A Measurement of Monoenergetic Neutrons from 9 Be(p,n) 9 B

Transcription

1 Journal of the Korean Physical Society, Vol. 32, No. 4, April 1998, pp A Measurement of Monoenergetic Neutrons from 9 Be(p,n) 9 B J. H. Kim, H. Bhang, J. H. Ha, J. C. Kim, M. J. Kim, Y. D. Kim and H. Park Department of Physics, Seoul National University, Seoul J. S. Chai and Y. S. Kim Cyclotron Application Laboratory, Korea Cancer Center Hospital, Korea Atomic Energy Research Institute, Seoul H. Y. Lee and S. A. Shin Department of Physics, Ewha Womans University, Seoul J. Y. Huh, C. S. Lee and J. H. Lee Department of Physics, Chung-Ang University, Seoul (Received 23 December 1997) We measured monoenergetic neutrons from 9 Be(p, n) 9 B with a 35-MeV proton beam from the MC-50 cyclotron, which is mainly used for the medical purpose, at the Korea Cancer Center Hospital. The ground state and the excited states of 9 B were identified from the time of flight measurments of the neutrons. We obtained an energy resolution of about 660 kev for 33-MeV neutrons for two types of scintillation counters (NE213 and BC408) using a 0.5-mm-thick 9 Be target. I. INTRODUCTION The MC-50 cyclotron at Korea Cancer Center Hospital (KCCH), which is an AVF (azimuthally varying field) type, has been used mainly for medical purposes, such as neutron therapy for cancer patients by using the 9 Be(p, n) 9 B reaction and radioisotope production [1]. At present, this is the only ion accelerator in Korea with a beam energy over 10 MeV. Recently some of beam time has been used for an experimental nuclear study [1,2]. We measured the monoenergetic neutrons from 9 Be(p, n) 9 B by using the 35-MeV proton beam from the MC-50 cyclotron at KCCH in order to study the energy resolution of scintillation detectors for fast neutrons. The energy resolution depends on not only the detector characteristics but also the cyclotron itself, e.g., the beam pulse width, the extracted beam-energy resolution, etc. The energy resolution of neutrons is important for future studies of various nuclear reactions involving neutrons. Typically, fast neutrons of a few tens of MeV are produced by the nuclear reactions 7 Li(p, n) 7 Be and 9 Be(p, n) 9 B. Also, we adopted 9 Be as our target in order to generate fast neutrons. In the case of 9 Be(p, n) 9 B, the jungho@ieplab.snu.ac.kr, telefax: reaction Q-value is MeV. Since 35-MeV protons were used as the beam, the zero-degree neutrons from the ground state of 9 B could have kinetic energy of about 33 MeV. II. EXPERIMENTAL SETUP Our experimental setup is schematically drawn in Fig. 1. We used a 35-MeV proton beam with a beam current of about 100 na from the zero-degree beamline, which was originally used for neutron therapy. The protons extracted from the cyclotron were bent upwards at the 70-degree bending magnet inside the gantry. Inside the gantry, a 1-cm-thick berillium target and a tungsten collimator had been installed for neutron therapy. During the experiment, we closed the tungsten collimator and used it as a beam dump. A 9 Be target with a thickness of 0.5 mm was located upstream in the gantry, and the scintillation counter was located at a distance of 8 m from the experimental target at zero degrees. The maximum available distance was 8 m because of the limited space in the treatment room. Also we placed a 1.2-cm-thick aluminum plate just after the bending magnet and used it as a charged particle -462-

