Fujiwara, Noboru

Higo, Toshiyasu

Iwashita, Yoshihisa

Matsuki, Seishi

Okihana, Akira

Yanabu, Takuji

Kyoto University Research Information Repository

TitleEfficiency Calculation and Measurement of Large NE213Neutron Counter (Commemoration Issue Dedicated toProfessor Sakae Shimizu on the Occasion of his Retirement)Author(s)Higo, Toshiyasu; Matsuki, Seishi; Iwashita, Yoshihisa;Okihana, Akira; Fujiwara, Noboru; Yanabu, TakujiCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1979), 57(1): 13-22Issue Date1979-03-31URL http://hdl.handle.net/2433/76822RightType Departmental Bulletin PaperTextversionpublisherKyoto University         Bull. Inst.  Chem. Res., Kyoto Univ., Vol. 57, No. 1, 1979                 Efficiency Calculation and Measurement                 of Large NE213 Neutron Counter                         Toshiyasu 'HIGO, Seishi MATSUKI, Yoshihisa IWASHITA,                       Akira OKIHANAt, Noboru FUJIWARAtt, and Takuji YANABU*                                        Received October 4, 1978               A 14 cm diameter and 19 cm depth counterwith liquid scintillator (NE213) was constructed             for the fast neutron detection. Design, performance and efficiencycalibration are described.            Calculated efficiency and experimental efficiency are compared at lowenergy region. They agree            within about 20%.                  KEY WORDS Large neutron counter /Its efficiency calibration/                                  I. INTRODUCTION             Efficiency calibration of a neutron counter is an old and new problem. For thin          neutron counters which detect neutrons up to 8 MeV, a simple analytical calculation of         total efficiency gives good agreement') with the experimental one. However, for thick          neutron counters, the effect of multiple scattering dominates and the simple analytical          calculation gives poor predictions.2>             For small counters, Verbinski et al.s' developed a Monte Carlo calculation code to         give the response function of a neutron counter with the NE213 liquid scintillator, and         the agreement between the prediction and the experimentally determined one was good."            In this paper, we report the neutron efficiency calculated by applying the Verbinski's          principle to our counter and also we report the calibration of the counter at low          energies. This counter was constructed for the detection of neutrons from the (2c, n)         reactions on various materials. The experiment was performed by using the 500 to 1500         MeV/c sr beam of the National Laboratory for High Energy Physics (KEK).5) H. CONSTRUCTION OF A NEUTRON COUNTER               The .design principle of the counter is as follows;          (1). to discriminate gamma rays from neutrons, the liquid scintillator NE213 should be used.         (2). to detect neutrons of 100 MeV with 20% efficiency, the depth of the scintillator               should be greater than 15 cm.        * AE.**T  V, ME Y7-, 5Z.W ; Institute for Chemical Research,              Kyoto University, Kyoto. t Present Address ; Kyoto University of Education, Kyoto. tt Present Address ; Nara Women's University, Nara. (13)        T. HIGO, S. MATSUKI, Y.  IWASHITA, A. OKIHANA, N. FUJIWARA, and T. YANABU                                            Reservoir                                            Stainless VesselLiq. NE213 m Light PM Guide f--190-------->                 Thin StainlessOptical Pyrex                 Fig. 1. Cross section and dimensionsof the neutron counter. 100-                          GAMMA RAY Z Z50--                O                               NEUTRON ------------------------------------------------------700750800 '..                                 CHANNEL              Fig. 2. Zero crossing time distribution of NE213 liquid scintillator                       with Ra-Be neutron source. (3) . to construct the counter economically, only one photomultiplier should be used.     Therefore, the scintillation light should be led to the photomultiplier via a light guide. (4). to make the counter feasible to detect charged particles, window of the counter      should be as thin as possible.    Cross section and dimensions are shown in Fig. L The liqiud scintillator NE213 of 2. 7 l was filled in the vessel. Not to degrade the characteristics of neutron-gamma ray discrimination, care was taken to protect the scintillator from oxygen, that is, the vessel was filled with Ar gas and then the NE213 was poured into the vessel. The inside wall of the vessel was coated with TiO2 paint and the light guide was wrapped with thin Al foil to raise the light collection efficiency. The photomultiplier used is 56DVP. Time resolution was measured to be less than 3. 6 ns for gamma rays.    Neutron-gamma ray discrimination was tested with a Ra-Be neutron source.The energy threshold was fixed at 2. 