind of detectors in which charge is collected and processed using somekind of electronics.

3.1 Sensitivity

The sensitivity of a detector is its capability of producing a usablesignal for a certain amount of radiation energy. No detector can besensitive to all types of radiation at all energies. Going outside the rangeof detectable energies results in a greatly decreased efficiency of detection ofsignal.

Detector sensitivity to a given type of radiation of a given energy dependson several factors:

1. the cross section area of the detector

2. the detector substance’s atomic mass

3. the inherent detector noise

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4. the protective material surrrounding the sensitive volume of the detec-tor

The cross section and detector substance’s atomic mass determine theprobability that the incident radiation will convert part or all its energy inthe detector into the form of ionisation. For neutral particles, this is much lessthe case, as they must first undergo an interaction which produces chargedparticles capable of ionising the detector medium. These interaction crosssections are usually much smaller so that a higher mass density and volumeare necessary to ensure a reasonable interaction rate, otherwise the detectorbecomes essentially transparent to the neutral radiation.

Even if the ionisation is produced in the detector, however, a certainminimum amount is necessary in order for the signal to be usable. Thislower limit is determined by the noise from the detector and the associatedelectronics. The noise appears as a fluctuating voltage or current at thedetector output and is always present whether there is radiation or not.

Another limiting factor is the material covering the entrance window, tothe sensitive volume of the detector. Because of absorption, only radiationwith sufficient energy to penetrate this layer can be detected. This thicknessthus sets a lower limit on the energy which can be detected.

3.2 Detector Response

In addition to detecting the presence of radiation, most detectors are alsocapable of providing some information on the energy of the radiation. Thisfollows since the amount of ionisation produced by radiation in a detector isproportional to the energy it loses in the sensitive volume. If the detector issufficiently large, such that the radiation is completely absorbed, then thisionisation gives a measure of the energy of the radiation. But this informationmay or may not be preserved depending on the design of the detector.

In general, the output signal of electrical detectors is in the form of acurrent pulse. The amount of ionisation is then reflected in the electricalcharge contained in this signal. Assuming that the shape of the pulse doesnot change from one event to the other, the integral is directly proportionalto the amplitude or pulse height of the signal. The relation between theradiation energy and the total charge or pulse height of the outputsignal is referred to as the response of the detector.

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3.3 Energy Resolution. The Fano Factor

3.3.1 Energy Resolution

The most important factor of a detector designed to measure the energy ofincident radiation is the energy resolution. This is the extent to whichthe detector can distinguish between two close lying energies. Ingeneral, the resolution can be measured by sending a monoenergetic beamof radiation into the detector and observing the resulting spectrum. Onewould always like to have a very sharp delta-function peak. But in reality,we observe a peak with finite width. This width arises because of fluctuationsin the number of ionisations and excitations produced.

The resolution is usually given in terms of full width at half maximum ofthe peak (FWHM). Energies which are closer than this interval are usuallyconsidered unresolvable. This is as shown in the figure. If we denote thiswidth as ∆E, then the relative resolution at energy E is

Resolution = ∆E/E. (3.1)

Figure 3.1: Energy Resolution Peak for 55Fe

This equation is usually expressed in percent. The resolution of the de-tector shown in Fig 3.1 is 2.53%, is calculated by

Resolution = ∆E/E =149eV

5.9keV= 0.0253

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3.3.2 Fano Factor

In general, the resolution is a function of the energy deposited in the detector,with it improving with higher energy E. This is due to the Poisson or Poisson-like statistics of ionisation and excitation. Indeed, it is found that the averageenergy required to produce an ionisation is a fixed number, w, dependent onlyon the material, popularly known as the work function of the material. Fora deposited energy E, therefore, one would expect on the average J = E/wionisations. Thus as energy increases, there is a higher probability that thenumber of ionisation events increase, this is assuming that the completeenergy is still absorbed and does not pass through the material (depends onthe thickness of the material).

The energy dependence of resolution R can then be seen to be (usingPoisson distribution)

R = 2.35σ

J= 2.35

√J

J= 2.35

√

w

E(3.2)

where the factor 2.35 relates the standard deviation of a Gaussian to itsFWHM. Thus we conclude that the energy dependence of resolution variesinversely as the square root of the energy.

Figure 3.2: Relation between the standard deviation σ and the full width athalf maximum (FWHM)

As illustrated in the Fig 3.2, note that the factor 2.35 comes from,

FWHM = 2√

2 ln 2σ = 2.355σ (3.3)

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Now, if the complete energy of the radiation is absorbed as is the case fordetectors used in spectroscopy experiments, the naive assumption of Poissonstatistics is incorrect. And indeed, it is observed that the resolution of manysuch detectors is actually smaller than that calculated from Poisson statis-tics. This difference could be due to the fact that, we assume that the totalenergy deposited is constant, but in reality it need not actually be completelyabsorbed. The total number of ionisations which can occur and the energylost in each ionisation are thus constrained by this value. Statistically, thismeans that the ionisation events are not totally independent (a criterion forusing Poisson statistics) so that Poisson statistics is not applicable. Fano

was the first to calculate the variance under this condition and found it tobe

σ2 = FJ, (3.4)

where J is the mean ionisation produced and F is a value known as the Fano

Factor.The factor F is a function of all the various fundamental pro-

cesses which can lead to an energy transfer in the detector. Thisincludes all reactions which do not lead to ionisation as well. It is thus anintrinsic constant of the detecting medium. Theoretically, F is very difficultto calculate accurately as it requires a detailed knowledge of all the reactionswhich can take place in the detector. Thus a modified equation of energydependence on resolution is given by

R = 2.35

√FJ

J= 2.35

√

Fw

E(3.5)

Typically for many detectors such as semiconductors or gases, F < 1. This,ofcourse, greatly increases the resolution of these type of detectors.

