The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Photon Counting Method for high flux Pulsed Laser-Compton Backscattered Photons
based on Poisson modelII

23Toshiteru Kil, Hiroyuki TOYOKA WA , Hideaki OHGAKI , Takashige BABA , Tatsushi SHIMA and Yasuki NAGAI

3

Institute of Advanced Energy, Kyoto University, Gokasho, Uj i, Kyoto 611-0011, Japan
1Quantum Radiation Division, Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, !BARAK! 305-8568, Japan

2Department of Applied Physics, Tokyo Institute ofTechnology, 2-12-1 0-okayama, Meguro, Tokyo 152-8551, Japan
3

Research Center for Nuclear Physics, Osaka University, 10-1 Mihogaoka, lbaraki, Osaka 567-0047, JapanAbstract

mnWe tested a new photon counting method based onPoisson model to evaluate the absolute photon flux ofpulsed photon generated by the Compton scattering of laser
light on high-energy pulsed electron. The Monte Carlosimulation code in which the pile-up phenomenon is taken
into account is developed to avoid the under estimate of thephoton flux. The calculated response functions of the BGO
scintillation detector well agreed with measured ones.

1 Introduction
The Laser Compton backscattered (LCS) photons
generated by the Compton scattering of laser light onelectron are quasi-monochromatic and polarized ones andtheir energy is variable. Thus the LCS photons are applied
for nuclear physics, detector calibration and so on. For theseapplications, it is very important to measure the absolute
photon flux. But these photons are produced in a shortperiod, and each pulses form radiation detector can't beseparated, because electron bunch length is very short.
Hence in this study a new photon counting method based on
Poisson model are developed.

-P(n) = - e mn!

(2),where m is the average number of the scattered photons per
pulse given by m=Np. Therefore the pulse height spectrums
will agree with the Poisson distributions, if the energy
resolution of the detector is infinite and the detection
efficiency is 100%. Thus the absolute photon flux can bemeasured by fitting the measured pulse height spectrumwith response functions. Response functions of the large
BGO scintillation detector for the multi-photon injection inwhich resolution of the detector, the time interval of theeach scattered photons and pulse response of the circuits aretaken into account are shown in Fig.l.

N=S

2 Principle of the calculation

The scattering probability of the laser photon on theelectron is proportional to the cross-section of the Compton
scattering. Hence probability of the photons are scatteredfollows the binominal distribution. The probability
distribution of the number of the scattered photons is given

P(n)=N'. pn(l-p)N-n (1),(N -n)!

where n is the number of the scattered photon, p is thescattering probability and N is the product of the numbers ofparticles contained in the photon and the electron bunch. Incase of the LCS, p is very small and N is quite large, thus
the binominal distribution are approximately written in thePoisson distribution,

ChannelFig. I The gamma ray response function of the BGO
scintillation detector for the multi-photon injection.
N is the number of the photon injected to thedetector.
The response function for the LCS photons, which aregiven by adding the response functions for the multi-photoninjection is shown in Fig.2.

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The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999-;;.,·iThe pulse height spectrum ·measured by the BGO
scintillation detector is shown in Fig.5 (thick line). Theresponse function for the LCS photons is also shown inFig.5 (thin line). In this case, the response function tominimise the residual error is that for 4.75 photos/pulse. As
shown in Fig.5, the response function can reproduce themeasured spectrum quite well.

Response function for 5photons/pulse

0.1=Cli0.01]
Cli.::~~0.001;;.,.t:
600

800

...."'.s.2:
....
..::=
Cli
l::t::

3 Experiment and results

ETL 240-SOOMeV
Storage Ring TERAS
lmax 500mAr800

1000

It is found that the response function based on thePoisson model can reproduce the measured pulse heightspectrum of the BGO scintillation detector quite well. Thismethod can be applied for the photon counting.

·[1] T. Tomimasuetal., IEEE Trans., NS-30,3133(1983)[2] H. Ohgaki et al., IEEE Trans., NS-38, 386 (1991)

200.----r----,---~----.----.

150"'

100.~

600

4 Conclusion

The time distribution of the LCS photons is shown inFig.4. The width of the time distribution is concerned withthe pulse width of the Q-switch Nd:YLF laser.

2000

200Fig. 5 Measured pulse height spectrum (thick line) andresponse function (thin line).

Fig. 3 Experimental setup

J05 References

BGO

010-4

Channelscintillator50

to- 110"'

Experiment was carried out using 3.5MeV LCS photons
provided by the storage ring TERAS[l] at ElectrotechnicalLaboratory (ETL) and Q-switch Nd:YLF laser[2]. Fig. 3shows the experimental setup. The backscattered photons
are collimated with lead collimator. The time difference ofmaster clock of the Q-switch laser and the LCS photons wasmeasured with thin plastic scintillation detector. The LCS
photons were detected by the BGO scintillation detector
with a diameter of2 inches and a thickness of 10 inches.

0to- 2Cli

Fig. 2 The response function for the LCS photon. The
average incident photons per pulse are 5.

-==

10-1

Cli

1000

Channelu

measured pulse height spectrum
response function for 4.75photons/pulse

0.0001

4000

Time differnce[ nsec IFig. 4 Time difference between master clock of the Qswitch Nd:YLF laser and the scattered photons.

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