SCI-O11. Design of a Compton Spectrometer Experiment for Studying Electron Response of a Scintillator

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1 88 The 1 st NPRU Academic Conference SCI-O11 Design of a Compton Spectrometer Experiment for Studying Electron Response of a Scintillator P. Limkitjaroenporn and W.Chewpraditkul Radiation Physics Laboratory, Department of Physics, Faculty of Science King Mongkut s University of Technology Thonburi, Bangkok 114, Thailand Abstract The scintillation response of a plastic scintillator (NE12) has been characterized using Compton coincidence technique. The coincidence technique allowed detection of nearly monoenergetic internal electrons resulting from the scattering of incident 662 kev gamma rays within a primary NE12 detector. Scattered gamma rays were detected using a secondary NaI(Tl) detector in a coincidence mode. The electron response nonproportionality of NE12 plastic scintillator was measured using this technique for electron energies ranging from 28 kev to 436 kev, by varying the scattering angle. The NE12 showed a good proportionality of light yield within 3% at energies between 9 kev and 436 kev. Below 9 kev, the light yield decreases by about 2% upon lowering the electron energy to 28 kev. The electron energy resolution of NE12 was also measured using this technique in energy range from 68 kev to 436 kev. It has been found that the energy resolution varies from 25.2% at 68 kev to 11.8% at 436 kev. Keywords: Plastic scintillator, Electron response, Energy resolution 1. INTRODUCTION Scintillation detectors are widely used in experimental nuclear physics, high energy physics, nuclear medicine,medical imaging, environmental studies, geological exploration and many other fields of use. Important requirements for the scintillation materials used in these applications include high light output, fast decay time of the light pulse, high stopping power, good energy resolution, good proportionality of light yield, minimal afterglow and low production costs. A good reviews on development of inorganics scintillators and inorganic scintillation detectors systems have been published by van Eijk[1],Moszynski[2], and recently by Lecoq et al.[3]. Plastic scintillators are widely used in detection and spectroscopy of charged particles and neutrons. Although its light output is lower than that of most inorganic scintillators, it has many advantages e.g. fast decay time of light pulse, non-hygroscopic, easy to fabricate in any shape and low costs. Light yield nonproportionality can be characterized as a function of either photon energy (photon response) or electron energy (electron response). Photon response is defined as the ratio of light yield to the energy deposited by photon. Similarly, electron response is defined as the ratio of light yield to the energy of the electron. While electron response is believed to be an intrinsic characteristic of a scintillation materials, very few studies have reported this scintillator characteristic. Porter at al.[4] have studied the electron response of NaI(Tl), anthracene and plastic(pilot B) scintillators using photomultiplier tube. They measured electron response nonproportionality by using external electron source(magnetic

2 The 1 st NPRU Academic Conference 89 spectrometer-energy selected electrons).the light yield response from external electron sources may be influenced by the surface effects in scintillators. The Compton Coincidence Technique(CCT) has been implemented for the purpose of characterizing NaI(Tl) light yield nonproportionality [5],[6]. Subsequently, the electron scintillation response of CaF 2, BGO, LSO, CsI(Tl), CsI(Na) and YAP were characterized using the CCT [7], [8]. This technique allowed a more accurate characterization of the electron response because the energetic electrons are produced internally in the scintillator by γ-rays. In this paper, the CCT has been firstly used to measure the electron response of plastic scintillator (NE12) 2.54x2.54 cm 3 covering energies from 28 kev to 43 kev. Consequently, the electron energy resolution has been determined over the energy range from 68 kev to 436 kev using the CCT measured energy spectra. 2. COMPTON COINCIDENCE TECHNIQUE A schematic of the Compton Coincidence Technique(CCT) used for measuring electron scintillation response is shown in Fig.1[6]. In this technique, the tested scintillator is exposed to a collimated beam of monoenergetic γ-rays of known energy E γ.some of the γ- rays will undergo a Compton scattering within the scintillator, transferring energy E e to an electron and exit the scintillator at some angle θ with energy E' γ, given by E γ Eγ = 2 1+ (Eγ / m c )(1 cosθ) where E γ is initial γ-ray energy and m c 2 is the rest mass energy of an electron. The Compton electron energy within the tested scintillator can be calculated by using conservation of energy, (1) E e = E γ E γ (2) The response of the scintillator to Compton electrons is recorded in coincidence with the Compton scattered photon detected by a secondary high-purity germanium(hpge) detector at a given angle defined by the next collimator. In CCT technique, the Compton electrons within the primary scintillation detector approximate a monoenergetic internal electron source and can be used to characterize the nonproportionality of scintillator light yield as a function of electron energy by using variable angle of the HPGe detector. Fig. 1 Schematic of the Compton Coincidence Technique(CCT)

