Measurement of neutron-induced proton-production energy spectra with NE213 scintillator

Radiation Measurements 47 (2012) 596e608

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Measurement of neutron-induced proton-production energy spectra with NE213 scintillator Tsuyoshi Kajimoto a, *, Nobuhiro Shigyo a, Kenji Ishibashi a, Hiroyuki Arakawa a, Robert C. Haight b, Nikolaos Fotiades b a b

Kyushu University, Motooka, Nishi-ku, Fukuoka 819-0395, Japan Los Alamos National Laboratory, Los Alamos, NM 87545, USA

h i g h l i g h t s < We measured neutron-induced proton-production energy spectra. < Pulse shape discrimination of the NE213 scintillator was useful to the analysis. < Measured spectra were compared with calculations with the PHITS and the FLUKA codes.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 9 December 2011 Received in revised form 27 April 2012 Accepted 27 June 2012

Many benchmark data are required for the improvement of theoretical model implemented in a Monte Carlo code for particle transport. To acquire the benchmark data, we measured energy spectra of protons emitted from graphite, aluminum, and iron targets bombarded with continuous-energy neutrons, which enable simultaneous measurements at the incident energies from 100 to 600 MeV at a time. The neutron flux incident on the target was measured with a 238U fission ionization chamber. Protons emitted from the target were measured with three DEeE detectors consisting of a thin NE102A scintillator and a thick NE213 liquid scintillator. In the analysis, the pulse shape discrimination of the NE213 scintillator enable us to distinguish events for a charged particle stopping in the scintillator from events for a charged particle penetrating the scintillator. Experimental results were compared with calculations by the PHITS code coupled with the JENDL-HE file, the Bertini model implemented in the PHITS code, and the PEANUT model in the FLUKA code. Ó 2012 Elsevier Ltd. All rights reserved.

Keywords: Proton energy spectrum Neutron incidence NE213 scintillator PHITS FLUKA

1. Introduction A Monte Carlo transport code such as the PHITS code (Niita et al., 2010), the FLUKA code (Ferrari et al., 2005), and the MCNPX code (Pelowitz, 2005), is used in various fields of nuclear physics, material science, and medical applications, because various quantities such as heat deposition, track length and particle flux and so on can be deduced by setting the precise geometrical configuration in the code. These codes deal with the transport of all particles over wide energy ranges, using physics models and nuclear data libraries. Accurate description for nucleon-induced reactions above 20 MeV is required for applications such as radiation cancer

* Corresponding author. Present address: Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8527, Japan. Tel.: þ81 82 424 7613; fax: þ81 82 424 2453. E-mail address: [email protected] (T. Kajimoto). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.radmeas.2012.06.020

treatment, dosimetry at commercial aircraft altitudes and in space, transmutation of nuclear waste with accelerator driven systems, and single-event effects in electronics. In the simulation code, an intranuclear cascade model is included for the reaction. The model is good at particle ejections which have high energy and have forward peaked angular distributions and is used at particle energies above 20 MeV. More improvement of the model is demanded for the developments and many experimental values are required for the benchmark test of the model. Many data have been measured for proton-induced reactions (Chrien et al., 1980; Förtsch et al., 1991; Kin et al., 2005) because the experiment is easier than that of neutron-induced reactions. There are only two works measuring neutron-induced neutron-produced spectra for the incident energy above 50 MeV (Hjort et al., 1996; Sagrado et al., 2011). For neutron-induced proton-emission reactions, many works have been reported, however, the data above 100 MeV is scarce (Franz et al., 1987, 1990; Bevilacqua et al., 2010, 2011).

T. Kajimoto et al. / Radiation Measurements 47 (2012) 596e608

To improve the accuracy of calculations, models are required to be tested and benchmarked. We attempted to measure neutroninduced neutron-production double differential cross sections (Kajimoto et al., 2011a). Furthermore, we measured not only neutron energy spectra but also proton energy spectra emitted from targets bombarded with continuous energetic neutrons between 100 and 600 MeV in this measurement at a time. In this paper, we describe analysis methodology for proton spectra and comparison with the measured proton spectra and calculations by the PHITS code (Niita et al., 2010) and the FLUKA code (Ferrari et al., 2005). Continuous energetic neutrons from a spallation target bombarded with 800 MeV protons were used as incident particles. The incident neutron flux was simultaneously measured with a 238U fission ionization chamber. The emitted proton was detected with DEeE detectors consisting of a thin NE102A scintillator and an NE213 liquid scintillator.

