Development of a quasi-monoenergetic neutron field using the Li(p, n) Be reaction in the MeV energy range at RIKEN

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1 Nuclear Instruments and Methods in Physics Research A 420 (1999) Development of a quasi-monoenergetic neutron field using the Li(p, n)be reaction in the MeV energy range at RIKEN Noriaki Nakao*, Yoshitomo Uwamino, Takashi Nakamura, Tokushi Shibata, Noriyoshi Nakanishi, Masashi Takada, Eunju Kim, Tadahiro Kurosawa High Energy Accelerator Research Organization (KEK), Tanashi Branch, Tanashi, Tokyo , Japan The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama , Japan Cyclotron and Radioisotope Center (CYRIC), Tohoku University, Sendai, Miyagi , Japan Received 21 April 1998; received in revised form 6 August 1998 Abstract A quasi-monoenergetic neutron field was developed using the Li(p, n)be reaction in the energy range from 70 to 210 MeV in the ring cyclotron facility at RIKEN. Neutrons were generated from a 10-mm-thick Li target injected by protons accelerated to 70, 80, 90, 100, 110, 120, 135, 150, 210 MeV. The neutron energy spectra were measured with an NE213 organic liquid scintillator using the TOF method. The absolute peak neutron yields were obtained by measurements of 478 kev γ-rays from Be nuclei produced in a Li target through the Li(p, n)be (g.s.#0.429 MeV) reaction. Two relative neutron fluence monitors, which were calibrated to the absolute peak neutron fluences by the Be measurement, were equipped along the neutron beam line during an irradiation experiment. This high-energy neutron field is very useful for neutron cross-section measurements, response measurements of neutron detectors, and shielding experiments Elsevier Science B.V. All rights reserved. Keywords: Quasi-monoenergetic neutron field; Neutron energy spectra; Ring cyclotron; Li(p, n)be reaction; Organic liquid scintillator; TOF method 1. Introduction Presently, some high-energy hadronic accelerator facilities are being planned or have started * Corresponding author. Tel.: # ; fax: # ; Noriaki.Nakao@kek.jp. Present address: National Institute of Radiological Sciences, Inage-ku, Chiba , Japan. Former name: Institute for Nuclear Study (INS), University of Tokyo. construction in our country. For the shielding design of these facilities, we must estimate the secondary neutrons produced by accelerated charged particles and their leakages through the shielding wall and maze, as well as neutron activation of the construction materials and instruments in the accelerator facility. It is therefore of essential importance to measure high-energy neutrons for obtaining cross-section and shielding data. However, these data are still very scarce for neutrons with energies greater than 20 MeV, and there is no /99/$ see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S ( 9 8 )

2 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Currently High Energy Accelerator Research Organization (KEK), Tanashi Branch. technique to detect high-energy neutrons with good accuracy, especially in the energy region above 100 MeV. For neutron cross-section and detector-response measurements, several neutron facilities in the energy region above 20 MeV have been in practical use, where the absolute values of the neutron fluences and energy spectra are estimated with good accuracy. Recently, three quasi-monoenergetic neutron calibration fields having an energy region of MeV were established using the Li(p, n) reactions in the following cyclotron facilities in Japan. (1) 40-MeV SF cyclotron facility at INS (Institute for Nuclear Study) University of Tokyo [1], (2) 45-MeV AVF cyclotron facility at CYRIC (Cyclotron and Radioisotope Center) of Tohoku University [2], (3) 90-MeV AVF cyclotron facility at TIARA (Takasaki Ion Accelerator for Advanced Radiation Application), JAERI (Japan Atomic Energy Research Institute) [3,4]. However, no established quasi-monoenergetic neutron calibration field exists in the energy region beyond 100 MeV. In this work we developed a quasi-monoenergetic neutron field having nine different energies between 70 and 210 MeV at the ring cyclotron facility of RIKEN (The Institute of Physical and Chemical Research). The Li(p, n)be reaction was employed to produce quasi-monoenergetic neutrons at RIKEN. Since the Li(p, n) reaction leading to the ground (0 MeV) and the first excited (0.429 MeV) states of Be nuclei has been well measured over the energy range up to 800 MeV [5 8], the Li(p, n)be (g.s.