3.3.3 Electronic Noise

In addition to the fluctuations in ionisation, a number of external factorscan affect the overall resolution of a detector. This includes effects from theassociated electronics such as noise, drifts, etc. Assuming all these sourcesare independent and distributed as Gaussians, the total ∆E is given by

∆E2 = ∆E2

det + ∆E2

elect + . . . . (3.6)

3.4 The Response Function

For measurement of energy spectra, an important factor which must be con-sidered is the response function of the detector for the type of radiation being

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detected. This is the spectrum of pulse heights observed from thedetector when it is bombarded by a monoenergetic beam of thegiven radiation.

The response function of a detector at a given energy is determined by thedifferent interactions which the radiation can undergo in the detector and itsdesign and geometry. For example, to see how the response functino changeswith radiation, consider a detector with gamma rays. Now, the principle in-teractions are photoelectric effect, Compton scattering and pair production.In the photoelectric effect, the gamma ray energy is transferred to the pho-toelectron which is then stopped in the detector. Since all photoelectronshave same energy, we would expect a sharp peak in the pulse height spec-trum. However, some gamma rays may also suffer Compton scattering. Asthe Compton electrons are distributed continuously in energy, it destroys theideal delta function response. In a similar manner, those events interactingvia pair production mechanism also contribute a different response.

Moreover, the response function can be improved by changing the geom-etry of the detector. A material of lower atomic number Z can be chosen,for example, to minimize backscattering and bremsstrahlung. Similarly ifthe detector is made to surround the source, backscattering electrons will becaptured thus decreasing the excape of these particles, etc.

3.5 Response Time

A very important characteristic of a detector is its response time. This is thetime which the detector takes to form the signal after the arrival ofthe radiation. This is crucial to the timing properties of the detector. Forgood timing, its necessary for the signal to be quickly formed into a sharppulse with a rising flank as close to vertical as possible. In this way a moreprecise moment in time is marked by the signal.

The duration of the signal is also of importance. During this period, asecond event cannot be accepted either because the detector is insensitive orbecause the second signal will pile up on the first. This contributes to dead

time of the detector and limits the count rate at which it can be operated.

3.6 Detector Efficiency

Two types of efficiency are generally referred to when discussing radiationdetection. Absolute efficiency and intrinsic detection efficiency. The abso-lute or total efficiency of a detector is defined as that fraction of

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events emitted by the source which is actually registered by thedetector, i.e.,

εtot =events registered

events emitted by source(3.7)

This is a function of the detector geometry and the probability of aninteraction in the detector. Thus the absolute efficiency can then be factoredinto two parts: the intrinsic efficiency εint, and the geometric efficiency εgeom.The total efficiency of the detector is then given by the product

εtot ' εintεgeom (3.8)

The intrinsic efficiency is that fraction of events actually hitting the de-tector which is registered, i.e.,

εint =events registered

events impinging on detector(3.9)

This probability depends on the interaction cross sections of the incidentradiation on the detector medium. The intrisic efficiency is thus a functionof the type of radiation, its energy and the detector material. For chargedparticles, the intrinsic efficiency is generally good for most detectors, sinceits a rare chance that a charged particle not to produce an ionisation. Forheavier particles, though, quenching effects may be present in some materialswhich drain the ionisation produced. The problem of efficiency is generallymore important for neutral particles as they must first interact to createsecondary charged particles.

The geometric efficiency, in contrast, is that fraction of the source radia-tion which is geometrically intercepted by the detector.

εgeom =events impinging on the detector

events emitted by source(3.10)

This, depends entirely on the geometrical configurations of the detectors andsource. The angular distribution of the incident radiation must also be takeninto account.

3.7 Dead Time

Related to the efficiency of the detector is also the dead time. This is thefinite time required by the detector to process an event which isusually related to the duration of the pulse signal. Depending on the

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type, a detector may or may not remain sensitive to other events during thisperiod. If the detector is insensitive, any further events arriving during thisperiod are lost.

On the other hand if the detector retains its sensitivity, then, these eventsmay pile up on the first resulting in a distortion of the signal and the subse-quent loss of information from both events. These losses affect the observedcount rates and distort the time distribution between the arrival of events.To avoid dead time effects, the counting rate of the detector must be keptsufficiently high such that the probability of a second event occurring duringthe dead time is small.

Figure 3.3: Extendable and Non-extendable Dead Times

Two different cases are usually distinguished.

3.7.1 Extendable Dead Time

In this case, the arrival of a second event during a dead time period extendsthis period by adding on its dead time τ starting from the moment of itsarrival. This is shown in Figure 3.3. This occurs in elements which remainsensitive during the dead time. In principle if the event rate is sufficientlyhigh, events can arrive such that their respective dead time periods all over-lap. This produces a prolonged period during which no event is accepted.The element is thus said to be paralyzed.