2. Experiment 2.1. Experimental arrangement This experiment was performed at the 4FP15L beam line at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center (LANSCE) (Lisowski and Schoenberg, 2006). Incident neutrons were produced by providing a pulsed 800 MeV proton beam from a linear accelerator on a thick water-cooled tungsten, spallation target (Target-4). Continuous energetic neutrons from Target-4 were delivered at 15 with respect to the proton beam, which was the 4FP15L beam line. The use of continuous energetic neutrons enabled us to measure data for incident neutrons with various energies at a measurement. The experimental arrangement is shown in Fig. 1. An absorber consisting of 10.2 cm thick polyethylene and 3.8 cm thick copper was inserted in the beam just downstream of the shutter to reduce the number of low energy neutrons (Sisterson and Ullmann, 2005). Disk-shaped targets were placed at 88.3 m downstream from Target-4. The characteristics of the targets are summarized in Table 1. Neutrons incident on the target were collimated to f 35.6 mm by a 1.8 m thick collimator composed of stainless steel and brass. The collimator was set at 2.7 m upstream from the target. A 238 U fission ionization chamber (Wender et al., 1993) was located at the position downstream of the collimator to measure the incident neutron flux. Protons emitted from the target were measured with three DEeE detectors. The original purpose of this experiment was the measurement of neutron energy spectra and the experiment was carried out not under vacuum but in the pressure of the atmosphere. Each DEeE detector was constructed with a 15  15 cm2

597

Table 1 Target characteristics. Densities and size of these targets were measured. Target

Density [g/cm3]

Thickness [cm]

Graphite Aluminum Iron

1.86 2.69 7.87

0.76 0.52 1.00

and 1.0 cm thick NE102A scintillator and a B 12.7 cm and 12.7 cm thick NE213 scintillator. Each DE detector was installed 2e3 cm away from the E detector. The DEeE detectors were installed at 15 , 30 , and 60 with respect to the beam direction, and distances from center of the target to surface of each DE detector were 64, 54, and 43 cm. Iron and lead bricks were located upstream from three DEeE detectors due to suppress background events. Data acquisition (DAQ) system of DEeE detectors and the fission chamber were separated. The experiment consisted of two measurements with and without the target, respectively referred as target-in and target-out measurements in this paper. The contribution of background events was evaluated experimentally by the target-out measurement. We simultaneously measured energy spectra of incident neutrons and emission protons. 2.2. Electronics Fig. 2 shows a simplified block diagram of the measurement circuit of DEeE detectors. The circuit included standard NIM and CAMAC modules. Data were recorded event by event via an electronic circuit connected to a personal computer. The system was controlled by a signal from E detectors and a T0 signal. The T0 signal was provided by the proton beam pick-off, one pulse for every micropulse. Micropulse spacing was constant at 1.8 ms. In order to measure not only charged particles but also uncharged particles, a signal from the DE detectors was not used to control the circuit. With T0’s absence, the system remains in a ready-to-measure state. With T0’s presence, a logic signal with length of 1.6 ms is sent to a coincidence module (COIN). An anode signal from a photomultiplier tube combined with the E detector is converted to a logic signal with a constant fraction discriminator (CFD1). The logic pulse from each E detector is combined at a Fanin/out module (FAN). If a logic signal from FAN arrives at COIN when the T0 long logic signal is on, COIN sends a trigger signal to a CAMAC crate for DAQ. Once the DAQ begins, the processes for further events are inhibited. The inhibition time per one process was 250 ms. Recorded data were charge signals from all DE and E detectors, a value of the time lag between signals of a T0 and from one of the E detectors, and the bit pattern for fired E detectors. These charge

Fig. 1. Geometry of present experiment. Carbon, aluminum, and iron disks were used as targets. Three DEeE detectors were set at 15 , 30 , and 60 with respect to beam axis, respectively. In order to obtain incident neutron flux, a 238U fission ionization chamber was installed upstream from the target.

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106

Charged particles

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Count

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1000

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ADC ΔE [ch] Fig. 3. ADC plot of the DE detector.

Fig. 2. Electronic circuit for recording DEeE detector data. The circuit consisted of standard NIM and CAMAC modules. The data were recorded event by event.

values were recorded using two charge-sensitive analog-to-digital converters (ADCs). Each ADC integrated an analog pulse with a specific gate width. The gate width of each ADC was set as containing the whole component of the analog pulse from each DE detector and E detector, and the slow component of the analog pulse from each E detector for the pulse shape discrimination. The gate width to the whole component was set to 500 ns. The gate to the slow component was delayed for 150 ns than the gate of the whole component and the gate width was set to 350 ns. A time-todigital converter (TDC) was used to acquire the time lag between signals of a T0 and from one of the E detectors. The full scale of the TDC was 2 ms. The bit pattern was obtained using a coincidence register (Coin. Reg.). For the fission chamber, the electronic circuit by Wender et al. (1993) was used. Recorded data included the charge amount of a signal from the fission chamber and the time lag between signals of a T0 and from the fission chamber. These data were also acquired event by event. 3. Analysis The neutron-induced proton production double differential energy spectrum (d2Y/dEdU) in a bin of center energy E, a width of dE, and a solid angle width of DU can be determined as

8

(1)

when Cin(E) and Cout(E) respectively the number of proton events in the energy bin in the target-in and the target-out measurements, which were obtained by data analysis of DEeE detectors: selection of proton events, conversion from channel value of the whole component ADC to deposition energy in the E detector: energy calibration of E detectors, determination of the emission proton energy, and determination of the incident neutron energy. Nin and Nout the number of incident neutrons and ε(E) the proton peak detection efficiency of the DEeE detector. 3.1. Selection of proton events Discrimination between events by a charged particle and an uncharged particle was derived using the ADC plot of the DE