#0.429 MeV) is a convenient reaction for neutron sources. The p-li neutrons give a sharp dominant monoenergetic peak with a small amount of low-energy continuum neutrons. Since there are no particle-emission stable states in Be above the first excited state, the formation of Be nuclei in a Li target is due to Li(p, n)be(g.s.#0.429 MeV). The absolute neutron numbers of the monoenergetic peak can, therefore, be measured by observing the MeV γ-ray emission in Li that follows the decay of Be. The energy spectra of source neutrons were measured with an organic liquid scintillator using the time-of-flight method. This neutron field was developed mainly for the following purposes: (1) measurements of the activation and spallation cross sections, (2) measurements of the responses and efficiencies for neutron counters, (3) measurements of particle production reactions and penetration through shields. 2. Neutron beam course and neutron sources A quasi-monoenergetic neutron field was developed at the E4 experimental room of the RIKEN ring cyclotron facility illustrated in Fig. 1. This room has a big charged-particle spectrometer, named SMART (Swinger and Magnetic Analyzer with a Rotator and a Twister), for nuclear-physics research; a part of it is used as a neutron beam line. Ions are accelerated in two steps with an AVFcyclotron and a ring-cyclotron, and are transported to the E4-room. The beam swinger permits us to bombard the accelerated particles onto a target in a scattering chamber at any angle up to 110. Quasi-monoenergetic neutrons were produced from a 10-mm-thick Li metal target (99.98 atm% enriched, 0.54 g cm) injected by 70, 80, 90, 100, 110, 120, 135 MeV/n H -ions or 150, 210 MeV protons through the Li(p, n)be reaction. Proton beam was focused on the center of the Li target within &2-mm-diameter. The beam intensity used is up to 100 na in order to suppress the activities induced in other experimental instruments. The protons that penetrated the Li target were focused by the PQ1 and PQ2 quadrupole-magnets, and were bent towards the beam dump by a PD1- dipole-magnet as shown in Fig. 1. A beam dump of lead is set in the beam duct through the PD1, and the whole PD1 is insulated so as to be used as a Faraday cup. Fig. 2 shows a side view of the neutron beam line along with the experimental

3 220 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Fig. 1. Overview of the E4 experimental room for the neutron-beam course at the RIKEN ring cyclotron facility. Fig. 2. Cross-sectional view of the neutron-beam course at the E4 experimental room at the RIKEN ring cyclotron facility.

4 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) arrangement. The neutrons produced at 0 from the target pass through a 3-cm-thick acrylic vacuum window and a 120-cm-thick iron collimator having 22-cm-wide22-cm-high hole, and reach the neutron measurement area. Concrete and iron shields are additionally equipped, as shown in Figs. 1 and 2, in order to shield the spurious neutrons produced at the PD1 beam dump. Although the total charges of protons through the beam dump were monitored with the current integrator, two relative neutron fluence monitors, an NE213 organic liquid scintillator (5.08-cmdiameter5.08-cm-long) near to the PD1 magnet (Monitor 1) and an NE102A plastic scintillator (2-cm-wide2-cm-high0.5-cm-thick) at the collimator exit (Monitor 2), were also equipped as shown in Fig. 2 because of an uncertainty in the amount of proton charges through the beam dump in a low-current experimental run. The counts of these neutron fluence monitors were calibrated to the absolute monoenergetic peak neutron fluence on the beam line after an estimation of the number of Be nuclei produced in the Li target, as described later. 