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3.7.2 Non-extendable Dead Time

The non-extendable case, in contrast, corresponds to an element which isinsensitive during the dead time period. The arrival of a second event dur-ing this period simply goes unnoticed and after a certain time the elementbecomes active again.

3.8 Types of X-ray Detectors

The major types of X-ray detectors are:

3.8.1 Proportional Counters

Proportional counters are one of the most common X-ray detectors. A pro-portional counter is somewhat like a light tube in reverse. Instead of applyingan electric charge to get light, we let X-ray photons fall on it and measurethe resulting electric charge. The detector consists of gas which reacts wellto X-rays at some applied high voltages. The incoming photons produceionisations by the processes like the photoelectric effect and maybe a little ofthe Compton effect. X-ray generally are not energetic enough to cause PairProduction effects.

These electrons are propelled by the electrode voltage, and measured bythe electronics (Charge Sensitive Preamplifier mainly) at the end.

The main advantage is that these have a large surface area, which meansthey can capture more X-rays than a smaller detector might.

A small section at the end of our project was dedicated to making anequivalent circuit for this kind of a proportional detector counter.

3.8.2 Microchannel Plates

Microchannel plates are essentially large X-ray photomultipliers. They aremade of layers of material divided into narrow channels. Incoming X-raysreact within one of the plate glass of metal layers via the Photoelectric effect.Thus the induced signal can be measured and thus help know the energy ofthe incoming X-ray.

3.8.3 Scintillation Detectors

Scintillators work by converting X-ray energy into visible light. Generallytheir bulk crystalline materials are NaI or CsI. As for scintillating gases,light production by activated alkali halides results from a complex sequence

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of excitations and de-excitations. These are mainly used for above 20keVenergies and are useful only for their large collecting areas, as their energyresolution is not very good.

3.8.4 Solid State Detectors

CCDs

Charge Coupled Devices, used popularly in digital cameras and video cam-eras, consist of Si doped with impurities to create sites where conductivity isdifferent. X-ray CCDs are used for measuring X-rays, while optical measurelight impacting on the surface.

There too, X-rays create a cloud of electrons when they react with Si/impurities.This cloud is moved using high voltages and again measured as electriccharge.

The main advantage is the accuracy in the measurement, though it hasa very small surface area.

PIN Diodes

This is the type of X-ray detectors used in the experiments that we performed.The PIN stands for P-Intrinsic-N, i.e. the diode is made of these three layers.The working of it is described in the subsequent chapter.

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Chapter 4

Study of PIN Detector Diodesand Processing Electronics

4.1 The PIN Photodiode

Figure 4.1: Overall View of PIN Diode

The PIN in PIN diode stands for P-Intrinsic-N. The efficiency of this isconsiderably improved by making the intrinsic region wide enough to absorbmost of the radiation. A cross section of the PIN photodiode is shown inFigure 4.2.

The PIN diode is by far the most commonly used photodiode, becausethe width of the depletion layer (the intrinsic region) can be controlled to

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Figure 4.2: Schematic cross-section (half device) of a PIN detector on n-typesubstrate

satisfy the requirements of high emission efficiency and improved responsespeed.

In this diode, a wide region formed from an intrinsic semiconductor issandwiched between the P and N regions. The intrinsic region has a relativelylow conductivity, thus a high resistivity, when compared to the P and Nregions. Therefore, most of the reverse-applied voltage apears across theintrinsic layer. This region is made long enough so that most of the incidentradiation on the diode is absorbed within the intrinsic region. Althoughthere will be a sacrifice of speed, the efficiency in the production of currentis considerably higher than that of the photodiode.

Figure 4.3: Block Diagram of the detection system setup

Additional increase of efficiency of photocurrent generation is obtained

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if the PIN photodiode is biased at a reverse voltage that is close to, butnot quite equal to, the avalanche breakdown voltage. Because of the highreverse voltage, avalanche multiplication of carriers takes place in the intrinsicregion, thereby resulting in internal current gain and increased sensitivity ofthe device. As a by-product of the high voltage, the electric field causes highacceleration of the carriers so that faster switching times are achieved.

4.2 The BEL PIN Diode

Figure 4.4: Detailed Pins of PIN Diode

The PIN diode shown in the picture is manufactured by BEL (BharatElectronics Ltd.) As we see in Figure 4.4 there are two pins visible. Thefirst pin can be seen having contact with the undermost layer. The secondis close to central contact.

Using a multimeter (the diode drop) measurement, we found out that atno photon incidence condition, the drop across these two pins was around0.42V.

Also we saw in the Figure 4.1 there are actually three pins for this diode.The third pin, not visible in Figure 4.4 is the gaurd ring pin. It can be clearlyseen in that figure. The diode drop across this pin and the first n pin wasfound to be around 0.445V. This was like a confirmation that the pins wedetected were correct.

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4.3 Introduction to XR100CR

Model XR-100CR is a high performance X-ray detector, preamplifier andcooler system, using a thermoelectrically cooled Si-PIN photodiode as an x-ray detector. It has one of the most high end technology being used in thefield of semiconductor X-ray detectors.

Figure 4.5: The XR100-CR

4.4 Internals in Brief

The main parts of the XR100-CR are the Beryllium window, the X-ray de-tector PIN diode, a cooler and finally the Charge Sensitive Preamplifier.