3 MeV. The result is shown in Fig. 2. Though the scintillator volume is large, the discrimination is good in this low energy region. III. EFFICIENCY CALCULATION    The FORTRAN program for the calculation of neutron counter efficiency developed (14)                Efficiency Calculation and Measurement of Neutron Counter by Verbinski et al. is called  05S.6) Yoshida et al.') have developed a program for the efficiency calculation based on the algorithm of the program 05S. Some differences exist between the 05S and that of Yoshida, which we call Code 1 hereafter, as the following. (1) . For the 12C (n, n)12C elastic scattering, simplified pattern of angular distribution is      used in 05S and is parametrised, but in Code 1, Legendre polynomial expansion      is applied to express the angular distribution. (2) . For the reaction 12C (n, pn)"B and "C (n, 2n)11C, the distribution of first emitted      particle is assumed p(0) =2-9/r in the c. m. system in 05S, but in Code 1,      isotropic distribution in the c. m. system is assumed. (3) . The spreading of light output due to the fluctuations of light collection efficiency,      photomultiplier gain and so on are taken into account in 05S, and Gaussian distri-     bution of light output is assumed, but in Code 1, no attention is paid to these      fluctuations. We improved further Code 1 and developed a new version, which we call Code 2, to calculate the counter efficiency. The main improvements are as follows; (1) . Energy spread of the incident neutron beam is taken into account by assuming      the Gaussian distribution. (2) . Finite resolution of the light output is taken into account. The actually observed      light output distribution is assumed to be Gaussian. The standard deviation c.2) of     light output is given by Q=Lo(L/Lo+O. 5)1/2(1)      where Lo is a parameter determined experimentally, and L is the light output      given by the relation between light output and energy loss31 (Fig. 3). Unit     of Lo and L is defined by the 1. 28 MeV gamma ray Compton edge of 22Na. The      light output is then calculated using the Neumann's method for each incident        neutron. (3). In Code 1, neutrons are assumed to enter the detector in parallel. In our experi-     ment, the neutron source is defined inside the circle of 1 cm diameter, and the • =10. z 1: oe .2.0.1: P a                   1 10                                Particle Energy(Mev)            Fig. 3. Relations between light output and charged particle energy loss                  in scintillator NE21332 One unit of light output is defined by the                   light output pulse corresponding to the Compton edge of 1.28                      MeV gamma-rays from 22Na source. ( 15 )        T.  HIGO, S..MATSUKI, Y. IWASHITA, A. OKIHANA, N. FUJIWARA, and T. YANABU      neutron counter, of which the diameter is 14 cm, is located at about 1 m apart      from the neutron source. Therefore, the divergence of the neutron beem is taken      into account in our program.    In Table I are listed the cross sections, light output yield and proton ranges in NE213 which are used in our calculation. These values are almost the same as those in 05S.    An example of efficiency curve of the neutron counter calculated by Code 2 is shown in Fig. 4. In this calculation, the distance- between the neutron source and the neutron counter is one meter. Point neutron source is assumed. Time resolution of the time of flight system is 3. 6 ns FWHM, so that the neutron beam is assumed to have an energy spread corresponding to this time resolution arround the specified value of the                Table I. References of Cross Section Data Used in Code 2                        ReferenceEnergy MeV            ENDF/B0-14 H05S Library14-100 dQ/dt205S Library 13. 7---62.5 .               KFK-7500^-10       v'°`alENDF/B10-44                      05S Library14--100              KFK-7500-40 ai1ENDF/B10-44                     05S Library -14-400 do/dQ05S Library               KFK-7500^-l4 a'"°rENDF/B10^ 14                     05S Library14-400 C ----------------------------------------------------------              do/dSQIsotropic on05R Library0-44                   05S Library14--              de/dQQisotropic                   05R Library0-14 Qn,n3a                               05S Library             Qn,n05S Libraryn. nn05S Library n.zn05S Library           Energy Loss-Light Yield Data05S Library Proton Range Data05S Library (16) Efficiency  Calculation and Measurement of ,Neutron Counter>                                 50-                             :7.340-                         w30-                            10-                        10 ~o 30 40 50 s0 70 80 90 100                                         NEUTRON ENERGY(MeV)                 Fig. 4. Efficiency calculatedby Code 2 up. to 100 MeV. neutron energy. Parameter Lo is fixed to 0. 1. Neutron threshold energy is 2: 35 MeV. Number of trial is 10000 and calculation time for one neutron energy is about 25 seconds using FACOM M190.                    IV. EFFICIENCY DETERMINATION    This experiment aims to estimate the efficiency of the neutron counter set at the same geometry as used in the (r-, n) experiment. Therefore, the neutron counter was set at 1 m apart from the target. Experimental setup is shown in Fig. 5.    