3. Estimation of the peak neutron yields The peak neutron yields of the quasi-monoenergetic neutron sources were estimated from the activities of the Be nuclei produced in the Li target during the proton irradiations. In order to determine the number of residual Be nuclei in the target, MeV γ-rays emitted from the decay of Be with a half-life of 53.3 day were measured with a high-purity Ge detector (CANBERRA Co., Ltd). The branching ratio of MeV γ-ray is 10.35%. The efficiency of the Ge-detector was determined with 3% accuracy. The ground and the MeV first excited states of Be are the only particle-emission-stable states of Be; therefore, the number of residual Be nuclei equals the number of peak neutrons released in the 4π direction [9]. The peak neutron yields from a thin Li in the forward direction (Φ ) can be determined by the following equations: Φ "N R(E ) (sr), (1) R(E )" (dσ/dω) (sr), (2) (dσ/dω)dω π where N is the number of Be nuclei produced in the Li target, R(E ) is the ratio, that is, a kind of index of the forwardness of the reaction, and dσ/dω is the angular-differential cross-section for Li(p, n)be(g.s.#0.429 MeV) reaction. The (p, n) reaction cross-section at 0 ; (dσ/dω) of 35.5$1.5 mb sr was reported in the energy range of MeV [6]. On the basis of the experimental data in the energy range of MeV, the R(E ) values were well fitted by Uwamino et al. [1] to the following function: R(E )"! E # E # E # E!0.8636, (3) where E is the proton energy in MeV. The error of R(E ) obtained by Eq. (3) is 6%. In this work, a 10-mm-thick Li target, which is rather thick, was used to produce a high intense neutron beam which was required for measurements of neutron activation cross-sections. Although protons lose their energy of a few MeV in this target thickness, using the above approximation that (dσ/dω) is constant, an effective R(E ) values was estimated as follows: R(E ) R " de dx dx de (sr), (4) dx dx where t is the target thickness, x and de /dx are the range and the stopping power, respectively, in Li for E, which were calculated using the SPAR code [10]. The denominator of Eq. (4) indicates the energy loss of primary protons in the target. The R values given by Eq. (4) are shown in Table 1, together with the energy losses in the target. On the other hand, an anglular straggling effect for primary protons in the thick Li target was calculated using the formula in Ref. [11] as a standard deviation (σ) of Gaussian distribution as shown in Table 1. For all proton energies, Table 1 indicates that the 3σ values are less than 1, and the

5 222 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Table 1 Parameters for estimation of peak neutron yields at 0 degree from 10-mm-thick Li (99.98% enriched) metal target via (p, n) reaction Proton energy (MeV) Energy loss (MeV) Straggling σ (Degree) R (sr) f (dσ/dω) (mb sr) Peak neutron yield (sr μc) E# E# E# E# E# E# E# E# E#10 Standard deviation of Gaussian distribution for angular straggling effect in the Li target. Correction factor for attenuation of produced peak neutrons through Li target. Read as cross section ratios between 0 and the angles at 3σ were estimated to be less than 1% using the angular distributions of cross section data given in Ref. [1]. The angular straggling effect of protons can, therefore, be neglected within 1% uncertainty. Neutrons from (p, n) reaction through the 2nd excited state (4.57 MeV) of Be can neither be estimated from the MeV γ-ray emission from Be nor be separated from the high-energy peak spectra because of comparatively large energy loss in the 10-mm-thick target used in this work. From the experimental results of the neutron yield by the Li(p, n)be (4.57 MeV) reaction at 0 from thin target, however, it decreases from 23% down to 3% with an increase in the neutron energy between 20 and 40 MeV [1], and is negligibly small in the neutron energies of 65 MeV [4] and 200 MeV [7]. Finally, the peak neutron yields from Li target at forward direction were determined as follows: Φ "N R f (sr), (5) where f is the correction factor for attenuation of produced neutron in the Li target. Using 0.5 b for the total cross-section of Li for MeV neutrons [12], the attenuation factor is &1.2%, and it decreases with an increase in the neutron energy. Here in this study, the f values were approximated to be 0.99 for all proton energies. The peak neutron yields per unit proton beam charge (μc) for all proton energies obtained from Eq. (5) by measuring the Be activity produced in the target are listed in the Table 1. The number of incident proton to the Li target were estimated with the following equation: " N R (6) N (dσ/dω) where N is the number of Li nuclei in the target. The peak neutron fluences at two measuring positions of 8.37 and 12.0 m from the Li target were estimated by considering the peak neutron attenuation through the 3-cm-thick acrylic vacuum window at 7 m from the Li target shown in Fig. 2 and the air beyond the acrylic window exit, and also by considering the neutron scattering at the collimator. The correction factors for these three effects, and neutron fluences estimated at these two positions are listed in Table 2. The penetration through the acrylic window and air, and the scattering effect at the collimator for peak neutrons were calculated using the MORSE-CG Monte Carlo code [13] combined with the DLC119/ HILO86 multi-group cross-section data set [14]. The errors of these factors are about 3% and 0.5% for acrylic window and air, respectively, considering the errors of cross-section data and the uncertainties of material compositions. The errors for collimator scattering effect are 3% for 8.37 m and less than 0.1% for 12.0 m. The contribution of

6 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) scattered neutrons at 8.37 m, just behind the collimator, was estimated to be 8 11%, while it was negligible at 12 m. The counts of the fluence monitors were calibrated with the thus-obtained peak neutron fluences. In the usual irradiation runs, the peak neutron fluences can be obtained through the counts of these fluence monitors. 4. Measurement and analysis of neutron energy spectra 4.1. Measurement The neutron-energy spectra were measured by the time-of-flight (TOF) method using a 12.7-cmdiameter12.7-cm-long NE213 organic liquid scintillator coupled with a R4144 photo-multiplier (HAMAMATSU photonics Co., Ltd). In order to obtain a good time resolution of the TOF measurement for high-energy neutrons, the neutron detector was placed both 12 and 20 m away from the Li target. The repetition period of the proton beam from the cyclotron was extended to about 1.0 μs by using a beam chopper to measure the neutron-energy spectra down to several MeV energy, even in the long neutron flight path. In the measurements, the currents of the proton beam were kept in the range of na and 5 10 na for 12 and 20 m measurements, respectively. The measuring electronic circuit shown in Fig. 3a was used in early measurements. The dynode signals of the detector were amplified with the delay-line amplifier (DLA) and inputted separately into the delay amplifier (DA) and the risetime-to-height converter (RHC). The pulse-height outputs from DA and RHC were used for twodimensional discrimination of neutron and γ-ray events. The neutron flight-time was measured with Fig. 3. Old (a) and new (b) block diagrams of the circuits for the TOF measurement using the NE213 scintillator. PM: photomultiplier, HV: high voltage power supply (John Fluke), PA: pre-amplifier (ORTEC 113), CFD: constant fraction discriminator (ORTEC 935), DLA: delay line amplifier (ORTEC 460), DA: delay amplifier (ORTEC 427A), RHC: rise-time-to-height converter (OKEN 7231C), TAC: time-to-amplitude converter (ORTEC 567), GG: gate and delay generator (KAIZU 1500), ADC: analog-to-digital converter (ORTEC AD811), DIV: signal divider, QDC: charge analog-to-digital converter (LeCroy 2249W), TDC: time-to-digital converter (LeCroy 2228A).