The Be window is used to pass the X-rays inside and block other kindof eletromagnetic radiation present in the atmosphere like infra-red, opticaland the ultra violet spectrum. Although the detector may not be highlysensitive to all these other radiations, it is still preferred to block it out andthus reduce the noise level. The thickness of the Be window used was 1 mili.e. 25µm.

Then comes the PIN diode. This is the actual X-ray detector. Thethickness of the detector’s (which we used) intrinsic region was 500µm.

After this, there is the cooler. The cooler includes a closed loop tem-perature control that regulates the cooler temperature. A two stage coolertypically shall cool to between 210 to 230 K when set for maximum cooling.Also mounted on the two stage cooler is the FET. The FWHM for 55Fe sourceof 5.9keV energy was found to be ≈ 150eV.

Finally, comes the Charge Sensitive Preamplifier (CSPA). This is the firstpart of the electronics, which converts the collected charge into a voltagesignal. The gain of this kind of CSPA is written in Volts/Coulomb. Now as

20

Figure 4.6: Internal Structure of Detector and Window

the charge collected is of the order of pF, hence, the gain is of the order of1012.

4.5 Connectors

4.5.1 Lemo Connector

It uses a connector called the 6-pin Lemo connector. This connector pinsdo the job of monitoring temperature, providing an HV Bias of about +100to +150 volts. Also, it provides power to the preamp; a dual power supplylike job; +/- 9 V. The power to the cooler too is fed through this LemoConnector.

4.5.2 BNC

The Bi Nodal Connector is used to provide an output from the XR-100CR.This output is then taken to the Shaper, and finally to an MCA. Basicallywhen using a PX2CR or a PX4, it is connected to the analog in of either of

21

the two.

4.6 Efficiency Curves

Figure 4.7: Efficiency Curve of XR100CR detector

As shown in the Figure 4.7, the efficiency v/s Kα lines of selected elements,i.e. basically varying with the energies of the X-rays is plotted. The efficiencyshown is the intrinsic efficiency discussed earlier in the characteristics ofdetectors.

The left hand side of the curves, are for different thicknesses of Berylliumwindows. The more the thickness the lesser the efficiency for the same energyX-ray. Thicknesses are specified in mil where

1mil = 25µm

This is rather expected as more of the radiation shall get absorbed by athicker Beryllium window.

Also, as we see on the right hand side of this graph, the many linesshown are for varying thicknesses of the active region. This active region is

22

nothing but the intrinsic region in the P-Intrinsic-N (PIN) detector. Thusas the thickness increases, more of X-rays get detected, hence increasing theefficiency.

4.7 Study of the Amptek PX4

Figure 4.8: PX4 Front View

4.7.1 Major Blocks

The input to the PX4 is the preamplifier output, already inbuilt into theXR100-CR detector cum CSPA. The PX4 digitizes the preamplifier output,applies real time digital processing to the signal, detects the peak amplitude(digitally), and bins this value in its histogramming memory, generating anenergy spectrum. The spectrum is then transmitted over a USB interfaceto the computer. The details about the software used are discussed in thefollowing subsections.

4.7.2 Connections

As we see in the Figure 4.8 we can see that the left hand side BNC is usedfor connection with the XR100CR. This is where the output from the CSPAis sent. Also can be seen the Lemo connector mentioned earlier. This isconnected to the XR100CR for all the power supply, bias and other suchutilities. At the back end of the PX4 is the USB connector. This directlycan be connected to the computer on the other side. Thus, the interface withthe PX4 is complete.

23

4.7.3 Software Interface

Figure 4.9: Software Screenshot

The software used is

MultiUSB ADMCA Analog and Digital

Version 1, 0, 0, 12Copyright c© 1998-2007 Amptek Inc.

Build Date: Jan 29 2007

As shown in the Figure 4.9 this is the software that we used for measuringand collecting data from the PX4. The PX4 is connected to the computer

24

through an USB cable and then this software is activated. The software hasprovisions to change the time of acquiring, to change the gain and otherthresholds. Also the number of channels can be changed. Another facilitywas to view the graph in logarithmic, or linear scale as we would want it.A logarithmic scale helped us understand the noise levels further. Shownin the Figure 4.9 is a logarithmic scale. As we can see, the right hand sidecontains the parameters like acquisition time, the gain, thresholds and otherdetails. Most of these settings were allowed to be defaults, after selecting theXR100CR we were working with.

4.7.4 Usage for the Experiments

The basic use of the PX4 was to convert the waveform seen as the output ofthe CSPA to a spectrum. The main change that occurs here is the changein the axes. The waveform is seen as Voltage v/s Time, while the spectrumas Counts v/s Channels. The Voltage axis (y) corresponds now to Channels(x) and Time (x) corresponds to Counts (y).

Now to relate these two, for certain values of voltage, the values are binnedand converted into a channel. Basically for a 1024 channel, we can assumeits similar to a 10-bit ADC. Thus converting from 0V to 5V (need not bethis, it is actually the saturation voltage of the CSPA) say to 0 to 1024. Forexample say, 2 Volts corresponds to channel number 435, then 4 volts wouldbe for 870. This is done by the PX4. Also the time axis is converted into thenumber of counts axis. That is for more the time you integrate, i.e. let theX-ray detector be exposed to radiation, the more the counts one observes. Asmall experiment was done to establish a relationship with counts and time.This is discussed in the next chapter.