Boundary line

700

Slow component ADC [ch]


 d2 Y 1 Cin ðEÞ Cout ðEÞ ; ¼  dEdU Nin Nout εðEÞdEDU

detector as shown in Fig. 3. We chose events by a charged particle which lost the partial energy in DE detector. The eliminated events included events for a charged particle not passing through the DE detector. We extracted events for a charged particle stopping in the E detector from the charged particle events with the pulse shape discrimination of the NE213 scintillator. It have been known that the pulse shape of events for a charged particle giving the partial energy in an NE213 scintillator and escaping from the NE213 scintillator becomes closer to that of events for g-ray-induced electrons (Byrd and Sailor, 1989; Nakao et al., 2001; Sasaki et al., 2002; Taniguchi et al., 2006; Satoh et al., 2006a). Because a Bragg peak is not observed for the escaping event and the average ionization density along the range is close to that of g-ray-induced events. It has been difficult to discriminate escaping events from gray-induced those. These escaping events have been eliminated together g-ray-induced events. We applied the elimination of escaping events with the pulse shape discrimination to the proton energy spectrum measurement. Fig. 4 shows a two-dimensional histogram of the whole- and slow-component ADCs for charged particle events. Escaping particle events overlapped g-ray-induced events on the histogram. We removed escaping particle events and selected events for a charged particle stopping in the E detector.

6 4

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Whole component ADC [ch] Fig. 4. Two-dimensional histogram of the whole- and slow-component ADCs for charged particle events.

T. Kajimoto et al. / Radiation Measurements 47 (2012) 596e608

3.2. Energy calibration of the E detectors We performed an experiment (Kajimoto et al., 2011a) for the energy calibration of the E detectors, and measured neutron response functions of the E detectors at the 4FP15L beam line. Each measured response function had a recoil-proton edge. As an example, a measured response function of the E detector for 58e62 MeV neutron incidence is shown in Fig. 6. Since the edge is produced from a recoil-proton scattered in the forward direction by an interaction of an incident neutron and a hydrogen, the

60

58 - 62 MeV neutron incidence 50 40

Count

We validated the effect of the pulse shape discrimination with two two-dimensional scatter plot of ADCs of the DE detector and the E detector. Fig. 5 shows the scatter plots without and with the pulse shape discrimination analysis. We can see two dogleg bands in the plot without the pulse shape discrimination. These bands are frequently observed in the DEeE method. These bands lying higher and lower DE ADC channels are formed by deuterons and protons, respectively. Events shaping bands are that a charged particle emitted from the target penetrates the DE detector, enters the E detector, and stops in the E detector or penetrates the E detector without an energy loss by a nuclear reaction. The charged particle with a lower energy stops in the E detector. With increasing the energy of the charged particle, energies deposited by the charged particle become smaller and larger in the DE detector and the E detector, respectively. Having an energy to penetrate the E detector, the charged particle gives not the full energy but the partial energy to the E detector. The deposition energy in the E detector is smaller with increasing the energy of the charged particle. Therefore, these bands bend. We can observe these penetrating events in the scatter plot without the pulse shape discrimination analysis. Since these penetrating events can not be found in the scatter plot with the pulse shape discrimination analysis, the pulse shape discrimination was an effect on the elimination of penetrating events. Particle identification was based on the DEeE technique. Events for a proton giving the full energy to the E detector were selected by specifying the region as shown in Fig. 5 and events for other charged particles were discriminated by the selection. There were events for a proton causing some energy loss by a nuclear interaction in the E detector. Since these events give the lower energy to the E detector than full energy events and appear in the region 1 shown in Fig. 5, these events were also removed by the selection. The pulse shape discrimination removed penetrating deuteron events overlapping proton full stop events.

Recoil-proton edge 30 20

Half height of edge 10 0 600

700

800

900

1000

1100

1200

1300

1400

Whole component ADC [ch] Fig. 6. Response function of the E detector for 58e62 MeV neutron incidence.

deposition energy at the recoil-proton edge represents the incident neutron energy. However, the edge was not observed for incident neutron energies above 120 MeV because high-energy protons penetrated the E detector. As shown in Fig. 6, calibration points were obtained from the half height of the recoil-proton edge in the response function at each incident neutron energy between 13 and 120 MeV. The whole component ADC in units of the channel was converted into proton deposition energy in the E detector.

3.3. Determination of emission proton energy The energy of protons emitted from the target was obtained by considering the energy loss before the proton arrives at the E detector. Since this experiment was performed not in a vacuum chamber but in the pressure of the atmosphere, the energy loss arose in the air and the DE detector by the ionization. Furthermore, the cut-off energy became larger because a lower energy proton loses the all energy in the air or the DE detector and can not reach the E detector. We considered the energy loss in materials: the air, the 1.0 cm thick DE detector, and the 0.1 cm thick aluminum case of the E detector. The air thickness was different at each DEeE detector. These material thickness was determined based on an assumption that the proton takes the shortest way from the center of the target to the center of the front plane of the E detector.

10

4000

599

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4000

With pulse shape discrimination

Without pulse shape discrimination

Extracted events 8

Deuterons 2000

Protons

6

8

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E ADC [ch]

E ADC [ch]

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Region 1 4

1000

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0 0

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Fig. 5. Two-dimensional scatter plots of the whole component ADC of the E detector and the ADC of the DE detector. Left and right panels show plots without and with the pulse shape discrimination analysis, respectively.