7 224 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) the anode signals from the detector as start signals and the trigger pulses of the beam chopper as stop signals. These three pulses were analyzed by CAMAC ADC and recorded in an event-by-event mode by the VAX data-taking system. The available dynamic range of the RHC is, however, comparatively small and the threshold of the pulse height becomes relatively high. In recent measurements for 70, 90 and 135 MeV p-li neutrons, therefore, the electronic circuit shown in Fig. 3b was employed to extend the dynamic range of the pulse-height distribution. By using the QDC (charge ADC), the charges of the total and slow (decay) components of the anode pulses were integrated for particle identification. The neutron flight time was measured by the time-to-digital converter (TDC). These three digital data from the CAMAC QDC and TDC were stored in a personal computer. All of these proccesses were controlled by the KODAQ data-taking system (Kakuken Online Data Aquisition System) [15] Analysis The neutron-energy spectra were converted using relativistic kinematics from the neutron flighttime distributions. The time of the reaction on the Li target was calibrated to the γ-ray burst from the target in the TOF distributions. Fig. 4a and b show two-dimensional distributions of 135 MeV p-li neutrons, which were used to analyze the output pulses from the scintillator obtained with the two circuits shown in Fig. 3a and b, respectively. The threshold of the pulse height on the measurement was relatively high as shown in Fig. 4a because of a narrow dynamic range of the RHC; on the other hand, much lower pulse heights could be measured using the new circuit as shown in Fig. 4b. The p, d and α particles were produced from neutrons, and the electrons were from γ-rays in the scintillator. Although most of the generated charged particles were clearly identified in both figures, the electron components generated from Fig. 4. Two-dimensional plots used to identify the particles produced in the scintillator for a 135 MeV p-li neutron source. The view of the rise-time versus pulse-height using the old circuit (a), the view (expanded in the low pulse-height part) of total component versus slow component of scintillation pulses using the new circuit (b).

8 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) γ-rays overlapped with the high-energy protons which escaped from the scintillator wall. They were not negligible for high-energy neutron incidence. The lowest light output, where electron component could be distinguished from the escaping protons was around 10 MeV-electron-equivalent (MeVee) in the case of old circuit. A light output threshold of 10 MeVee (10-MeVee-bias) was, therefore, employed in the analyses to eliminate the electron component. In fact, almost the higher part of the electron component consists of muons due to space radiations, and the contribution above 10 MeVee was less than 0.1%. On the other hand, in the case of new circuit, electron component could not be distinguished from the escaping-proton components even above 10-MeVee, and the contribution of muon events could not be eliminated. To obtain the lower neutron energy spectra, Co-equivalent bias (Co-bias) was also employed for 70, 90 and 135 MeV proton experiments, and both the electron and escaping-proton components were eliminated from the two-dimensional distribution and a one-dimensional pulse-height distribution was made by summing up all other components. The Co-bias was determined with a channel having times counts of the Compton edge due to the 1.17 and 1.33 MeV γ-ray from Co, and this corresponds to 1.15 MeVee. For the calibration for higher light output, Eq. (3) of Ref. [16] was used. Since 10 MeVee corresponds to about 15.5-MeV proton light output, it was determined with a halfmaximum channel of the recoiled proton edge of the MeV neutron response which was obtained by gating in the TOF distribution. Because there is no absolutely measured response function matrix for the NE213 scintillator in the neutron energy range above 100 MeV, and the available code to calculate the response functions of the scintillator for neutrons in this energy region is only the code by Cecil et al. [17], it was used to calculate the efficiency of 10-MeVee bias for the NE213 scintillator used in this work. To calculate the efficiency of Co-bias without escaping-proton effect, the calculation code, in which escaping proton events are not taken into account, was made by modifying the code of Cecil. The efficiencies for these two thresholds are shown in Fig. 5, compared with those calculated using the original code in which the escaping protons are taken into account. Fig. 5. Neutron detection efficiencies of a 12.7-cm-diameter by 12.7-cm-long NE213 scintillator calculated with a code by Cecil et al., and those calculated without escaping-proton effect by the modified code. In the analyses of this work, the efficiency by the original code was used for 10 MeVee bias, and the efficiency without escaping-proton effect was used for Co-bias.

