gamma-ray spectroscopy

Summer CourseInstrumental Neutron Activation

Analysis

July 7-18, 2008

Gamma-Ray Spectroscopy

Dr.Ir. Peter Bode Associate Professor Nuclear Science &

Engineering


Analysis

July 7-18, 2008

INAA:Semiconductor detectors

RNAA:Semiconductor detectorsScintillation detectors


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Principle of a semiconductor detector

Solid-state ionisation detectors


Analysis

July 7-18, 2008


N-type Ge:

Impurities such as P and As as electron donors

P-type Ge:

Impurities as B, Al, Ga as positive charge donors


Analysis

July 7-18, 2008


Semiconductor detector:

Junction diode with P and N type impurities on either side

Applying a reverse bias:

A P-I-N structure is formed


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

n-type silicon

e

h

-V 0

qe dV1

qh dV2

dq = (qedV1 + qhdV2 )/V

i = dq/dt

n+ contact

p+n junction

reverse bias, fully depleted

• silicon diode

• germanium detector



Analysis

July 7-18, 2008

Atomic

number Operating

temperature (K)

Band Gap, eV

Electron mobility,

(cm2V-1s-1)

Hole mobility

(cm2V-1s-1)

Energy to produce an electron-hole pair, eV

Si 14 77 300

1.16 1.12

2.1 .104

1500 1.1.104

500 3.76 3.61

Ge 32 77 0.67 3.6 . 104 4.2.104 2.96

GaAs 21 33 130

300

1.43

8500

400 4.51 4.2

CdTe 48 52 300 1.47 1100 100 4.43

Cd1-x Znx Te 48 30

52 300 1.44 (x=0)

- 2.26

(x=1); typically

1.5-2.2 for x=0.1 -0.2

1350 120 4.43 (x=0) –

(x=1); typically 5.0-5.5 for

x=0.1-0.2

HgI2 80 53 300 2.13 100 100 4.22 Some properties of semiconductor materials



Analysis

July 7-18, 2008

Schematic representation of a Ge-semiconductor detector,

Solid-state ionisation detector


Analysis

July 7-18, 2008



Analysis

July 7-18, 2008


Contacts:

n+: diffusion of Li-atoms 700 – 1000 m (dead layer)

p+: implantation of B-atoms 0.3 m


Analysis

July 7-18, 2008

•Different types of Ge semiconductor detectors



Analysis

July 7-18, 2008


Channel number pulse height

No.o

f p

uls

es

(*

10

00

)


Analysis

July 7-18, 2008



Analysis

July 7-18, 2008


Pulse height spectra obtained with Si(Li) detectors.Left: X-ray spectrum of 241AmRight: - spectrum of 241Am


Analysis

July 7-18, 2008

Different types of cryostats for use with Ge-semiconductor detectors



Analysis

July 7-18, 2008

Energy resolution

Usually:

Full Width on Half Maximum @ 1332 keV of 60Co

@ 122 keV of 57Co

@ 6 keV of 55Fe


Analysis

July 7-18, 2008

Energy resolution

State-of-the-art:

1332 keV: 1.58 – 2.0 keV, depending on crystal size

122 keV: 0.6 – 1 keV

5.9 keV: 0.2 – 0.5 keV


Analysis

July 7-18, 2008

Peak Shape

Ratio of :

FWHM/Full Width 0.1 M

FWHM/Full Width (1/50) M


Analysis

July 7-18, 2008

Gamma-ray peak shape and tail parameters


Analysis

July 7-18, 2008

Peak Shape

Ratio of :

FWHM/Full Width 0.1 M theoretically: 1.83

FWHM/Full Width (1/50) M theoretically: 2.38

Importance: symmetry !!!


Analysis

July 7-18, 2008

High energy (top) and low energy tail parameters


Analysis

July 7-18, 2008

High energy tail of pulser peak


Analysis

July 7-18, 2008

Calibration source activity


Analysis

July 7-18, 2008

Peak-to-Compton ratio

Defined as:

Ratio of peak height at 1332 keV and average peak height in energy range between 1040 and 1096 keV


Analysis

July 7-18, 2008

Peak-to-Compton ratio

State-of-the-art:

p/C ~ 50-100, depending on size of crystal:

pC = 34.75 + 1.068 (εCo-60) - 4.96.10-3 (εCo-60)2


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Efficiency

Absolute efficiency defined as:

Relative to the efficiency of a 3” x 3 ” NaI(Tl) detector,

defined as 1.2.10-3 counts/1332 keV photon,measured at a source-detector distance of 25 cm


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Determination of photopeak efficiency curve

Absolute: Using calibrated sources with known gamma-ray emission rates and activity values, traceable to Bq

Single gamma-ray emitting radionuclidesPoint sourcesExtended sources

Problem:Many sources contain 60Co and 88Y; corrections for coincidence effects require also the p/T curve