2 A Measurement of Monoenergetic Neutrons from J. H. Kim et al Fig. 1. Experimental setup of our experiment using the MC-50 cyclotron at KCCH. stopper. The charged particles from the reaction at the target were mostly spread out at the 70-degree bending magnet or stopped in this aluminum stopper. Neutrons from the reaction passed the aluminum plate and flew through air until they reached the detector. Table shows the specifications of the detectors which were used for neutron detection. We used two types of detector. One was made of a liquid scintillator (NE213) and had a 2 [φ] 4 cm[t] cylinder shape. The other was made of a plastic scintillator (BC408) and had a cm 3 cubic shape. A 2-inch PMT (H1161) was mounted on both types of detector. We used the pulse-shape discrimination (PSD) method to discriminate gammas from neutrons. The light emission of most scintillators has two components in its signal: one fast component and one slow component. Also, for some scintillators, the ratio of the fast to the slow component varies with the type of exciting radiation [3]. As a result, the overall decay time of the emitted light pulse varies, allowing the use of the PSD method. It is well known that a NE213 liquid scintillator is very effective for separating gamma rays from neutrons by using the PSD method [3]. We also applied the PSD method with a BC408 plastic scintillator which was adopted to see whether it could separate neutrons from gammas to any extent. In addition, a paper reported the feasibility of using the PSD method for neutron/gamma separation with C 6 H 6 type scintillators [4]; however, the author didn t note clearly which material he used. Surrounding the detector, we arranged thick lead blocks, which were about 10-cm thick, to shield against the room background gammas. Figure 2 shows the electronics diagram, including the PSD scheme. The same electronics scheme was used for both the NE213 and the BC408 detectors. The H1161 PMT has two anode outputs and one dynode output. One anode output was used for trigger timing (ADC gate and TDC start) after being discriminated with a constant fraction discriminator (CFD) to reduce the time walk effect due to the pulse-height differences between events. The other anode was used for the ADC input to measure the energy deposited by an incident particle. The dynode output was used for the input to the pulseshape analyser (PSA) module after being amplified and shaped using a pre-amplifier and a delay-line amplifier (DLA). The PSA module gave the timing of the trailing edge of the pulse at some fraction of the full pulse height. The output of the PSA module was used as the TDC stop. To measure the time of flight of the neutrons and the gamma-rays at the target, we used the RF timing of the cyclotron which could give the production time of the Table 1. The specifications of our neutron detector. Scintillator Effective Area Thickness PMT NE213 (liquid) 20.3 cm 2 (φ = 2 ) 4 cm H1161 BC408 (plastic) 100 cm 2 (10 10) 10 cm H1161

3 -464- Journal of the Korean Physical Society, Vol. 32, No. 4, April 1998 Fig. 2. Block diagram of the electronics used in this experiment, including the PSD method. neutrons or gamma rays from the target after substracting the timing offset. The ADC and the TDC information were processed with a CAMAC system and PC. Data acquisition was done using KODAQ [5], which could be used for the online monitoring, too. The raw data were recorded on the hard disk for off-line analysis. We stored about 500 events per second. III. DATA 1. Particle Identification The neutron/gamma separation was done by the PSD method. Figure 3 shows the neutron/gamma separation achieved with the NE213 scintillation detector. The x- axis represents the PSD timing, which is the time difference between the leading edge and the trailing edge of the pulse. The y-axis represents the ADC and corresponds to the energy deposited by the gamma-rays or the neutrons. The figure shows a strong correlation between the ADC and the PSD timing because we didn t try to optimize the PSD scheme. However, the separation of the neutrons and gamma-rays was good enough over the entire ADC range, as one can see from the figure, to do the TOF analysis. For the BC408 plastic scintillator, we could not see any separations. 2. Timing Spectra The TOF was determined by the timing difference between the detector timing and the cyclotron RF timing (T RF T d ). Figure 4 shows the time of flight (TOF) spectrum of scattered particles obtained by the NE213 detector. The solid line is the TOF spectrum of the gammas identified by the PSD spectrum and shows a sharp