9 226 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) It is, however, reported that comparing with the experimental results in the neutron energy above 20 MeV, the original Cecil code gives &10% higher efficiencies in low-light output bias [18] and that the discrepancies of response functions have been found for higher light output region [16]. The uncertainty of the code is, therefore, considered to be &10%, and more than 15% in the case of escaping-proton elimination. 5. Results and discussions 5.1. Absolute peak neutron fluences The absolute peak neutron fluences at 12 m from the target were experimentally estimated from the target activities using the parameters in Tables 1 and 2 and Eq. (5), and were also estimated by integrating over the peak region of the neutronenergy spectra resulted by the NE213 scintillator measurements. The fluences obtained by these two methods are listed and compared in Table 3, and their errors are listed in Table 4. From the results of the NE213 scintillator using 10-MeVee bias for 90 and 135 MeV in Table 3, the ratios of peak neutron fluences using old and new circuits agreed well within a few %, and it could be found that the contribution of the high-energy γ- ray and the muon events was negligibly small. The discrepancies between the two fluences given by the NE213 and target activity measurements were from a few percent up to 23% which is higher than the uncertainties of the calcualted efficiency. Although the results from the NE213 scintillator using Cobias were generally higher than those from target activity in 10 23% because of the errors for the efficiency without escaping-proton effect, the NE213 results using 10-MeVee bias agree within 8% difference as average with the target activity results. The number of incident proton on the Li target were also estimated from the target activity using Eq. (6), and were compared with those measured with the C.I. (current integrator) as shown in Table 3. All these measurements were carried out simultaneously with the NE213 measurements at 12-m distance, and the beam current was kept in na. In such a very low-beam current run, an accuracy of the C.I. measurement is generally quite low, and the comparison of the measured results between the C.I. and the target activity show discrepancies of 15% maximum, but only 6% as average. Totally in the present work, the target activity measurements could estimate more accurate peak neutron fluences and incident proton numbers. Table 2 Peak neutron fluences at 8.37 and 12.0 m from the Li target estimated with the values in Table 1. Correction factors are also shown for peak neutron attenuation through the acrylic window and air, and for contribution of neutron scattered by the iron collimator. Neutron attenuation by air was estimated beyond the acrylic window exit (703 cm from the target) Proton energy (MeV) Acrylic window (3 cm) 8.37 m 12 m Air (134 cm) Collimator Peak neutron fluence (cm μc) Air (397 cm) Collimator Peak neutron fluence (cm μc) E# E# E# E# E# E# E# E# E# E# E# E# E# E# E# E# E# E#03 Read as

10 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Table 3 Peak neutron fluences at 12 m measured by the NE213 scintillator and the integrated proton charges through the beam dump measured by Current Integrator (C.I.), comparing with those estimated by the Li target activity. Circuit used in the NE213 measurements, bias and efficiency in analyses are also shown. The escaping-proton effects are not taken into account in the efficiencies for Co-bias Proton energy (MeV) Peak neutron fluence at 12 m Parameters for NE213 Circuit Bias Efficiency (%) NE213 (cm) Target activity (cm) Proton number Ratio C.I. (μc) Target activity (μc) Ratio 70 New 10 MeVee E# E# Co-Bias E# E# Old 10 MeVee E# E# Old 10 MeVee E# E# New 10 MeVee E# E# Co-Bias E# E# Old 10 MeVee E# E# Old 10 MeVee E# E# Old 10 MeVee E# E# Old 10 MeVee E# E# New 10 MeVee E# E# Co-Bias E# E# Old 10 MeVee E# E# Old 10 MeVee E# E# Read as In a measurement of the Be activity produced in the Li target, the absolute peak neutron fluence at 12.0 m could be obtained within 7.5% accuracy, considering the errors tabulated in Table 4. On the other hand, the peak neutron fluences given by using the calibrated value of a fluence monitor have an error of &9%, since the ratio of the integratedbeam current and two fluence monitor counts fluctuated