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observed in the TDC histogram as shown in Fig. 7. The incident neutron energy En was derived from using the following equation:

Prompt -ray peak

8 9 > > < = 1 En ¼ mn c2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi  1 > > : 1  ½L=ðctn Þ2 ;

Count

105

where mn is the rest mass of neutron, c is the velocity of light, and L is the neutron flight length from Target-4 to the center of the target. tn is the neutron flight time and was delivered with the following equation:

104

103 0

(2)

tn ¼ 500

1000

1500

2000

2500

3000

3500

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TDC [ch] Fig. 7. TDC histogram for all events.

The energy loss was evaluated with stopping power values calculated by the SPAR code (Armstrong and Chandler, 1973). The flight time of protons was also calculated as considering the energy loss. The energy of protons emitted from the target was derived from adding the energy loss to the proton deposition energy in the E detector. We set the cut-off energy to be 40 MeV because a 40 MeV proton is able to arrive and gives sufficient energy to the E detector. The sufficient deposition energy is 13 MeV, which was the minimum value of obtained calibration points. The upper energy was set to 130 MeV to extract only full stop proton events. Since protons emitted from the target do not fly with the shortest distance necessarily, the flight path length of the proton slightly varies by the proton-produced position in the target and the position of protons incident on the E detector. The difference of the flight path length causes a different energy loss and a flight time. Maximum differences were deduced from the flight path and length obtained by varying these positions and became 0.4 MeV and 1.7 ns. The different energy loss made worse the proton energy resolution and the flight time difference led to deterioration of the energy resolution for incident neutrons to determine the neutron energy with the time-of-flight (TOF) technique. 3.4. Determination of incident neutron energy

(3)

where Lg is the prompt g-ray flight length which is the sum of length between Target-4 and the center of the target and distance between the center of the target and the E detector, Tg is the TDC value of the peak by prompt g-rays, and Tp is the TDC value of proton events. dT is the time per channel of the TDC. The TDC was calibrated with a time calibrator and dT became 0.50 ns. tp is the proton flight time between the center of the target and the E detector. The value of tp has been derived in the determination of emission proton energy. The energy resolution of incident neutrons was a function of the neutron flight time and was determined by the deviation of the neutron flight time. The deviation consisted of the full width at half maximum (FWHM) of the prompt g-ray peak and the difference of the proton flight time. The FWHM was derived from fitting a Gauss distribution to the prompt g-ray peak to be 3.5 ns. The difference of the proton flight time has been obtained in the determination of emission proton energy and the value was 1.7 ns as described above. The value of the energy resolution was 1.3% for 100 MeV and 4.4% for 600 MeV.

3.5. Number of incident neutrons The number of neutrons incident on the target were derived from analyzing data of the fission chamber. The analysis and the derivation were as reference (Wender et al., 1993). Furthermore, we corrected the number of incident neutrons by considering contributions lower energy neutrons from preceding micropulses overlapped neutrons from the last micropulse. This contribution was caused by use of the pulsed spllation neutron source and the TOF technique.

Target

Fission chamber i=0

Neutron energy [MeV]

Neutron energy [MeV]

The derivation of incident neutron energy was based on the TOF technique. A sharp peak by prompt g-rays from Target-4 can be

 Lg  þ Tg  Tp dT  tp ; c

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i=3

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Measured flight time: tn(0) [ns]

Fig. 8. Relation between measured flight time tn (0) and neutron energy for the last four micropulses (i ¼ 0, 1, 2, and 3). Left and right panels show the relation at the fission chamber position and the target position, respectively.

T. Kajimoto et al. / Radiation Measurements 47 (2012) 596e608

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Calculated fission count [-]

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C, 240-260 MeV neutron incidence ° 15 ° 30 ° 60

15

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Proton energy [MeV]

Measured flight time: tn(0) [ns] Fig. 9. Calculated fission counts for last three micropulses (i ¼ 0, 1, and 2). Fission counts were obtained with the expected neutron energy spectrum and neutroninduced fission cross sections of 238U.

Incident neutrons had a continuous energetic distribution by use of the spallation neutron source. The number of neutrons (f(E)/ dE) in centered at an energy E and a width of dE was determined as

Cf ðEÞf ðEÞ ; ¼ sf ðEÞrU εdE dE

fðEÞ

(4)

where Cf(E) is the number of fission events in the energy bin, sf(E) is the neutron-induced fission cross-section of 238U (Kajimoto et al., 2011b), rU is the area density of 238U deposited on a foil of the chamber, ε is the detection efficiency, f(E) is the correction factor for the contribution of neutrons from preceding micropulses, and dE is a width of incident energy in centered at the energy E. Fission events were discriminated from events for the a decay of 238 U and particle knockout reactions, which provided much less energy to the chamber than fission fragments. The neutron energy of fission events was determined by the TOF technique with relativistic kinematics. The energy resolution for the TOF measurement was determined by the deviation of the neutron flight time: the FWHM of the prompt g-ray peak to be 3.7 ns. The value of energy resolution was 1.3% for 100 MeV and 4.5% for 600 MeV. Lower energy neutrons from preceding micropulses overlapped neutrons from the last micropulse which triggered the DAQ system owing to the short repetition time (1.8 ms) of micropulse and the

Fig. 11. Signal-to-background ratios at the incident neutron energy between 240 and 260 MeV for the graphite target.