3 9 The 1 st NPRU Academic Conference 3. EXPERIMENTAL PROCEDURES The plastic scintillator (NE12) used in this study was supplied by Nuclear Enterprise, England, with dimension of 1x1 inch 3.The plastic scintillator was first wrapped with several layers of Teflon tape and mounted to the RCA 8575 PMT using DC 2 silicone grease. The PMT with plastic scintillator was wrapped with black tape and covered with thin aluminum housing. In this study, a 25 mci 137 Cs source(e γ =662 kev) was placed about 5 cm. from plastic scintillator with 1 cm diameter collimators as indicated in Fig. 1. The 2x2 inch 3 NaI(Tl) detector was used to detect the Compton scattered gamma rays. Ten difference angle(θ) were used with the CCT to produce ten electron energies ranging from 28 kev to 436 kev. NaI(Tl) Detector 137 Cs Plastic Scintillator PMT PA AMP PC PA TSCA MCA Board AMP DL AMP GATE ADC PC 32 Bit I/O Board Fig. 2 Nuclear instrumentation used in the CCT NE Cs (E γ = 662 kev) Counts Channel number Fig. 3 Energy spectrum of 662 kev γ-rays from 137 Cs source measured with NE12 scintillator.

4 The 1 st NPRU Academic Conference NE12 E e = 68.2 kev Counts Counts FWHM 52.% Channel number FWHM 22.5% NE12 E e = kev Channel number Counts Channel number NE12 E e = kev FWHM 15.2% Fig. 4 Energy spectra of Compton electrons with the indicated energies for NE12 scintillator measured with the CCT. A schematic of the nuclear instrumentation used in this study is shown in Fig.2. The signal from NaI(Tl) detector was passed to a preamplifier, amplifier and then to a timing single channel analyzer(tsca). The TSCA output was used to gate the plastic scintillator analog-to-digital convertor(adc). The electron energy spectra from the plastic scintillator in coincidence with the Compton scattered gamma rays were then recorded using a 32 Bit I/O

5 92 The 1 st NPRU Academic Conference board in the computer. A multichannel analyzer board(mca board) in the second computer was used to record the entire energy spectra of the Compton scattered gamma rays at various scattering angles. The coincidence electron energy spectrum for the plastic scintillator was a Gaussian shaped peak. For each electron peak, the centroid and full width at half maximum(fwhm) of the full energy peak were obtained from Gaussian fitting software of Canberra MCA. 4. RESULTS AND DISCUSSION Fig.3 presents the energy spectrum of 662 kev γ-rays from a 137 Cs source measured with NE12 plastic scintillator. One first notices that the pulse height distribution is shaped by the Compton scattered electrons and there is no full energy peak in the γ-ray spectrum. Fig.4 presents some of the Compton electrons with the indicated energies for NE12 scintillator measured in coincidence with the Compton scattered γ-rays at energies 68, 248 and 413 kev, respectively. 1 Light output (a.u.) Electron energy (kev) Fig. 5 Light output of NE12 scintillator as a function of electron energy.the solid line is the fit to points above 16 kev Light output (a.u.) Electron energy (kev) Fig. 6 Expansion of Fig.5. Light output as a function of electron energy,for energies up to 167 kev.

6 The 1 st NPRU Academic Conference 93 Fig.5 shows the relative light output(proportional to the centroid of the electron peak) as a function of the electron energy. The solid line is determined by the points above 16 kev. Expansion of the low energy region is shown in Fig.6. The CCT measured electron response for NE12 plastic scintillator is shown in Fig.7. These results are normalized to unity at 436 kev. These results indicated a nearly proportional electron response above 9 kev (~3% change). Below 9 kev, the electron response decreases more significantly indicating a larger light yield nonproportionality for the low energy electrons. The electron response decreases about 2% over the measured energy range. Below 28 kev, accurate electron response measurements were not achievable due to photomultiplier thermal noise and the relatively low light yield of plastic scintillator. Consequently, the electron response below 28 kev is not reported in this study Electron response, Re Electron energy (kev) Fig. 7 Electron response of NE12 plastic scintillator measured using CCT and normalized to unity at 436 kev. 1 η e = 29.64e -.2E e Energy resolution (%) 1 Total Geometric Component, η g Electron energy resolution, η e Electron energy (kev) Fig. 8 Electron energy resolution (η e ) of NE12 plastic scintillator determined using CCT. Also shown is the geometric component (η g ) due to finite solid angle subtended by the NaI(Tl). The total NE12 electron energy resolution is shown for comparison.