The commonly used reaction d+ +3He was used to determine the efficiency. Deuteron beams from the Kyoto University Cyclotron (Ed=14 MeV), and from the Van de Graaff accelerator (Ed=4 MeV) were used. Deuterated polyethylene, (CD2) n, was used as a target and its thickness was 400 ftg/cm2 or 50 fig/cm2. 3He detector, a 9 mm diameter SSD, was set 45 mm apart from the target. To make sure the neutron-3He coincidence, 3He detector should cover the angular range of 3He associated with neutrons which enter into the neutron counter. An example of angular range covered by the SSD is shown in Fig. 6. Dotted circle in the figure shows the angular range of the associated 3He and circle of 2, 3, and 4 show the angu-lar range subtended by the SSD. As seen from the figure, the SSD covers the area of 3He even if the deuteron beam changes its position. Experimentally, the neutron count-er is set for the moment at some polar angle and then the 3He detection angle is varied to survey the relevant polar angle by observing the variation of the coincidence rate.                          Vacuum_.                                                     ScatteringChamber •                                     Si 55D                      -------- --'----'—Beam              Faraday Cap f/- / ~• - CD2 Target 5mm All V Neutron Counter                   Fig. 5. Experimental setup for efficiency determination. (17)    T. HIGO, S. MATSUKI, Y. IWASHITA, A.  OKIHANA, N. FUJIWARA, and T. YANABU 4 • Ls's? 2 -10 Beam Spot (4x 2 mm2) 45'50' 55• et     Fig. 6. Area 1 describes the angular range of 3He associated with neutrons            which enter into the neutron counter. Area 2 (or 3, 4) describes the           angular range subtended by SSD if beam spot position is at 2 (or 3,           4) in the beam spot. cn - //r-s\ F•. o_   U" 3 24 26 28 30 32 34e • z'                                                    •0                           U•               3 2 1 0 1 2 3cm,       DownUp       Fig. 7. o Coincidence rate as a function of 0, the angle of SSD to the              deuteron beam direction.              • Coincidence rate as a function of azimuthal angleof the neutron                 counter.         Si SSD TFA D'scn------------G G f 3He gate TAC-------------'IPHAITOF Spectrum T P C Delay F A xt. Strobe                                                               -Neutron -Gamma      (Bias)Discri fast -' F A Scint PMTAC SCA m stowTSCA                                                                Neutron P A  - jDDLA fEIA-1-----------PHA pulseHeight                                                         Spectrum              SCA m            Fig. 8. Block diagram for efficiency determination experiment.                        ( 18 ) Efficiency Calculation and Measurement of Neutron Counter Then the neutron counter is moved azimuthally to fix the position by observing the variation of coincidence rate. These conditions are shown in Fig. 7.    Electronic block diagram is shown in Fig. 8. The detection threshold of the neutron counter was set at 2. 35 MeV. For this purpose, we measured first the light output of the neutron counter from the gamma-ray source of "Co and 137Cs. And then the con-stant k and a in the following relation was estimated from the light output corresponding to the Compton edge of gamma-rays in these sources.           Light output=k X (electron energy loss) a. Second, the experimental relation3) between the proton energy loss and light output for the NE213 was used to determine the bias energy. (see Fig. 3) The light output resolu-tion was determined as shown in Fig. 9 by using the 6 MeV neutron beam. Parameter L0 in Eq. (1) was determined to be 0. I. The efficiency was determined as follows; N                              0 •U           104 L=0 L,=0. it T                                        6MeV                          1 2 3 4 5 Pulse Height           Fig. 9. Dot ; experimental pulse height distribution of 6 MeV neutrons.                  Broken line ; response function calculated by code 2, L0=0.                  Solid line; response function calculated by code 2, L0=0. 1. 70- 60' 50-o - ?-40--                U -Z -                   u30 L w 20- 10•_         051015                                   NEUTRON ENERGY(MeV)                    Fig. 10. Total efficiency determined experimentally.                          solid line ; efficiency calculated by Code 2. (19 )       T.  