long flight path. The measured neutron flight time tn(0) is given for the last micropulse (i ¼ 0), the real flight time tn(i) in other micropulses is

tn ðiÞ ¼ tn ð0Þ þ Ms i;

(5)

where i is the preceding number from the last micropulse (i ¼ 0) and Ms is the micropulse spacing to be 1.8 ms. Fig. 8 shows relations between measured flight time: tn(0) and the neutron energy for the last four micropulses (i ¼ 0, 1, 2, and 3) at the fission chamber position and the target position. The neutron energy range becomes narrow as the increase in the preceding number. We disregarded the effect of these neutrons to the derivation of proton events because the cut-off energy set to 40 MeV was sufficiently larger than the energy of these neutrons. For the fission chamber, neutron-induced fission reactions of 238U could not be induced below 1.5 MeV. Neutrons from last three micropulses (i ¼ 0, 1, and 2) contributed to the fission events. In order to estimate the contribution, a neutron energy spectrum with the wide range above a few MeV at the fission chamber position was required. The energy spectrum, which could not be

1

Peak efficiency [-]

Incident number [neutrons/MeV]

0.9

9

10

0.8

0.7

30 deg. 0.6

0.5 40

108 100

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15 deg. 60 deg.

60 80 100 Proton energy [MeV]

120

Neutron energy [MeV] Fig. 10. Number of neutrons incident on the target.

Fig. 12. Proton peak efficiencies calculated by the PHITS code coupled with the JENDLHE file.

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experimentally measured because of the constant micropulse spacing, was calculated with the PHITS code. The number of fission reactions was deduced with the expected neutron spectrum and neutron-induced fission cross sections of 238U. Obtained fission counts for last three micropulses (i ¼ 0, 1, and 2) are shown in Fig. 9. We obtained a ratio of fission counts by neutrons from last micropulse (i ¼ 0) to that by neutrons from last three micropulses (i ¼ 0, 1, and 2) in each incident energy. The ratio (i ¼ 0) had values from 0.975 to 0.995. Ratios of neutrons from the micropulse for i ¼ 1 and 2 were 5  104e2.5  102 and 1  104e4  104, respectively. The contribution of neutrons from the micropulse for i ¼ 1 and 2 was not large because the bin width of incident energy becomes smaller with the increase in the preceding number. Furthermore, we evaluated the contribution by means of experimental neutron response functions measured at the 4FP15L beam line. Measured neutron response functions had two recoilproton edges by neutrons from last two micropulses (i ¼ 0 and

C, 15 , 97.5-102.5 MeV inc.

1). In each incident energy, a ratio of neutrons from micropulses for i ¼ 0 and 1 was deduced from fitting these edges with two response functions calculated with the SCINFUL-QMD code (Satoh et al., 2006b; Kajimoto et al., 2011b). These ratios were consistent with above estimates to within 1%. The number of neutrons incident on the target is shown in Fig. 10. Obtained numbers have error-bars. Details for the error will be described below. 3.6. Background subtraction The target-out measurement was performed in the same configuration, but without the target. Events obtained in the targetout measurement, background events, underwent the same analysis procedure as signal-events. Fig. 11 shows signal-to-background ratios at the incident neutron energy between 240 and 260 MeV for the graphite target. The proton energy range was limited owing to the cut-off energy and the energy with which a proton could

C, 30 , 97.5-102.5 MeV inc.

C, 60 , 97.5-102.5 MeV inc.

C, 30 , 117.5-122.5 MeV inc.

C, 60 , 117.5-122.5 MeV inc.

C, 30 , 135-145 MeV inc.

C, 60 , 135-145 MeV inc.

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C, 30 , 155-165 MeV inc.

C, 60 , 155-165 MeV inc.

C, 15 , 175-185 MeV inc.

C, 30 , 175-185 MeV inc.

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C, 30 , 190-210 MeV inc.

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Exp. JENDL-HE Bertini PEANUT

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Proton energy [MeV] Fig. 13. Double differential energy spectra of protons emitted from the graphite target for 97.5e102.5, 117.5e122.5, 145e155, 175e185, and 190e210 MeV neutron incidences. The empty points stand for measured data. Curves indicate calculations by the PHITS code with JENDL-HE file (solid), Bertini model of the PHITS code (dashed), and the PEANUT model of the FLUKA code (chain).

T. Kajimoto et al. / Radiation Measurements 47 (2012) 596e608

penetrate the E detector. In the case that the yield of protons emitted from the target is sufficiently obtained, there is a tendency that the ratio become small with decreasing the detector angle. Since the distance between the beam axis and the detector became small with increasing the detector angle in our detector setup, we consider that the main contribution to the background is protons produced from an interaction between an incident neutron and a nucleus consisting of the air on the beam line. 3.7. Proton peak detection efficiency and solid angle Proton peak detection efficiencies were required because we extracted events for a proton giving the full energy to the E detector and stopping in the E detector by the analysis. The efficiency could be obtained by measuring or calculating response functions of the E detector to protons for each incident energy and deriving the ratio of a peak part to the all parts in the response function which consists of peak and tail parts. The peak is formed by protons which give the all energy to the E detector. The tail part in the response function consists of events for a proton penetrating the E detector

C, 15 , 240-260 MeV inc.