7 94 The 1 st NPRU Academic Conference.1 Porter at.al.[4] measured the electron response of plastic(pilot B) scintillators,for and.6 thick samples, coupled to a PMT. They exposed the detectors to a magnetic spectrometer-energy-selected electrons and measured a nonproportionality effect of ~5% and ~2%, respectively for.1 and.6 thick samples, in the energy range from 2 to 1 kev and a flat behavior in the energy range from 1 to 35 kev. The electron energy resolution, η e defined as the ratio between the full width at half maximum and the peak position, for the electron energy range from 68 to 436 kev, was also measured. In Fig.8 the electron energy resolutions are plotted versus the electron energies. The geometric broadening η e of the measured coincidence spectrum due to the range of electron energies associated with the finite solid angle subtended by the NaI(Tl) detector in CCT was also determined in analysis and is shown in Fig. 8. It is observed that the geometric component is significant at electron energies below about 15 kev. The energy resolution of plastic scintillator η e decreases exponentially with electron energy. Below 68 kev, accurate energy resolution measurements were not achievable due to photomultiplier noise and the relatively low light yield of plastic scintillator. Consequently, the energy resolution below 68 kev is not reported in this study. 5. CONCLUSIONS In this study the electron response for plastic scintillator(ne12) has been measured using CCT for electron energies ranging from 28 kev to 436 kev. The electron response of plastic scintillator was observed to increase monotonically with increasing energy. The light yield becomes proportional to within 3% at energies above about 9 kev. Below 9 kev, the electron response decreases by about 2% upon lowering the electron energy to 28 kev. The electron energy resolution for plastic scintillator has been measured using CCT for electron energies ranging from 68 kev to 436 kev. The measured energy resolution decreases exponentially with increasing electron energy. It varies from 25.2% at 68 kev to 11.8% at 436 kev. Further investigations of the intrinsic energy resolution and light yield non proportionality of the crystals are in progress, especially with inorganic scintillators by using Compton Coincidence Technique(CCT). 6. REFERENCES [1] van Eijk, C. W. E. (21). Inorganic-scintillator development. Nucl. Instrum. Methods. Phys. Res., A46, [2] Moszynski, M. (23). Inorganic scintillation detectors in γ-ray spectrometry. Nucl. Instrum. Methods. Phy.s Res., A55, [3] Lecoq, P., Annenkov, A., Gektin, A., Korzhik, M.,& Pedrini, C. (26). Inorganic scintillators for detector systems. The Netherlands: Springer. [4] Porter, F. T., Freedman, M. S., Wagner, F., Jr., & Sherman, I. S. (1966). Response of NaI, anthracene and plastic scintillators to electrons and the problems of detecting low energy electrons with scintillation counters. Nucl. Instrum. Methods, 39, [5] Rooney, B. D., & Valentine, J. D. (1996). Benchmarking the Compton Coincidence Technique for measuring electron response nonproportionality in inorganic scintillators. IEEE Trans. Nucl. Sci., 43(3), [6] Rooney, B. D., & Valentine, J. D. (1994). Design of a Compton spectrometer experiment for studying scintillator nonlinearity and intrinsic energy ersolution. Nucl. Instrum. Methods, A353, 33-7.

8 The 1 st NPRU Academic Conference 95 [7] Taulbee, T. D., Rooney, B. D., Mengesha, W. & Valentine, J.D. (1997). The measured electron response nonproportionalities of CaF 2, BGO, and LSO. IEEE Trans. Nucl. Sci., 44, [8] Mengesha, W., Taulbee, T. D., Rooney, B. D., & Valentine, J. D. (1998). Light yield nonproportionality of CsI(Tl), CsI(Na), and YAP. IEEE Trans. Nucl. Sci., 45,