4 A Measurement of Monoenergetic Neutrons from J. H. Kim et al Fig. 5. Time structure of the RF signal and the detector output. The detector output was used for the start signal, and the RF signal of the cyclotron for the stop signal of the TDC. Fig. 3. Neutron/gamma separation achieved with NE213 using the PSD method. One can see clear separation of neutrons (right) from gammas (left). A small background is seen on the right side of the neutron line. peak, as expected. The dashed line is that of the neutrons and shows a rather broad structure due to factors discussed later in this paper. In the gamma spectrum, one can see many peaks, and we should determine which peak is the gamma from the target. To check this point, we removed the experimental target, and we found that only the peak (at around channel 1290) labeled by γ mostly disappeared. Hence, we could determine the reference position of our TOF spectrum because the velocity of the gamma is constant. The additional peaks in the gamma-gated spectrum were found to be gammas from the beam dump in the gantry and background in the beam transport. The RF frequency of the MC-50 cyclotron for a 35- MeV proton beam is about 22 MHz, which corresponds to a period of 45 ns. As shown in Fig. 5, the period of 45 ns is shorter than the flight-time difference between the gamma and the neutron from the 9 Be(p, n) 9 B reaction for an 8-m flight. The fastest neutron from the target is about 33 MeV, and the flight-time difference is about 73 nsec. Thus, the timing difference between the gamma and the fastest neutron should be determined by F T n F T γ = 2T RF + (T γ T n ), where F T n(γ) and T n(γ) are the flight time and the peak position in the timing spectrum of the neutron (or gamma), respectively, and T RF is the RF period. For the same reason, the TOF spectrum of high-energy neutrons overlapped with those of low-energy neutrons and gave a large background in the neutron spectrum (dotted line in Fig. 4). To remove this overlapping and to study the energy resolution of the detector system, we imposed an additional ADC cut (ADC > 500). As one Fig. 4. Time of flight (TOF) spectra for gammas (solid line) and neutrons (dotted line) from the target as obtained by the NE213 detector. Each spectrum was drawn after particle identification using PSD spectrum. Fig. 6. Neutron-energy spectrum measured by the NE213 scintillation detector. One can see the ground state and the excited states of 9 B clearly.

5 -466- Journal of the Korean Physical Society, Vol. 32, No. 4, April 1998 Table 3. The various the contributions of the finite neutron-energy resolutions. All quoted values are one standard deviation. δl Detector δt OF δe t (%) l T OF E Total(%) BC NE state and the excited state of 9 B. The sharp peak appearing at the leftmost side is self-timing events due to the finite width of the RF logic signal. Fig. 7. Same as Fig. 6, but measured with the BC408 scintillation detector. can see from the PSD plot (Fig. 3), this ADC cut also rejects most of the gamma events. 3. Neutron Energy Spectra The neutron velocity was calculated from the time of flight difference between the gammas and the neutrons in the spectrum of Fig. 4. Using relativistic kinematics, the energy of neutrons was obtained from the velocity and the mass of the neutron after particle identification. Figure 6 shows the calculated neutron-energy spectrum detected with the NE213 liquid scintillator. The peak position of the fastest neutrons was 33.4 MeV, which is consistent with the expected energy of 32.6 MeV for the ground state of 9 B. In the figure, we can identify the neutrons from the excited states of 9 B at the position of 30.5 MeV, which is also consistent with the expected excitation energies of MeV for the 1st excited state and MeV for the 2nd excited state [6]. The overall spectrum is consistent with the previous experimental result with a similar TOF distance [7]. Fig. 7 shows the same figure as Fig. 6, but measured with the plastic scintillation detector (BC408). In this figure, we can also see the clear separation of the ground Table 2. The overall neutron energy resolution obtained in this experiment. The σ means one standard deviation (68% confidence level) of a Gaussian fitting of the ground-state peak. Detector Neutron Energy FWHM σ Resolution (MeV) (MeV) (MeV) (%) BC NE IV. SUMMARY Table shows a summary of the results. We got an overall energy resolution of 2% for the neutrons of 33 MeV in one standard deviation (68% confidence level). There are many factors which contribute to the finite energy resolution of the neutron-energy measurement. Some of them come from the detection side, and the others from the cyclotron itself. For the detection side, these are the intrinsic timing resolution of the detectors, the time-walk effect of the electronics, the finite depth of the detector, beam-energy straggling inside target, etc. For the cyclotron side, we should consider the beam pulse width, the RF timing resolution, the beam energy resolution, etc. Using a non-relativistic approximation, the relative uncertainties in the energy can be written as δe E = (δl l ) 2 ( ) 2 δt OF + + T OF ( ) 2 δet E where δt OF 2 = δtd 2 + δt b 2, δt d being the intrinsic timing resolution of the detector, and δt b the pulse width of the beam. δt OF was dominated by δt b because the intrinsic resolution of the detector was on the order of 100 ps, but the beam pulse width was 2 ns. Also, δe t is the beam-energy straggling inside target, and δl is the depth of detector. Here, the time-walk effect was corrected during the off-line analysis. The detailed procedure for the time-walk correction was shown in Ref. 5. The beam-energy resolution was not considered. Table shows the contribution of each term. The summation of all these factors is about 430 kev (1.3%). The estimated energy resolution is smaller than the experimental energy resolution of 660 kev (2%) for both NE213 and BC408, which means that some additional factors of about 500 kev (1.5%) are missing. One of these factors could be the beam-energy spreading. The energy resolution for the 35-MeV proton beam of this cyclotron is known to be about 500 kev (1.4%). If we adopt this value for the beam-energy resolution, the experimental resolution for the neutrons is exactly reproduced.