603

or inducing some energy loss by a nuclear interaction in the E detector. The efficiency was estimated by the PHITS code. Since nuclear reaction models included in the PHITS code are not established in the low energy region, the efficiency was calculated with Japanese Evaluated Nuclear Data Library High Energy Data (JENDL-HE) (Watanabe et al., 2011). In the calculation condition, protons from a point source were assumed to enter the whole face of the E detector uniformly. The solid angle of the E detector was easily determined with the distance from the point source to the front face of the E detector and the radius of the E detector. We arranged materials: the air, the DE detector, and the frame of the E detector in front of the E detector. The thickness of each material was set same as the experiment and the calculations were performed for each detector angle. The efficiency was obtained from the full energy peak in the deposited energy distribution in the E detector. Fig. 12 shows efficiencies of each DEeE detector. The upper limit of proton energy was set to 130 MeV because a proton penetrates the E detector above the energy. The lower limit was set to 40 MeV equal to the

C, 30 , 240-260 MeV inc.

C, 60 , 240-260 MeV inc.

C, 30 , 290-310 MeV inc.

C, 60 , 290-310 MeV inc.

C, 30 , 340-360 MeV inc.

C, 60 , 340-360 MeV inc.

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C, 30 , 480-520 MeV inc.

C, 60 , 480-520 MeV inc.

C, 15 , 580-620 MeV inc.

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Proton energy [MeV] Fig. 14. Same as Fig. 13, but for 240e260, 290e310, 340e360, 385e415, 480e520, and 580e620 MeV neutron incidences.

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cut-off energy which was determined from the minimum value of the obtained calibration points. The proton range in the E detector becomes longer with the increase in the energy. The increase in the range leads to induce a nuclear reaction in the E detector. Besides, in the case that a proton enters close to the edge of the E detector, the effective thickness becomes smaller and the proton is easy to escape from the E detector without depositing the full energy. The escaping effect depends on the distance between the point source and the front surface of the E detector. The escaping effect is larger as the decrease in the distance. Above 45 MeV, the effect is larger than the effect by nuclear reaction and efficiencies at 15 become larger than the others in the higher energy region. Efficiencies decrease with the increase in the proton energy because of these effects. 3.8. Uncertainties of the experimental results The uncertainties consisted of statistical and systematic errors. The statistical error was deduced from numbers of proton events

Al, 15 , 97.5-102.5 MeV inc.

and fission events. The systematic error was mainly caused by neutron-induced fission cross sections of 238U and the contribution of neutrons from preceding micropulses in the fission chamber analysis, and the proton peak efficiency. The error by neutrons from preceding micropulses was 1%, which was deduced from validity of the estimation with experimental neutron response functions measured at the 4FP15L beam line as mentioned above. The systematic uncertainty in neutron-induced fission cross-sections of 238U below 260 MeV was estimated to be 5% from the difference in experimental cross-sections reported by Lisowski et al. (1991) and Shcherbakov et al. (2002). Since the fission cross-sections have not been reported above 260 MeV, the uncertainty was set to 10%. It is difficult to determine the uncertainty of efficiencies because the efficiencies have not been experimentally validated. Furthermore, the energy resolution of the detector is also important to obtain the accurate efficiency without measuring the efficiency, however, the energy resolution could not be obtained in this experiment. We set the large uncertainty to peak efficiencies and the value was 10%.

Al, 30 , 97.5-102.5 MeV inc.

Al, 60 , 97.5-102.5 MeV inc. Exp. JENDL-HE Bertini PEANUT

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Proton spectrum [protons/MeV/sr/neutron]

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Al, 15 , 135-145 MeV inc.

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Al, 30 , 155-165 MeV inc.

Al, 60 , 155-165 MeV inc.

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Proton energy [MeV] Fig. 15. Same as Fig. 13, but for the aluminum target.

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4. Monte Carlo calculations We obtained proton energy spectra emitted from targets for the neutron incidence. For comparison, we calculated the data with the PHITS code version 2.30 and the FLULKA code version 2011.2.12 with the implemented theoretical models, the Bertini model (Bertini, 1969; Niita et al., 2001) in the PHITS code and the PEANUT model (Ferrari et al., 2005; Battistoni et al., 2007) in the FLUKA code. Furthermore, spectra were obtained with the PHITS code coupled with the JENDL-HE file. The default settings in calculations with the Bertini model were adopted to the physical option. Libraries for the neutron- and the proton-incidence were used in calculations with the JENDL-HE file. The coalescence mechanism was only activated and other physical options were default in calculations of the PEANUT model. In all calculations, the cut-off energy of particles was set below 20 MeV. In the calculation configuration, we set the neutron beam and target configurations as the experiment. The intensity of neutron beam was assumed to be not dependent on the position and be

Al, 15 , 240-260 MeV inc.

605

uniform. Neutron beam with an energy width in centered at an energy was induced to the target owing to the use of continuousenergy neutrons in this experiment. The energy width was same as the measured data. Emitted protons were estimated with ringtype detectors (Satoh et al., 2011) centered upon the target. The space between the target and detectors was filled with an ideal vacuum. The statistical uncertainty was below 5% for each proton energy bin. 5. Results 5.1. Proton energy spectra The double differential energy spectra (DDPSs) of protons emitted from the graphite, aluminum, iron targets for the neutron incidence are shown in Figs. 13e18 with calculated values. The incident neutron energy of each DDPS has a width because continuous-energy neutrons were used as incident particles. The highest neutron energy is limited by the time resolution of TDCs of

Al, 30 , 240-260 MeV inc.