The file generated by this software is saved as an .mca file which hasall information regarding the detected spectrum. It first has the mode, thechannels, the thresholds, and then the peak informations, etc. But the partof our concern was the values of counts from channels 0 to 1023, as we workedin a 1024 channel environment.

These count values were isolated, and then given a serial count, by writingsimple C++ codes to do this. Thus it enabled us to replot these figures insoftware like gnuplot. Thus all calculations such as the total number of countsabove a certain noise threshold, and other such details could be obtained.

4.7.5 Alternatives

After all this, one may feel that the PX4 is an indispensible part of the entireX-ray detector setup. But this is not so. An instrument called the PX2CR

25

developed by Amptek Inc. also can be used. This too, has an interface witha separate (not inbuilt) Multi Channel Analyser and then can be connectedto a computer, through a USB or serial port. It too had a different software.

But apart from this, an instrument called the Pulse Height Analyser witha Multi Channel Analyser can serve the purpose. Although, there is not asstraightforward a way then to process the obtained data. It would haveto be noted down channel wise from 0 to 1023, which would have been apretty tough job. But the point is that, we can obtain the FWHM, and theresolution of the detector using this kind of instruments too.

26

Chapter 5

Count Rate observed withvarying Time and Distance

5.1 Variation of Counts with Time

Figure 5.1: Counts v/s Channels for different Time

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 200 400 600 800 1000 1200

"fe-55-6.3cm-50s.txt2""fe-55-6.3cm-100s.txt2""fe-55-6.3cm-150s.txt2""fe-55-6.3cm-250s.txt2""fe-55-6.3cm-500s.txt2""fe-55-6.3cm-750s.txt2"

27

5.1.1 Introduction

As the first part of knowing more about the XR-100CR device, the Amptek’sSi PIN X-ray detector cum Charge Sensitive Preamplifier, we observed thevariation of counts for different integration time. The integration time isthe time for which the detector is exposed to the X-rays and is the durationduring which the actual counting occurs. The device was connected throughthe PX4 to the computer and the software ADMCA Analog and Digitalsupplied by Amptek Inc. was used. Although, conceptually, a PHA (PulseHeight Analyser) could have been used instead. But as software (for thePX4) facilitated ease of storage of data, and use it later for processing.

5.1.2 Procedure

The source 55Fe was kept at some distance from the detector window, whichwas later found to be 6.3cm. All observations were carried out for thisdistance. The integration time was varied from 50s to 750s, in suitablychosen steps. An initial study of the background helped us know that thenoise levels are not hampering our main signal. Accordingly calculationswere made to take care of the noise issues. All count values less than andequal to two were not considered. A simple C++ program was written toset this threshold of two counts and also to sum up all the values of countsfor each channel, running from 0 to 1023.

5.1.3 Observations

Table 5.1: Counts vs TimeTime Counts Error(sec) (σ)50 2224 47.2100 4214 64.9150 6712 81.9250 11083 105.3500 22339 149.5750 33694 183.6

As we see, 2 main peaks were observed where the counts were integrated.A Kα peak and another smaller Kβ peak. The number of counts were thustabulated as shown in Table 5.1. The Kα suggests that this is the first

28

electron that got ejected from the K-shell of the atom, and Kβ indicates thesecond electron.

The total counts were measured as mentioned before using a simple C++program, which counted the value of the counts in each channel, and thenadded them all up. The error bars were put by statistics as the square rootof the mean value.

Figure 5.2: Counts v/s Time Graph

0

5000

10000

15000

20000

25000

30000

35000

40000

0 100 200 300 400 500 600 700 800 900

"countofcountsgraph" u 1:2:3

0

5000

10000

15000

20000

25000

30000

35000

40000

0 100 200 300 400 500 600 700 800 900

"countofcountsgraph" u 1:2:3f(x)

5.1.4 Conclusion

The expected variation of counts with time was linear. So, we fit the obtaineddata points for an equation

counts(t) = A + Bt (5.1)

It was found that, the line fitted, passed through the origin and was intolerable error bar limits. The values of the constants were

A = 0 and B = 45counts/second

29

From this we can conclude that for the given apparatus and conditions,the measured counts per second of the detector is 45, given by the value ofB, and the variation of the number of counts with time is linear.

30

5.2 Variation of Count Rate with Distance

Figure 5.3: Energy Spectrum of 55Fe at diffrent Distances

0

50

100

150

200

250

300

350

400

450

0 200 400 600 800 1000 1200

coun

ts

channel number

"05cm-100s""10cm-100s""15cm-100s""20cm-100s""25cm-100s""30cm-200s""35cm-200s""40cm-300s""45cm-400s""50cm-400s"

5.2.1 Introduction

Another phase in the study about the source and the characteristics of theXR-100CR device, the Amptek’s Si PIN X-ray detector, we observed thevariation in count rate with varying distance of the source from the detectorwindow. The device was connected through the PX4 to the computer andthe software ADMCA Analog and Digital made by Amptek Inc. was used.