between 2% and 5% during all of the experiments. The Monitor-2 scintillator was more reliable than the Monitor-1 scintillator because it was equipped at the collimator exit and was much less influenced by spurious neutrons from the beam dump Neutron energy spectra The neutron-energy spectra analyzed using 10-MeVee bias were obtained down to &20 MeV neutron energy, and for 70, 90 and 135 MeV proton incidences the neutron spectra was obtained down to &7 MeV using Co-bias. Those results obtained at 12 and 20-m distances were relatively in good agreement although the peak energy resolutions were different. For 70, 90 and 135 MeV results, a little discrepancies in the energy spectra in higher energy region could be found between Co-bias and 10-MeVee bias analyses as described in the previous section because of efficiency errors. Since the peak fluences obtained using 10-MeVee bias analyses were closer to the results obtained from target activity, the results using 10-MeVee bias were used as neutron-energy spectra above 20 MeV. The neutron-energy spectra below 20 MeV for Co-bias, however, were smoothly connected to those above 20 MeV for 10-MeVee bias because the escapingproton effect can be neglected below 20 MeV neutron energy as shown in Fig. 5. The neutron-energy spectra measured with the TOF method are shown in Figs. 6 and 7 for MeV and MeV p-li neutrons, respectively. These figures clearly show that each neutron spectrum has a sharp monoenergetic peak at an energy of 3 to 4 MeV lower than each proton energy and a small amount (about one-tenth per MeV) of low-energy continuum. The absolute

11 228 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Table 4 Error estimations for neutron fluences (Φ ) measured with (a) the target activity and (b) the NE213 scintillator (a) Target activity Compositions Error (%) Results Error (%) Gamma-ray counting statistics (1.0 Ge-efficiency 3.0 Number of Be 3.2 Number of Be 3.2 R-values 6.0 Angular straggling effect (1.0 Φ at 0 deg. 6.9 Φ at 0 deg degree cross section 4.2 Proton number 8.1 Φ at 0 deg. 6.9 Factor for attenuation through acryl 3.0 (8.37 m) (12 m) Factor for attenuation through air (8.37 m) (12 m) Factor for scattering at collimator Φ at position Fluence monitor fluctuation 5.0 Φ at position Neutron yield at 0 degree from the target. Neutron fluence at measurement position estimated by the target activity. Neutron fluence at measurement position estimated by the fluence monitor. (b) NE213 scintillator Compositions Error (%) Results Error (%) Counting statistics (0.1 Counting loss correction 3.0 Detection efficiency 15.0 Bias 3.0 Φ at position 15.6 values of these spectra were obtained by normalization to the peak neutron fluences at 12-m distance estimated by the target activity shown in Table 2. The physical characteristics of these neutron energy spectra for nine proton energies shown in Figs. 6 and 7 are tabulated in Table 5. The energies and FWHMs of the neutron peak were estimated by fitting the Gaussian distribution to the measured neutron energy spectra. The neutron peak widths become narrower with increasing proton energy due to lower energy losses in a 10-mm-thick Li target up to 135 MeV, while on the other hand, those of the higher energy neutrons such as 150 and 210 MeV p-li neutrons were much wider than the proton energy losses in the target due to poor energy resolutions. For 70, 90 and 135 MeV p-li neutrons, which were measured using the new measuring circuit system, the neutron-energy spectra were obtained down to &7 MeV; the others were down to &20 MeV using the old measuring circuit system. 6. Summary A quasi-monoenergetic neutron field having nine different neutron peaks between 70 and 210 MeV was established at the RIKEN ring cyclotron facility. The source neutrons were generated from a 10- mm-thick Li target injected by protons of nine

12 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Fig. 6. Energy spectra of neutrons from a 10-mm-thick Li target injected by 70, 80, 90, 100 and 110 MeV protons. The peak fluences were normalized to those at 12 m estimated from the target activity.

13 230 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Fig. 7. Energy spectra of neutrons from a 10-mm-thick Li target injected by 120, 135, 150 and 210 MeV protons. The peak fluences were normalized to those at 12 m estimated from the target activity.