6 A Measurement of Monoenergetic Neutrons from J. H. Kim et al V. DISCUSSION We measured the monoenergetic neutrons from the reaction 9 Be(p, n) 9 B by using BC408 and NE213 scintillation detectors. The neutron energy was measured with the time of flight methods, and we got an energy resolution of about 2% which was dominated by the beamenergy straggling inside target, the beam pulse width, the distance from the target to detector, and the beamenergy spreading. To improve the neutron-energy resolution in this energy range, we may have to reduce the beam pulse width or improve the energy spreading of the beam. However, currently, the flight path of the neutron should be lengthened considerably in connection with the thin target. Also, the beam pulse separation should be large enough to enlarge the dynamic range of the neutron-energy measurements. The results of this experiment show the possibility of measuring various physics quantities, e.g., the total and the differential cross sections, and the angular distributions of various neutron reactions involved with the MC-50 cyclotron. ACKNOWLEDGMENTS We appreciate very much the generous support from the staff members of the Korea Cancer Center Hospital. H. B. acknowledges the partial supports from the Ministry of Education through the Basic Science Research Institute of Seoul National University and from the Ministry of Science and Technology through the 97 Atomic Energy Research and Development Project. J. S. C. acknowledges the support from the Ministry of Science and Technology. Y. D. K. acknowledges the support from the Korea Science and Engineering Foundation. REFERENCES [1] Conference Proceedings of Kongneung Symposium on Nuclear Physics 96, edited by C. S. Lee, J. C. Kim and J. S. Chai (1996). [2] C. S. Lee, J. C. Kim, H. T. Chung, J. H. Ha, Y. K. Kim, S. J. Chae, J. S. Chai, Y. S. Kim and J. D. Lee, J. Korean Phys. Soc. 27, 474 (1994). [3] W. R. Leo, Techniques for Nuclear and Particle Physics Experiments (Springer-Verlag, Berlin, Heidelberg, 1994), Chap. 7. [4] M. Ahmed, Nucl. Instru. Meth. 143, 255 (1977). [5] Y. D. Kim, H. Bhang, O. Hashimoto, K. Maeda, K. Omata, H. Outa, H. Park and M. Youn, Nucl. Instru. Meth. A372, 431 (1996), and references therein. [6] Table of Isotopes, 8th Edition. [7] J. W. Watson, F. J. Wilson, C. A. Miller and D. O. Wells, Nucl. Instru. Meth. 164, 129 (1979).