Measurement of Double Differential Cross Sections of ... - Ipen

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Yoshioka, S. Matsuyama, N. Hirakawa, T. Nakamura, Su. Tanaka, S. Meigo, H. Nakashima, Sh. Tanaka and N. Nakao, Nucl. Instrum. Methods A428 (1999) 454.
Measurement of Double Differential Cross Sections of Secondary Heavy Particles Induced by Tens of MeV Particles M. Hagiwara1), T. Sanami2), M. Baba1), T. Oishi1), N. Hirabayashi1), M. Takada3), H. Nakashima4), and S. Tanaka4) 1) Cyclotron and Radioisotope Center, Tohoku University, Sendai, Japan 2) High Energy Accelerator Research Organization (KEK), Tsukuba, Japan 3) National Institute of Radiological Sciences, Chiba, Japan 4) Japan Atomic Energy Research Institute, Ibaragi, Japan Abstract. Measurement of differential fragment production cross sections for proton- and neutron-induced reactions is described. The double-differential fragment production cross-section for 70-MeV protons and the yields for 75 MeV neutrons were obtained with a specially designed Bragg curve spectrometer (BCS). An experiment is also described for proton-induced reactions employing the E-TOF method to obtain complementary and more detailed information than neutron-induced reactions.

(E-TOF) method [3] having the capability of mass identification even in the energy region where BCS is not applicable, while the solid angle is very small.

INTRODUCTION Energy and angular double-differential crosssection data for fragment (secondary charged particles heavier than the Li particle) production of proton- and neutron-induced reactions are of importance for dosimetry and the evaluation of radiation effects such as single event upset (SEU) by cosmic-rays because of their large local ionization. However, experimental data on the fragment production are very scarce due to experimental difficulties in fragment detection, i.e., low yield and large energy loss in samples. For that reason, most of the past experimental data were obtained by the activation method, which does not provide energy and angle information. Furthermore, theoretical calculations treating fragment production are few and uncertain. Therefore, it is important to obtain reliable experimental data of differential cross sections for fragment production.

BCS has been used mainly for fragment measurement in heavy charged-particle-induced reactions but not applied to neutron-induced reactions. We designed the BCS with special care for applying to a neutron beam in addition to a charged-particle beam, and we were successful in identifying the fragments by proton- and neutron-induced reactions. The BCS proved very promising for fragment detection even in neutron-induced reactions, but there are still some problems that should be solved. An E-TOF method is restricted only to charged-particle-induced reactions due to the small detector solid angle. The dynamic range of fragment energy will be higher than in the BCS. This paper describes energy-angular fragment production measurement in proton- and neutroninduced reactions and extension to proton-induced reactions employing the E-TOF method.

For fragment detection, we adopted (1) a Bragg curve spectrometer (BCS), providing almost all information on the particle with a single counter in a large solid angle [1,2] and (2) an energy-time-of-fight

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BRAGG CURVE SPECTROMETER (BCS)

APPLICATION OF THE BCS TO PROTON-INDUCED REACTIONS

Figure 1 shows a schematic diagram of the BCS developed. It is a cylindrical gridded ionization chamber (GIC) [4,5] filled with an Ar + 10%CH4 gas at a pressure of ~200 Torr. In the case of protoninduced reactions, fragments produced from targets in the vacuum chamber enter the detector along the axis through a thin film window and ionize the gas in the BCS. The free electrons drift to the anode by the electric field keeping the shape of the Bragg curve. The time distribution of the anode signal corresponds to the reversal of the ionization distribution (Bragg curve) by the fragment. Therefore, the fast part of the anode signal is proportional to the Bragg peak value, which is in proportion to the atomic number (Z) of the fragment. The integration of the entire anode signal represents the total charge, which is proportional to the fragment energy. Therefore, the BCS can provide information on the energy and the atomic number of fragments using only the anode signal.