31

5.2.2 Procedure

Figure 5.4: Zoomed-in view of Energy Spectrum of 55Fe at diffrent Distances

0

20

40

60

80

100

120

140

460 480 500 520 540 560 580 600

coun

ts

channel number

"05cm-100s""10cm-100s""15cm-100s""20cm-100s""25cm-100s""30cm-200s""35cm-200s""40cm-300s""45cm-400s""50cm-400s"

The source 55Fe was kept at a varying distances from the detector window.All observations were carried out for a time interval of 100 seconds. Butwe noticed that as the distance increased beyond approximately 35cm thenumber of counts decreased rapidly. Thus to compensate with that, weincreased the integration time to double and then later four times the original100 seconds. But as we observed earlier, the variation of the number ofcounts with time is linear, it was easy to perform the required calculations

32

Table 5.2: Distance vs Counts (Normalised to 100 seconds)Distance Counts Error(cm) (σ)05 6008.94 558.74810 1484.95 141.67115 615.604 104.51420 311.948 68.827825 172.615 50.082330 114.281 47.79135 69.1146 32.629640 48.7361 30.27445 33.8646 21.43650 25.0417 21.0621

for normalizing the number of counts to counts/100s.Again an initial study of the background helped us to confirm that the

noise levels are not hampering our signal. Accordingly a threshold was setto deal with the noise issues. This was done in the C++ program itself. Allcount values less than or equal to 2 were eliminated, and not considered forcounting in the total number of counts.

For this experiment, we took three sets of readings, for each value of dis-tance. Thus, from these, we calculated a mean, and then found the standarddeviation σ which was taken as the value of the error bars. Notice that theerror bar for the first reading is very high. This we assume could be due tofactors like the geometric efficiency of the detector.

5.2.3 Observation

The source 55Fe, is kept at different distances from the XR100CR Si-PINdetector and the counts are measured for a certain amount of time. All thecounts seen in the table though are for a fixed time interval of 100s, whichwas easy as the total number of counts was found to be linearly proportionalto time. Thus counts per 100seconds is mentioned in the tables. The detectoroutput, processed by the PX4 is shown in Fig 5.3.

The zoomed-in view of the channels containing the Iron peak (∼ chan-nel 507) is shown in Fig 5.4.

33

5.2.4 Calculation

We tried to fit the data with various fitting equations as shown in Table 5.3.Also shown is their respective reduced χ2 values. This was done using thesoftware GNUPlot, a command-driven interactive function and data plottingprogram under the GNU General Public License.

Table 5.3: Fitting the data by EquationsFitting Equation Reduced χ2

f(x) = A + Bx

+ Cx2 + D

x3 + Ex4 + F

x5 0.00587102f(x) = A + B

x+ C

x2 + Dx3 + E

x4 0.00500529f(x) = C

x2 1.19257f(x) = A + C

x2 0.0840252f(x) = C

x2 + Gx6 1.04004

f(x) = Cx2 + F

x5 0.989627f(x) = C

x2 + Ex4 0.877687

The value of the reduced χ2 indicates the goodness of the fit. The closerit is to 1, we say the better is the fit. Hence the best fit is when we use theequation of the form

f(x) =C

x2+

F

x5

where C = 123131 ± 9866(8.013%)F = 3730690 ± 2212000(59.28%)

Also the fit for the equation

f(x) =C

x2

where C = 132241 ± 10076(7.619%)Notice the huge errors on the value of the constant F. Although this is

pretty threatening to the fitted curve, it can be ignored saying that, as thevalue of the distance increases, i.e. the value of x increases, the contributionto f(x) due to F

x5 decreases rapidly. Hence the reduced χ2 value of the fitf(x) = C

x2 is also very close and hence acceptable.The plot of the data and the fitted equation is shown in Fig 5.5. In the

plot

f(x) =C

x2

g(x) =C

x2+

F

x5

34

Also in real applications we shall be encountering very very large distancesand hence the effect due to close proximity of the source from the detector canbe neglected. There are other factors such as geometric efficiency mentionedearlier, which play a major role when the source is kept close.

5.2.5 Conclusion

We observe that as the distance x increases the contribution of the value ofFx5 decreases rapidly so much that we can ignore the contribution from thisfactor.

Thus finally we can conclude by saying that the variation of total numberof counts is inversely proportional to the square of the distance and followsthe square law. Also the approximate values for which

f(x) =132241

x2(5.2)

Hence the final equation for count rate (counts per sec), under similarconditions is Equation 5.3, obtained by just dividing by 100s due to thelinearity of counts with time.

f(x) =1322.41

x2(5.3)

35

Figure 5.5: Data points vs the Fitted Equation

0

1000

2000

3000

4000

5000

6000

7000

0 10 20 30 40 50 60

Cou

nts

Distance (cm)

"distance_counts"f(x)

g(x)

36

Chapter 6

BARC Charge SensitivePre-Amplifier

The basic block diagram of a CSPA is shown below in Figure 6.1.

Figure 6.1: Block Diagram of CSPA

37

After characterisation of the variation in counts for varying distances andtimes, we decided to test a Charge Sensitive Preamplifier. The CSPA usedis developed by BARC, and is still in the research stages. So, we thoughtof laying out a characteristic, wherein, we could try and find the gain of thepreamplifier.

6.1 Block Diagram

We connect the preamplifier to a research pulser. The block diagram for theconnections are shown in Figure 6.2. The research pulser output was fed asthe test input to the CSPA, and the output was noted on the oscilloscope.

