measurement of the absolute efficiency, with a precision better than 2%, of a pmt working in single...
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![Page 1: Measurement of the absolute efficiency, with a precision better than 2%, of a PMT working in single photoelectron mode Philippe Gorodetzky APC lab, Paris,](https://reader035.vdocuments.us/reader035/viewer/2022062716/56649ddc5503460f94ad3270/html5/thumbnails/1.jpg)
Measurement of the absolute efficiency, with a precision better than 2%,
of a PMT working in single photoelectron mode
Philippe GorodetzkyAPC lab, Paris, France
VLVT09, Athens, October14, 2009
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How to calibrate ?
Comparison to a reference
Calibratedsource
Calibrateddetector
(photodiode)
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1) Calibrated Source
• GOOD : Only 1 measurement– Time variations of no importance
• BAD : Control of the spatial variations of the source. ==> IMPOSSIBLE– Angle & surface of emission (Liouville)
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2) Calibrated Detector
• GOOD : Spatial variations of no importance if comparison made in same conditions
• BAD : Need 2 measurements=> time variations important, but can
be controlled through a third detector ==> POSSIBLE
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Needs
• NIST photodiodes have a gain of about 0.5. So the light flux has to be reduced ~106 times
• Avoid geometry problems in illumination=> exactly same geometry for PMT and
calibrated detector
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How (is the calibration done)2 steps
• Mapping of the photocathodesRELATIVE
• Comparison to a NIST photodiode @ 1 position
ABSOLUTE
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Mapping of the photocathodes
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Very few photons
We illuminate with an LED of the good wavelength, and pulse at one kHz in order for the ADC to follow. To make a single photoelectron spectrum, while we acquire on the ADC, we lower the quantity of photons sent per pulse until the obtained peak has a stable position*. Then the number of events in that peak lowers while the number of events in the pedestal increases. When do we stop lowering the light?
The spectrum is mainly a one pe (the bump), but there is still a « peak » at 2 pe, very weak, which will be troublesome when a discriminator is set between the pedestal and the 1 pe, and we count with a scaler: each 2 pe counts double, and the result will be wrong.
scaler
scaler
•One can also use an oscilloscope and watch when the base-line under the pulse begins to fill
SINGLE PHOTO-ELECTRON
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We use Poisson: and P0, P1, P2 are the respective populations of the
pedestal, of the 1 pe and of the 2 pe
P0 = (m0 / 0!)e-m = e-m
P1 = (m1 / 1!) e-m = m e-m = m P0
P2 = (m2 / 2!) e-m = (m2 / 2) e-m = (m / 2)P1 = (m2 / 2)P0
If one wants that P2 = 1% x P1, (m / 2)P1 = 0.01P1, then m / 2 = 0.01, and m = 0.02
Now, the ratio between P0 and P1: P0 / P1 = P0 / (mP0) = 1 / m will be 1 / 0.02 = 50
In our case, as soon as the pedestal is 50 times more important than the 1 pe, the 2 pe will be less than 1% of the 1 pe
Usually, one takes: pedestal = 100 times 1 pe. Then we are sure not to pollute the measurement. Now we can set the discriminator threshold to be in the bottom between the pedestal and the 1 pe (at 0.25 of the 1 pe), and we just have to count in two scalers the pulses sent to the LED and the discriminator output. Exit the ADC: one can pulse until 100 kHz, which allows comfortable statistics in a few minutes.
Also, the threshold being in a valley, the measurement will not be very sensitive to a small variation of the threshold, or of the gain due to HV small changes.
€
Px =mx
x!e−m
SINGLE PHOTO-ELECTRON
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Mapping of the photocathodes
Reduce the light per pulse & adjust the gain
Optical fiber
•One can also use an oscilloscope and watch when the base-line under the pulse begins to fill
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Mapping of the photocathodes
In red: Coïncidences between generator & PMT discriminator
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Mapping : « PMT-JY »
The photocathode is naked ( = 51 mm)
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Mapping of the photocathodes.Here, absolute
Better efficiency if we use only the central part=> diaphragm of 20 mm
Full pmt (40 mm diameter)
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Absolute measurement
• PMT and 1 photodiode at the same time
• BUT : very different gains : 1 vs 107
=> how to divide a light flux by 107 ?
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Absolute measurement
• Use of integrating spheres to reduce light
• Measurement of the light flux reduction
• Measure the PMT efficiency
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http://www.labsphere.com/data/userFiles/A%20Guide%20to%20Integrating%20Sphere%20Theory&Apps.pdf
SINGLE PHOTO-ELECTRON
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If one measures 1 nW in the second diode (noise = 1 pW) and 0.775 mW in the first :
Ratio = 7.75 105
Calibration of the system
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ANALYSIS
100 kHz: 14.425 nW in NIST
As the ratio = 7.75 105
One sends on the PMT: 14.425 nW / 7.75 105 = 1.861 10-5 nW
Energy of a photon @ 378 nm: E = h = hc/
E = 6.026 10-34 x 3 108 / 378 10-9 = 4.783 10-19 J
One knows that 1 J = 2.090 1018 photons
So 1.861 10-5 nW ==> 1.861 10-14 x 2.090 1018
= 38912 photons / sec.
In one measurement of 100 sec, we have sent on the PMT: 3891200 ph
We have measured 686797 pe ==> efficiency (discri) = 686797 / 3891200 = 17.65%
We have to add 8.8% (discri) so efficiency = 19.2 % at 377 nm (PMT center), and 15.8% for full pmt, instead of 22% given by Photonis
Discri
Calibration of the PMT
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Absolute measurement
Uncertainties :
• Flux reduction (ratio) : 3 % (2 NIST diodes)
ΔR/R = (ΔI/I + Δα/α)udt + (ΔI/I + Δα/α)o1
• Efficiency measurement : 1.7 % (1st NIST cancels out)
Δε/ε = ΔR/R - (ΔI/I + Δα/α)udt
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3 Leds
NIST Photodiode
trans-impedance amp.
Integrating sphere
4 cmAmplification of TTL pulses in 40 V pulses with a risetime of 2 ns collimator
If one wants a more collimated photon beaminstead of Lambertian distribution
Another way to look at the set-up: the first sphere is a "perfect" splitter (to the NIST and the first diaphragm)followed by a very stable light reducer.
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One application: AntaresAnd why not NESTOR, or km3 ?
They calibrate their system with atmospheric muons,but do not know very well (!!!) the efficiency in the back of the tube
X,Y,Z, , movement in a black box
The light source:
- integrating sphere
- collimator
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Another application: JEM-EUSO 36 pixel Hamamatsu PMT
Efficiency of pixel 22, at 378 nm, and looking only at anode 22 = (17.9 ± 0.32)%
Then, the sum of all 9 pixels gives the full pixel 22 absolute efficiency at 378 nm when looking at all 9 anodes: (29.8 ± 0.54)% where the relative uncertainty has the same causes than above, and is 1.8%.
Assuming a collection efficiency of 70% (Hamamatsu), one gets a quantumefficiency of 40%Less than 1% of the counts are in coincidence in any combination of 2 pixels
So, it is not a cross-talk, but a point spread function of 5.8 mm diameter, twice thediameter of the PSF of the lenses, that is 4 times its surface.
Hence, Hamamatsu is designing a new PMT, with a better focus (a 64 pixels)
We illuminate the center of pixel 22 with a spot of 1 mm size.