The experiment was performed at the AVF cyclotron laboratory of the National Institute of Radiological Science (NIRS). The fragments emitted from the polypropylene target (4 µm thick, 0.9 g/cm3) to a 30-degree direction were measured. The proton beam current was measured by a Faraday cup installed downstream of the target. The proton energy and beam current were 70 MeV and ~4 nA, respectively. In the measurement circuit, the energy signal and the Bragg peak signal were obtained from the anode signal by processing with a long time constant (6 µsec) and a short time constant (0.25 µsec), respectively. To reduce background events and the dead time of ADC, only coincidental data between the anode and cathode were accumulated. The coincidence time is set to 6 µsec, which is equivalent to the maximum traveling time of electrons from the cathode to the grid. They are collected as two-dimensioned data using the KODAQ handler [7] with a CAMAC system.

Ar+10%CH4 gas 200 Torr

Figure 2 shows the measured two-dimensional spectrum on the energy vs. Bragg peak, and Bragg peak spectrum over separation limits. Good separation between each fragment and S/N ratio is confirmed. In this case, lighter fragments are produced mostly and heavier products are very few. The turning blows at the maximum energy point in Fig. 3 are caused by the fragments that have ranges longer than the

Drift space 27cm Drift to anode by E field

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FIGURE 1. Schematic diagram of the developed Bragg

curve spectrometer (BCS). The distance between the cathode and the grid is 27 cm, and between the grid and the anode is 0.5 cm. The wire radius and spacing of the grid are 0.1 mm and 1 mm, respectively. Thus, the grid inefficiency is 2.6% [6]. Ring electrodes are arranged in 3-cm steps to achieve a homogeneous electric field. To apply the BCS to neutron-induced reactions, we put samples inside the chamber to decrease the energy loss of fragments and enlarge the detection solid angle. We adopt tight neutron collimation, high-Z element (Ta) electrodes having small fragment production rate, and an additional shield electrode to reduce backgrounds due to neutron irradiation of the chamber body and counting gas.

FIGURE 2. Energy vs. Bragg peak two-dimensional spectra (top) and Bragg peak spectrum above separation limits (bottom) for polypropylene (4 µm) induced by 70-MeV protons.

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En=75MeV Be spectra

En=75MeV Li spectra

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rate of the fragment using a thick sample (carbon: 100, 200 µm) and the backgrounds using nickel (100 µm) and aluminum (6 µm). In this work, we tested thin samples to minimize energy loss of fragments. An experiment was performed using the 7Li(p,n) neutron source at TIARA [11]. To measure fragments from carbon, we used thin samples of polypropylene (10 µm) whose thickness makes energy loss correction less than 10% of its energy for lithium. In addition, 200 µm carbon, 500 µm silicon, and 10 µm gold foil were employed. The gold foil is used as backing for the polypropylene sample since the sample should act a cathode electrode also. Each sample set inside the cathode plate was irradiated directly by neutrons collimated by a ~3-m-long neutron collimator, and an additional ~60-cm-long collimator was used to avoid neutrons hitting the BCS structure. The neutron peak energy was ~75 MeV. The proton beam current incident to Li target was ~1 µA.

En=75MeV B spectra C (200µm)x1000 Si (500µm)x1000 Poly 10µm Au 10µm

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FIGURE 3. Comparisons of the yield of Li, Be, and B for carbon (200 µm), silicon (500 µm), polypropylene (10 µm), and Au (10 µm) and induced by 75-MeV quasimonoenergetic neutrons.

Figure 5 shows the two-dimensional spectrum on the energy vs. Bragg peak for a neutron-induced reaction on a 200-µm-thick carbon sample. The spectrum was obtained by only ~2 hours of irradiation. The electronics circuit is the same as the case for protons. Fragments heavier than the α particle are separated distinctly, though the separation of the Bragg peak was much inferior compared with a protoninduced reaction due to the effects of the various emission angles.

cathode-grid distance. It is meaningful to extend the measurable energy range by developing a correction method for this effect. Energy calibration was performed by replacing a sample with a 241Am α source. The DDX for Li, Be, and B from carbon obtained by the polypropylene sample are shown in Fig. 4. As observed in Fig.4, the present results show a similar shape for Li with the data by C. T. Roche et al [3] but a larger one than LA150 [8] and the QMD calculation by PHITS code [9]. Li Spectra

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FIGURE 4. DDX for lithium, beryllium, and boron emission from carbon at 30 degrees by a 70-MeV proton. FIGURE 5. Energy vs. Bragg peak two-dimensional spectra (top) and Bragg peak spectrum above separation limits (bottom) for carbon (200 µm) induced by 75-MeV quasimonoenergetic neutrons.