Al, 60 , 240-260 MeV inc.

Al, 30 , 290-310 MeV inc.

Al, 60 , 290-310 MeV inc.

Al, 30 , 340-360 MeV inc.

Al, 60 , 340-360 MeV inc.

Al, 30 , 385-415 MeV inc.

Al, 60 , 385-415 MeV inc.

Al, 30 , 480-520 MeV inc.

Al, 60 , 480-520 MeV inc.

Al, 15 , 580-620 MeV inc.

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Proton energy [MeV] Fig. 16. Same as Fig. 14, but for the aluminum target.

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the E detector and the fission chamber to be 600 MeV. The outgoing energy region of DDPSs was between 40 and 130 MeV owing to the cut-off energy and the energy with which a proton could penetrate the E detector. At incident energies below 150 MeV, spectra are affected by protons emitted strongly in the beam direction. We can observe forward emission edges in spectra at 15 and 30 . The effect forming the edge decreases with the increase in the detection angle and we can not observed the edge in spectra at 60 . The energy at which the edge appears increases with the increase in the incident energy and strays from the energy range between 40 and 130 MeV. We can find flat spectra at incident energies above 180 MeV. The shape of DDPSs calculated by the PHITS code with the JENDL-HE file reproduces experimental those. However, 15 spectra for the graphite target at incident energies between 200 and 300 MeV show a different tendency to experimental values. DDPSs by the Bertini model are shaped differently from experimental results at incident energies below 200 MeV. At edges in 15

spectra, there are discrepancies between measured and calculated values. The slope differs from experimental that for 60 spectra at incident energies between 100 and 200 MeV. Calculated spectra above 250 MeV agree with measured spectra. The trend of DDPSs by the PEANUT model shows agreement with measured values. For DDPSs of graphite and aluminum targets at 15 , calculations present a stronger forward emission edge than experimental results.

5.2. Energy integrated spectra Fig. 19 illustrates proton yields obtained by integrating the DDPS over the energy region of protons between 40 and 130 MeV as a function of the neutron energy for each target. DDPSs for lower incident energies and forward angles have the component of more forward-ejected protons. This component has a higher end-point energy as the incident neutron energy increases. Therefore, we can find a peak below 200 MeV.

Fe, 15 , 97.5-102.5 MeV inc. Fe, 30 , 97.5-102.5 MeV inc. Fe, 60 , 97.5-102.5 MeV inc. Exp. JENDL-HE Bertini PEANUT

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Fe, 30 , 135-145 MeV inc.

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Fe, 30 , 155-165 MeV inc.

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Fe, 15 , 175-185 MeV inc.

Fe, 30 , 175-185 MeV inc.

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Proton energy [MeV] Fig. 17. Same as Fig. 13, but for the iron target.

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Fe, 15 , 240-260 MeV inc. 10-4 Exp. JENDL-HE Bertini PEANUT

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Proton energy [MeV] Fig. 18. Same as Fig. 14, but for the iron target.

Below 250 MeV, calculated values at 15 by the PEANUT model reproduce the tendency of measured values than the others. However, spectra at 60 by the PEANUT model underestimate experimental values. For graphite and aluminum targets at 15 , calculations with the JENDL-HE file differ from experimental results. Calculations with the Bertini model do not reproduced measured values for forward focused emission and for the graphite target. In the energy region above 400 MeV, all calculations for the graphite target at 15 and 30 overestimate experimental values. The maximum overestimation of the JENDL-HE file, the Bertini model, and the PEANUT model became 83%, 75%, and 78%. Above 400 MeV, experimental values for the graphite target at 60 are as large as those at 30 . This angle dependency could not be found in all calculations. The investigation of the discrepancies between calculations and measured data is now underway. 6. Summary In order to validate theoretical models used in Monte Carlo codes and evaluated nuclear data, we measured proton energy spectra

produced at 15 , 30 , and 60 from graphite, aluminum, and iron targets bombarded with continuous-energy neutrons at the WNR facility of the LANSCE. Continuous-energy neutrons enabled simultaneous measurements at various incident energies. The energy spectrum of incident neutrons were measured with the fission chamber. DEeE detectors which consisted of a thin NE102A plastic scintillator and a thick NE213 scintillator were used for measuring outgoing protons emitted from the target. The pulse shape discrimination of the NE213 scintillator was able to discriminate events for a charged particle stopping in the scintillator from events for a charged particle penetrating the scintillator. Proton energy spectra at incident energies from 100 to 600 MeV were obtained. The energy range of outgoing protons for obtained spectra was between 40 and 130 MeV. Measured energy spectra were compared with calculations by the PHITS code coupled with the JENDL-HE file, the Bertini model in the PHITS code, and the PEANUT model in the FLUKA code. There are discrepancies between calculations and measured values and the investigation of discrepancies is now underway. These measured results will be useful as benchmark data for investigating the validity of a Monte Carlo simulation. It will be