Figure 6.2: Block Diagram

One important point to note was that, the pulse was getting loaded by theCSPA, and the value was decreasing. This is because of the input impedancenot being large enough. When the input impedance is large, the currentpassing through it is negligible and thus the voltage drop on it is small

38

too. Hence, loading effect is not seen. Buffer stages (maybe of opamps) arerecommended so as to have nearly infinite input impedance.

6.2 Observations

The observation table is shown in Table 6.2. The table contains readingsfrom different points. One from the Research Pulser (when open), labelledas RP; the next being the loaded pulse output, labelled as LPO; and thefinal listing being that of the BARC Preamp Output, labelled as BPO. Thevarious columns as seen in the table are

Pulse Height It is the height of the pulse as measured on the oscilloscope.

Pulse/sec It is the frequency of the pulser output.

Rise Time (tr) It is the time taken by the pulse to travel from ground levelto peak level. It is measured in nano-seconds.

Decay Time (τd) It is the time taken by the pulse to fall from the peaklevel to 1/e times peak level, for an exponentially decaying curve. It ismeasured in micro seconds.

Fall Time (tf) It is similar to decay time, but we have used it for the outputpulse of the BARC preamp, which is not a exponential curve. Wedefined it as the time taken by the pulse to fall from the peak level toground level. It is measured in micro seconds.

Delay Time (tdly) It is the time lag between the arrival of the loaded pulseand the arrival of the BARC preamp output pulse. It is measured innano-seconds.

Polarity It denotes the polarity of the pulse. Usually we feed negativepolarity pulse to the BARC preamp and the output is positive polaritypulse.

The Research Pulser characteristics are as follows

• Rise time (tr) = 500ns

• Decay time (td) = 5 µs

• Normalise = 000

This is a set of knobs, with the help of which pulse amplitudes can beset with great precision.

39

SN Instru- Pulse Pulses Rise Decay Fall Delay Polarityment Height perSec Time Time Time Time

(Volts) (Hz) tr (ns) τd (µs) tf (µs) tdly (ns) (+/-)

1 RP 0.1000 100 500 5 -LPO 0.0336 108.7 704 5.416 -BPO 0.321 108.7 1124 17.23 +660 +

2 RP 0.2000 50 500 5 -LPO 0.0675 54.7 714 5.452 -BPO 0.579 54.7 1246 15.8 +754 +

3 RP 0.3000 100 500 5 -LPO 0.0983 107.9 746 5.452 -BPO 0.579 107.9 1170 15.7 +720 +

4 RP 0.4000 100 500 5 -LPO 0.1310 108.1 666 5.592 -BPO 1.007 108.1 1094 15.7 +710 +

5 RP 0.5000 100 500 5 -LPO 0.1630 108.1 682 5.526 -BPO 1.167 108.1 1032 16.5 +722 +

Table 6.1: RP, LPO and BPO

• Attenuation = ×1.4

Its the property of the pulse to have some attenuator in it. This toocould be selected from ×1.2 to ×2.

• Polarity = Negative

The table of the loaded pulse as seen in the CRO versus the output ofthe BARC Preamp also seen in the CRO, is shown in Table 6.2.

There are a few things, we would like to discuss here. Firstly, the risetime and decay time are the pulser characteristics. The variation in these,could cause a variation in the final CSPA output. Decreasing rise time,means sharper pulse, and thus the amplitude levels may vary. Also, a largerdecay time may mean that the fall time, i.e. the time taken by the outputto stabilise, to a zero level after the undershoot may increase too.

40

Input OutputmV V33.7 0.31267.5 0.597101 0.82135 0.99169 1.13203 1.25237 1.35271 1.46303 1.57330 1.64362 1.75401 1.85433 1.91470 1.98497 2.01

Table 6.2: LP0 vs BPO

6.3 Conclusion

We fit an equation to the linear region of the graph, from x = 135 to x = 400.The equation obtained is

f(x) = Ax + B

where A = 0.00321044± 0.00006117(1.905%)B = 0.583293± 0.01718(2.946%)The fitted graph is shown in Fig 6.3.

Hence the gain of the BARC preamp is 0.0032V/mV or 3.2V/V . However,the standard way of expressing gain of CSPA’s is volt/coulomb. In order tobring into effect this conversion; as the test input had a capacitor of value1pF connected to it, the effective gain was found to be 3.2× 1012V/C, in thelinear region of the curve.

The region below x = 135 shows nonlinear characteristics. The regionabove x = 400 is saturated. The maximum output possible from the BARCpreamp is ∼ 2 Volts.

41

Figure 6.3: Experimental vs Fitted Equation

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

2.2

0 50 100 150 200 250 300 350 400 450 500

BARC

Pre

amp

Out

put (

V)

Loaded Pulse (mV)

"loadedpulse"f(x)

42

Chapter 7

Equivalent Circuit of aProportional Detector using aTransformer

7.1 Introduction

As described earlier in the types of detectors, one of them is a ProportionalCounter X-ray Detector. This circuitry is used for making a simple equivalentof a Proportional Detector which can be later used in modelling the back endelectronics for this type of detector.

As shown in the figure, we have the high voltage (HV), through a dividernetwork, and is connected to the equivalent circuit of the detector. This iswhat is actually done too, in case of real detectors. After this, the capacitoris used to decouple the DC bias and couple the AC signal sent out by thedetector. The voltage is converted to charge and thus, we say that the inputto the CSPA (Charge Sensitive PreAmplifier) is a charge.