14 N. Nakao et al./nucl. Instr. and Meth. in Phys. Res. A 420 (1999) Table 5 Characteristics of the neutron energy spectra measured with the TOF method shown in Figs. 6 and 7. Peak fluences were normalized with the values at 12 m distance given in Table 2 Proton energy (MeV) Lower limit of spectrum (MeV) Peak neutron energy (MeV) Peak FWHM (MeV) Peak range (MeV) Peak neutron fluence at 12 m (cm μc) E# E# E# E# E# E# E# E# E#03 Read as different energies of 70 to 210 MeV. The neutronenergy spectra down to about 7 20 MeV were obtained by the TOF method, and the absolute peak fluence could be determined within &7% accuracy with the γ-ray spectrometry of Be nuclei produced in the Li target. This neutron field has already been used for measurements of the neutron detector responses and activation cross-sections of C and Bi [19]. We expect that this highenergy neutron field will be further used for other neutron experiments. Acknowledgements We would like to thank Dr. Takashi Ichihara, Mr. Shin Fujita and Mr. Shunji Nakajima of RIKEN for their co-operative help and advises on the experiments. We also thank Dr. Yasushige Yano and other cyclotron staff members for their co-operative operation of the pulsed proton beam. References [1] Y. Uwamino, T.S. Soewarsono, H. Sugita, Y. Uno, T. Nakamura, T. Shibata, M. Imamura, S. Shibata, Nucl. Instr. and Meth. A 389 (1997) 463. [2] M. Takada, T. Nakamura, M. Baba, T. Iwasaki, T. Kiyosumi, Nucl. Instr. and Meth. A 372 (1996) 253. [3] M. Baba, T. Kiyosumi, T. Iwasaki, M. Yoshioka, S. Matsuyama, N. Hirakawa, T. Nakamura, Su. Tanaka, R. Tanaka, Sh. Tanaka, H. Nakashima, S. Meigo, Proc. Int. Conf. Nuclear Data for Science and Technology, Gatlinburug, 1994, p. 90. [4] N. Nakao, H. Nakashima, T. Nakamura, Sh. Tanaka, Su. Tanaka, K. Shin, M. Baba, Y. Sakamoto, Y. Nakane, Nucl. Sci. Eng. 124 (1996) 228. [5] J. D Auria et al., Phys. Rev. C 30 (1984) [6] J.W. Watson et al., Phys. Rev. C 40 (1989) 22. [7] J. Rapaport et al., Phys. Rev. C 41 (1990) [8] T.N. Taddeucci et al., Phys. Rev. C 41 (1990) [9] S.D. Schery, L.E. Young, R.R. Doering, Sam M. Austin, R.K. Bhowmik, Nucl Instr. and Meth. 147 (1977) 399. [10] T.W. Armstrong, K.C. Chandler, ORNL-4869, [11] J.B. Marion, F.C. Young, Nuclear reaction analysis, North-Holland, Amsterdam, [12] V. McLane et al., Neutron Cross sections, vol. 2, Academic Press, New York, [13] G.R. Straker, P. N. Stevens, D. C. Irving, V.R. Cain, ORNL-4585, Oak Ridge National Laboratory, [14] R.G. Alsmiller Jr., J.M. Barnes, J.D. Drischler, ORNL/TM-9801, Oak Ridge National Laboratory, [15] K. Omata, Y. Fujita, N. Yoshikawa, M. Sekiguchi, Y. Shida, INS-Rep.-884, [16] N. Nakao, T. Nakamura, M. Baba, Y. Uwamino, N. Nakanishi, H. Nakashima, Sh. Tanaka, Nucl. Instr. and Meth. A 362 (1995) 454. [17] R.A. Cecil, B.D. Anderson, R. Madey, Nucl. Instr. and Meth. 161 (1979) 439. [18] S. Meigo, Nucl. Instr. and Meth. A 401 (1997) 365. [19] E. Kim, T. Nakamura, A. Konno, Y. Uwamino, N. Nakanishi, M. Imamura, N. Nakao, S. Shibata, Su. Tanaka, Nucl. Sci. Eng. 129 (1998) 209.