Analysis

July 7-18, 2008


Relative:

Using mix of sources with well-established gamma-ray intensity ratios

1 source for entire energy range, e.g. 152Eu

2-5 sources, e.g. 182Ta + 133Ba + 75Se + 24Na + … Problem:Intensity ratios not always well established


Analysis

July 7-18, 2008


Relative:

1 source:advantage: simpledisadvantage: do not always fully cover entire energy range; inter/extra-polation disputable in 80-150 keV range

3-5 sources:advantage: better coverage all energy rangesdisadvantage: more cumbersome, problems with non-matching parts


Analysis

July 7-18, 2008

Determination of efficiency curves

Relative:

Using mix of sources with well-established gamma-ray intensity ratios

1 source for entire energy range, e.g. 152Eu

2-5 sources, e.g. 182Ta + 133Ba + 75Se + 24Na + …


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

New Tools for Nuclear Spectroscopy

Better and bigger Ge detectors High count rate electronics

High-resolution scintillation detectors (LaBr3(Ce))

Position-sensitive (strip) detectorsMonte Carlo modelingImage processing


Analysis

July 7-18, 2008

Bigger Ge- Detectors

100 10000.1 %

1 %

10 %

100 %

3000

Absolute photopeak efficiency

75 cm3 (17 %) 4 cm

560 cm3 well

Photon energy, keV

0.3 % 20 %

3 % 90 %


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Typical improvement in detection limits

20 %

100 %

well125 cm3

Arbitrary units

0,01 0,1 1

0,01 0,1 0,25-0,3 1

0,01 0,1 0,15-0,25 1

Bigger Ge-Detectors

well560 cm3

0,01 0,1 1

0,07 - 0,1


Analysis

July 7-18, 2008

New Tools for Nuclear Spectroscopy

LaBr3(Ce) scintillation spectra

P.Doorenbos et.al., IEEE Transactions 51 (2004) 1289Developed and Patented by T.U.Delft: produced by Saint Gobain, France


Analysis

July 7-18, 2008

Preamplifiers

- Resistor feedback- Pulse optical feedback

high resolutions (planar detectors)

- Transistor feedbackhigh count rates


Analysis

July 7-18, 2008

NIM bin and power supply

- Adequate capacity

- standard: +/- 24 V +/- 12 V +/- 6 V


Analysis

July 7-18, 2008

High Voltage supply

Typically (+/-) 3-5 kV

Different power supplies for Ge and NaI(Tl) detectors

dV/dt networks

LN2 switchoff option


Analysis

July 7-18, 2008

Spectroscopy Amplifiers

Analogue systems

Digital systems

- Gaussian shaping- Triangular shaping- Gated-integrated shaping


Analysis

July 7-18, 2008

Baseline retoration and Pole-zero setting


Analysis

July 7-18, 2008

Spectroscopy Amplifiers

Essential:

- Amplifier and HV at opposite outermost places in NIM-BIN- Match amplifier’s output DC level to ADC’s input DC level

- High time (shaping) constant: better resolution, lower

throughput Often typically set at 3 s for most (coaxial) detectors 6 s for planar detectors


Analysis

July 7-18, 2008

ADC

- Linear Approximation or Wilkinson ADC- Successive Approximation ADC

Wilkinson ADC’s : best linearity

Wilkinson ADC: dead time depends on channel no.

Succ. Appr. ADC: fixed dead time per channel.

Settings: zero level, lower level, upper level, conversion gain


Analysis

July 7-18, 2008

Pulse generators

- Used for dead time/pile-up correction

- Fed into the test input of the preamplifier

- Essential: input peak shape must match detector signal (rise time and fall time (1000-2000 s )


Analysis

July 7-18, 2008

Dead-time stabilizer

Loss free counting

Digital dead-time stabilizer


Analysis

July 7-18, 2008

Signals

Use oscilloscope

Track:

- Output signal preamplifier

- Output signal spectroscopy amplifier


Analysis

July 7-18, 2008

Bad signals


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Low energy tailing:

- wrong baseline restoration- incomplete charge collection

High energy tailing:- wrong baseline restoration

Peak broadening:- increased noise- incomplete charge collection

Shifts/instability:- proportional: gain- constant: DC problems


Analysis

July 7-18, 2008

Spectral shape


Analysis

July 7-18, 2008

Photoelectric effect (1)

The photon energy is transferred to an atomic electron

KL

e-

h

3

4

, h

Ze Cross section

Energy photo-electron: be EhE

Photo-electron


Analysis

July 7-18, 2008

Compton effect (1)h

h’

Ee-

Energy conservation:

Energy Compton electron:

Angular correlation:

)cos1(1

)cos1()(

2

2

2

cmhcmh

E

e

ee

'hvEhv e

Partial energy transferto a ‘free’ electron

2tan1)cot( 2

cm

h

e


Analysis

July 7-18, 2008

h

h’

Ee-

Angular distribution of the scattered photons

0

90

180

270

0 1 2

10 MeV

1 MeV0.1 MeV

0.01 MeV

richting invallend

foton

Compton effect (3)


Analysis

July 7-18, 2008

Only if

• E > 1022 keV

Mainly if• high Z

Pair production

Annihilation


Analysis

July 7-18, 2008

Interactions of photons

10 -2 10 -1 10 0 10 1 10 2

Photon energy (MeV)

0

20

40

60

80

100A

tom

ic n

um

ber

Z

Photo-Electric Effect

dominantPair Production

dominant

Compton Effect

dominant C =

= C


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Photon interacts with crystal:

• Absorption of all energy by photoelectric effect

• Absorption of all energy by multiple scatteringand subsequent absorption by photoelectric effect


Analysis

July 7-18, 2008


3. Absorption of part of the energy of the photon due to scattering effects and escape of scattered photon from the crystal


Analysis

July 7-18, 2008


3. If energy of photon > 1022 keV: pair production effects:

-All energy absorbed- One of the electrons escapes from the crystal: only E- 511 keV deposited- both scattered electrons escape from the crystal:E-1022 keV deposited


Analysis

July 7-18, 2008


4. Al low energies:

All energy transferred to Ge electrons, but scattered Ge electrons escape from crystal:

E-(Ge-k) keV deposited in crystal


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Coincidence effects


Analysis

July 7-18, 2008

Coincidence effects

Coincidence count rate: Rc = D.1.2

Random coincidence count rate: Rr = 2..R1.R2

Rr = 2..D2. 1.2

1, 2 : total counting efficiency

(= full energy photopeak efficiency x peak/Total ratio)

Relevant:True coincidence effects: Absolute efficienciesRandom coincidence effects: Count rate + absolute efficiencies

In all cases needed: peak-to-Total ratio


Analysis

July 7-18, 2008

Measuring the p/T curve

Simple: using e.g. 65Zn or 137Cs

Complex: modeling using MCNP


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Coincidence effects


Analysis

July 7-18, 2008

Compton suppression systems


Analysis

July 7-18, 2008

Compton suppression spectometerTiming Electronics

Anti-coincidence

Spectroscopy electronics

Analogue to Digital

converter

Multi-channel pulse-height

analyzer

Annular scintillation detector shield with

plug detector

Sample


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

Gamma-ray spectrum of 137Cs, recorded with and without Compton suppression.

Compton suppression spectrometer


Analysis

July 7-18, 2008

Compton suppression spectrometer


Analysis

July 7-18, 2008

Selecting a detector


Analysis

July 7-18, 2008

P.Bode, J.Radioanal.Nucl.Chem. 222 (1997) 117-125 + 127-132


Analysis

July 7-18, 2008

Errors in gamma-ray spectroscopy

- Source-to-detector distance

- Filling height and source-self attenuation effects

- Coincidence summing effects


Analysis

July 7-18, 2008

Steps to be taken for an NAA laboratory

Vial with comparator

Vial with comparator

Vial with sample

Neutron fluence rate

Estimation neutron fluence rate at sample position

Not necessarily the arithmetic mean of the comparator values !


Analysis

July 7-18, 2008

Source-detector distance

Source-end-cap distance e.g. R cm

End-cap thickness + end-cap-crystal distance e.g 0.6 cmDistance to average point of complete absorption of photon energy e.g. 2.5 cm


Analysis

July 7-18, 2008

Steps to be taken for an NAA laboratory

Don’t correct just for difference in distance to end-cap! Include distance to average interaction point inside the Ge crystal (typically at ~ 1.5-2.5 cm (depends on size of detector; add 0.5 cm crystal-end cap distance)

sample standard

10 cm

0.5 cm

0.2 cm

0.5 cm2.5

cm

Correction for count rates: {10+ (0.5/2)+0.5 + 2.5}2/{10+(0.2/2)+0.5 + 2.5}2 = 1.023

Assuming uncertainty in interaction depth 1 cm: correction is then 1.025

Uncertainty of correction is difference between these corrections, viz. 0.2 %


Analysis

July 7-18, 2008

Source-detector distance

0 5 10 15 201.0

1.5

2.0

2.5

3.0

3.5

Co

rre

ctio

n f

act

or

Source-to-endcap distance (cm)

Source height 0.1 cm 0.2 cm 0.3 cm 0.5 cm 1 cm 2 cm 5 cm


Analysis

July 7-18, 2008


Analysis

July 7-18, 2008

The mot optimal choice


Analysis

July 7-18, 2008

gamma-ray spectroscopy

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