7.2 A203

The A-203 chip given by Amptek Inc. is a Charge Sensitive PreAmplifier(CSPA). This was used directly in this case. The output from the equivalentdetector circuit was connected to the detector input of the chip. The generalspecifications of the chip are shown as below

General

Operating Voltage: +10 to +18 V DC

43

Figure 7.1: Block Diagram of Equivalent Circuit

Operating Current: 1.4 mA Quiescent at 15 VTemperature: −550 to +700 C operational

Charge Sensitive Preamplifier Output

Rise time: 50nsFall time: 30µs

AC Output Impedance: 50Ω

7.3 Operation

In more detail is the HV divider network and the equivalent circuit shown inFig 7.3.

The first 2.2nF capacitor is used to stabilise the HV, as even small fluc-tuations in the HV could be sensed as signal by the capacitor, present beforethe A203 chip. The detector circuitry is highly sensitive to the order of tensof pC and hence, a good stability is a must. The 2MΩ resistance is used tocharge the capacitor. This R-C low pass filter is chosen by design criteria,

44

Figure 7.2: Detailed Circuitry of Equivalent Detector

like having a high time constant and thus neutralising the sharp variationsin high voltage which is fed to the other circuitry.

The next 20MΩ resistance acts like a load resistance, in series with thedetector. Thus when a photon falls on a detector, (in the general case)ionisation occurs, the charge is collected at anode due to HV and then thedifference in this current is felt at the 20MΩ resistance. This signal is con-verted to charge using the next 0.01µF capacitor. This in turn is fed to theA203 chip.

7.4 Observations

7.4.1 Noise Analysis with Switching On of Instruments

The output is tabulated as shown in the Table 7.1.Notes:

• The pulser had an offset signal of 2mV when all its dials were set to0 position and thus the output when it was switched on. This strictly

45

Figure 7.3: Actual Circuitry of Equivalent Detector

cannot be considered as random noise.

• Shift in the baseline and resettling back is noticed when the HV isswitched on

• All noise levels are very small, and hence there is a finite scope formeasurement error.

7.4.2 Output of CSPA with Loaded Pulse

All noise levels were measured step by step, as each instrument was switchedon. Finally pulses were fed from the research pulser to the transformer’s oneend. The loaded pulse amplitude was noted. Also, the output at both levelsof HV was noted, i.e. when HV was 0V and 1.5kV . This is tabulated inTable 7.2

Notes:

• Notice that the loaded pulse is nearly 25% of the open pulse, this is dueto comparable input impedance of the transformer. A buffer stage at

46

Table 7.1: Noise AnalysisVs HV Pulser Noise

State State State Vpk−pk

OFF OFF OFF 0.4mVON OFF OFF 4mVON ON OFF 14mVON ON ON 130mV

Table 7.2: CSPA Output for Different Input PulsesPulser Input Loaded Pulse Output (HV=0V ) Output (HV=1.5kV )in mV (-) in mV (-) in V (+) in V (+)

15 4 0.9 0.830 7.5 1.75 1.5440 10 2.25 1.9550 12.5 2.9 2.668 17 3.4 3.180 20 3.8 3.596 24 4.0 3.9110 27 4.0 4.0

this point, for example an opamp, would increase the input impedanceto a high value and thus would prevent this loading.

• Also, the output voltage when HV is 1.5kV is slightly lesser than theoutput when HV is 0V .

• The output reaches saturation at 4.0V and the peak does not increaseany further.

7.4.3 Transformer Characteristics and Loading

Also a special note was made on the loading due to the transformer. It wasobserved that the loaded pulse measured nearly 25% of the open pulse. Thiswas further verified with a few more values as shown in the Table 7.3. Alsois seen that the transformer had a 1:1 ratio. The core used was a ferritetoroidal core, as shown in the picture at the end of the report. Each primaryand secondary had 4 windings.

47

Table 7.3: Loaded Pulse v/s Open PulsePulser Input Loaded Pulse Transformer Outputin mV (-) in mV (-) in mV (+)

130 34 34270 68 68400 100 100530 130 130660 170 170800 200 200

7.4.4 Noise Overriding Signal due to HV

Another interesting phenomenon observed was the introduction of a largeriding noise due to HV being switched on. A set of readings was speciallytaken at small values of pulser output. The following Table 7.4 shows thevariation of noise overriding on the output signal for very small inputs.

Table 7.4: Noise Analysis Overriding on OutputPulser Input (Open) CSPA Output (Average) Noise Variation

in mV (-) in mV (+) in mV1.3 80 ±253 175 ±255 250 ±308 420 ±2510 500 ±20

7.5 Conclusion

Thus we have characterised this equivalent circuit of a detector using a trans-former. It makes it easier to calibrate the back end electronics, as the factorslike humidity, temperature and other such natural factors will not affect theequivalent circuit.

48

Bibliography

[1] Dalla Betta & Pignatel, E.A.,Nuclear Instruments and Methods in Physical Research,A 395 (1997)

[2] Philip A.Charles, Frederick D. SewardExploring the X-ray Universe

[3] W.R. LeoTechniques for Nuclear and Particle Physics Experiments

[4] R.Decher, E.A.,X-ray and gamma ray astronomy detectors

[5] Glenn F.KnollRadiation Detection and Measurement

[6] www.amptek.com

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