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10-2

C, 15

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100 200 300 400 500 600100 200 300 400 500 600100 200 300 400 500 600

Incident neutron energy [MeV] Fig. 19. Integrated proton yields for graphite, aluminum, and iron targets in the energy region of protons between 40 and 130 MeV as a function of incident neutron energy. The empty points stand for measured data. Curves show calculations by the PHITS code with the JENDL-HE file (solid), the Bertini model of the PHITS code (dashed), and the PEANUT model of the FLUKA code (chain).

possible to measure accurate double differential cross sections by use of a thinner target and a vacuum chamber in order to reduce energy loss in the target and the air. Acknowledgment This study has benefited from the availability of the Los Alamos Neutron Science Center at the Los Alamos National Laboratory. This facility is funded by the US Department of Energy under contract DE-AC52-06NA25396. References Armstrong, H.W., Chandler, K.C., 1973. Oak Ridge National Laboratory. ORNL-4869. Battistoni, G., Muraro, S., Sala, P.R., et al., 2007. The FLUKA code: description and benchmarking. In: Albrow, M., Raja, R. (Eds.), Proceedings of the Hadronic Shower Simulation Workshop 2006, Fermilab 6e8 September 2006. AIP Conference Proceeding, vol. 896, pp. 31e49. Bertini, H.W., 1969. Phys. Rev. 188, 1711e1730. Bevilacqua, R., Pompa, S., Simutkin, V.D., et al., 2010. Radiat. Meas. 45, 1145e1150. Bevilacqua, R., Pompa, S., Simutkin, V.D., et al., 2011. Nucl. Instrum. Methods A 646, 100e107. Byrd, R.C., Sailor, W.C., 1989. Nucl. Instrum. Methods A 274, 494e500. Chrien, R.E., Krieger, T.J., Sutter, R.J., et al., 1980. Phys. Rev. C 21, 1014e1029. Ferrari, A., Sala, P.R., Fasso, A., et al., 2005. FLUKA: A Multi-particle Transport Code. CERN-2005-10, INFN/TC_05/11, SLAC-R-773. Förtsch, S.V., Cowley, A.A., Lawrie, J.J., et al., 1991. Phys. Rev. C 43, 691e700. Franz, J., Rössle, E., Sauerwein, C., et al., 1987. Nucl. Phys. A 472, 733e758. Franz, J., Koncz, P., Rössle, E., et al., 1990. Nucl. Phys. A 510, 774e802.

Hjort, E.L., Brady, F.P., Drummond, J.R., et al., 1996. Phys. Rev. C 53, 237e242. Kajimoto, T., Shigyo, N., Ishibashi, K., et al., 2011a. J. Korean Phys. Soc. 59 (2), 1721e1724. Kajimoto, T., Shigyo, N., Sanami, T., et al., 2011b. Nucl. Instrum. Methods A 665, 80e89. Kin, T., Saiho, F., Hohara, S., et al., 2005. Phys. Rev. C 72, 014606. Lisowski, P.W., Schoenberg, K.F., 2006. Nucl. Instrum. Methods A 562, 910e914. Lisowski, P.W., Gavron, A., Parker, W.E., et al., 1991. Proceedings of the Specialists Meeting on Neutron Cross Section Standards for the Energy Region above 20 MeV, Uppsala, Sweden, pp. 177e186. Nakao, N., Kurosawa, T., Nakamura, T., et al., 2001. Nucl. Instrum. Methods A 463, 275e287. Niita, K., Takada, H., Meigo, S., et al., 2001. Nucl. Instrum. Methods B 184, 406e420. Niita, K., Matsuda, N., Iwamoto, Y., et al., 2010. PHITS: Particle and Heavy Ion Transport Code System, Version 2.23. JAEA-Data/Code 2010e022. Pelowitz, D., 2005. MCNPX User’s Manual Version 2.5.0. Los Alamos National Laboratory report LA-CP-05-0369 (April 2005). Sagrado, I.C., Lecolley, J.F., Lecolley, F.R., et al., 2011. Phys. Rev. C 84, 044619. Sasaki, M., Nakao, N., Nakamura, T., et al., 2002. Nucl. Instrum. Methods A 480, 440e447. Satoh, D., Sato, T., Endo, A., et al., 2006a. J. Nucl. Sci. Technol. 43, 714719. Satoh, D., Sato, T., Shigyo, N., et al., 2006b. JAEA-Data/Code. 2006-023. Satoh, D., Moriguchi, D., Kajimoto, T., et al., 2011. Nucl. Instrum. Methods A 644, 59e67. Shcherbakov, O., Donets, A., Evdokimov, A., et al., 2002. J. Nucl. Sci. Technol. Suppl. 2, 230e233. Sisterson, J.M., Ullmann, J., 2005. Nucl. Instrum. Methods B 234, 419e430. Taniguchi, S., Nakao, N., Yamakawa, H., et al., 2006. Nucl. Instrum. Methods A 562, 954e957. Watanabe, Y., Kosako, K., Kunieda, S., et al., 2011. J. Korean Phys. Soc. 59 (2), 1040e1045. Wender, S.A., Balestrinit, S., Brown, A., et al., 1993. Nucl. Instrum. Methods A 336, 226e231.