APPLICATION OF THE BCS TO NEUTRON-INDUCED REACTIONS In a previous work [10], we succeeded in measuring fragments induced by 65-MeV quasimonoenergetic neutrons and estimated the counting

Figure 3 shows the results of Li, Be, and B energy spectra from polypropylene, thick carbon, thick silicon,

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and gold foil. Events from gold foil can be regarded as background for the data from polypropylene. It means that we can obtain sufficient statistics in this condition. In the case of the silicon sample, the yield is low in comparison with polypropylene in spite of the thick sample. In either case, the energy spectrum of He, Li, Be, B, and C could be observed with an acceptable S/N and counting rate.

experiments, the BCS proved to be applicable to not only proton-induced reactions but also neutroninduced reactions. We intend to improve the measuring method as follows with the refinement of data treatment employing a new data-acquisition method: (1) extension of the dynamic range, and (2) correction for the effects of the emission angle. We are also preparing fragment production measurements for proton-induced reactions using the E-TOF technique to extend the energy range and improve data quality.

ENERGY-TIME OF FLIGHT METHOD (E-TOF) We are developing a method of fragment measurement for proton-induced reactions using the E-TOF method. In this method, the mass numbers are derived by combing the energy and TOF. The entire energy range of fragments will be covered. These data will be useful for complementing the BCS data with limited dynamic range. Up to now, we identified the fragments from polypropylene (4 µm thick) up to A=12 using MCP coupled with a 50 µg/cm2 carbon foil for a start detector and SSD for a stop detector (E detector) with a 0.7-m flight path, as shown in Fig. 6.

ACKNOWLEDGMENTS The authors express their thanks to the operation crew of the NIRS, TIARA, and CYRIC cyclotron for their cooperation.

REFERENCES 1. C.R. Gruhn, M. Binimi, R. Legrain, R. Loveman, W. Pang, M. Roach, D.K. Scott, A. Shotter, T.J. Symons, J. Wouters, M. Zisman, R. Devries, J.C. Peng and W. Sondheim, Nucl. Instrum. Methods 196 (1982) 33. 2. N.J. Shenhav and H. Stelzer, Nucl. Instrum. Methods 228 (1985) 359. 3. C.T. Roche, R.G. Clark, G.J. Mathews and V.E. Viola, Jr, Phys. Rev. C 14 (1976) 410. 4. N. Ito, M. Baba, I. Matsuyama, S. Matsuyama, and N. Hirakawa, Nucl. Instrum. Methods A337 (1994) 474 5. T. Sanami, M. Baba, K. Saito, N. Hirakawa. Nucl. Instrum. Methods. A440 (2000) 403. 6. O. Bunemann, T.E. Cranshaw, J.A. Harvey, Can. J. Res. A27 (1949) 373. 7. K. Omata and Y. Hujita, INS-Rep-884, Institute for Nuclear Study, University of Tokyo, 1991. 8. M. B. Chadwick, P. G. Young, S. Chiba, S.C. Frankle, G. M. Hale, H. G. Hughes, A. J. Koning, R. C. Little, R. E. MacFarlane, R. E. Praeel, and L.S. Waters, Nucl. Sci. Eng., 1331 (1999) 293. 9. H. Iwase, K. Niita, T. Nakamura, J. Nucl. Sci. and Tech. 39 No.11 (2002) 1142. 10. T. Sanami, M. Baba, M. Hagiwara, T. Hiroishi, M. Hosokawa, N. Kawata, N. Hirabayashi, T. Oishi, H. Nakashima and S. Tanaka. J. Nucl. Sci. and Tech. Suppl. 4 (2004) 502. 11. M. Baba, Y. Nauchi, T. Iwasaki, T. Kiyosumi, M. Yoshioka, S. Matsuyama, N. Hirakawa, T. Nakamura, Su. Tanaka, S. Meigo, H. Nakashima, Sh. Tanaka and N. Nakao, Nucl. Instrum. Methods A428 (1999) 454.

FIGURE 6. Energy vs TOF two-dimensional spectrum for polypropylene (4 µm) induced by 70-MeV protons.

SUMMARY We developed a BCS and applied it to proton- and neutron-induced reactions aiming at the measurement of fragment production cross sections. Through these

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