HIGO, S. MATSUKI, Y. IWASHITA, . OKIHANA; N. FUJIwARA, and T. YANABU Table IL Error Summary of Efficiency Determined Experimentally Ed=14 MeV Cyclotron beam Ea =4 MeV Van de Graaff beam E. (MeV) 0, (lab) AE, (MeV)E. (MeV) 6, (lab) 4E, '(MeV) 5.9 90°<0.9 3.4 90° <0.38 7.975° <1.2 5.0 60 <0.45                                    Considered errors 3Counting statistics< 3 < 3Target thickness estimation*< 3 < 3D (d, n) 3He reaction cross section** <10. <- 1Integrated beam charge< 1 < 2Solid angle error< 2 < 1Al absorption error***< 1 <10Error caused by bias setting error< 5 <12Estimated total error<12      * Error in d-  elastic scattering cross section data9)'i01 to estimate target thickness     ** Error in D (d, n) 3He reaction cross section data.11)        For Ed=4MeV, this cross section was estimated using interpolated data of Legendre        expantion coefficients for angular distribution of the cross section. *** Errors in estimating neutron flux attenuation by Al wall of the chamber. This error        arises from neutron total cross section data12) for Al. s„,,= (number of coincidence counts)/                                   (neutron flux into the neutron counter) The experimental results of the total efficiency are shown in Fig. 10. Errors and other parameters are listed in Table II.                          V. DISCUSSIONS    In Fig. 9 are shown the neutron pulse height spectrum together with the prediction by Code 2. As seen in the figure, light output resolution should be taken into account in the Monte Carlo calculation. The efficiency calculated with and without the light output resolution are shown in Fig. 11. Light output resolution was fixed as L°=0, 1                                                          e '~ ,o T Y           UI u_ W~. O 510                                     NEUTRON E ERGY(MeV)         Fig. 11. • (X) Efficiency calculated by Code 2. Finite (infinite) resolution                  of incident neutron energy and light output is assumed. ( 20 )                     Efficiency Calculation and Measurement of Neutron Counter                   Table III. Dependence of Efficiency on Neutron Bias Energy.  Eesp is                            Efficiency Determined Experimentally and E„,„r is the                         One Calculated by Code 2 En(MeV)Bias(MeV)s (%) c,,,,,, (%) 2.049 I 39 3.442.353232 2.72824 2.05951 5.02.355846 2.74840      hereafter. Figure 4 shows the efficiency curve in the energy range up to En=100 MeV.         Comparison is made in Fig. 10 between the experimentally determined efficiency      and calculated one with Code 2. In Table III are shown the dependence of efficiency      on neutron bias energy. The experimentally determined efficiency and calculated one      are listed. The calculated values are in moderate agreement with those of experimental         ones.          Experimental efficiency estimation of such a large counter is rather scarce. At      about 76 MeV, Narbonis' reported the agreement between calculation and experiment       less than 10%. Love et a1.4) reported the comparison of response function between      calculation and experiment at about 2. 7 MeV and 14 MeV, and got agreement less      than 10%. From these results, including our experiment, Monte Carlo calculation can      predict neutron efficiencies which agree with experiments less than 20% up to 100 MeV.                             ACKNOWLEDGMENTS          We would like to acknowledge Dr. K. Shin and his collaborators for offering us the      Monte Carlo program and for kind suggestion. We would like also to thank Professor       H. Itoh for the arrangements of this experiment. The (it-, n) reaction experiment was      done with the collaboration of Saga University, Kyushu University and the Institute for Chemical Research, Kyoto University. The authors would like to acknowledge the       collaborators for their encouragement of this work. Thanks are also due to the crews      of the cyclotron and Van de Graaff accelerator of Kyoto University for their kind      cooperation. The calculation was done with FACOM M190 at the Data Processing      Center, Kyoto University and with Central Computer HITAC8800 at Data Handling      Division of National Laboratory for High Energy Physics (KEK) .                                REFERENCES (1) Eberhard Finckh in "Nuclear Spectroscopy and Reactions (Part B),” J. Cerny Ed.,                Academic Press, New York and London, 1974, p. 573. (2) Noel R. Stanton, COO-1545-92 (1971), unpublished (3) V. V. Verbinski, W. R. Burrus, T. A. Love, W. Zobel, N. W. Hill, and R. Textor, Nucl. ( 21 )    T. HIGO, S.  MATSUKI, Y. IWASHITA, A. OKIHANA, N. FUJIWARA, and T. YANABU     Instr. & Meth. 65, 8 (1968). (4) T. A. Love, R. T. Santoro, R. W. Peele, and N. W. Hill, Rev. Sci. Instr. 39,541 (1968). (5) to be published. (6) R. E. Textor and V. V. Verbinski, ORNL-4160, unpublished (7) M. Yoshida, master's thesis, Kyoto Univ. (1976), unpublished (8) Ph. Narboui, Ph. N. 70-12, ORSAY, unpulished (9) A. S. Wilson, M. C. Taylor, J. C. Legg, and G. C. Phillips, Nucl. Phys, Al26, 193 (1969). (10) J. E. Brolley, Jr., T. M. Putman, L. Rosen, and L. Stewart, Phys. Rev., 117, 1307 (1960). (11) J. E. Brolley, Jr., T. M. Putman, and Louis Rosen, Phys. Rev., 107, 820 (1957). (12) BNL325, 3rd ed, Vol II (1976). ( 22 )

Efficiency Calculation and Measurement of Large NE213 Neutron Counter (Commemoration Issue Dedicated to Professor Sakae Shimizu on the Occasion of his Retirement)

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Efficiency Calculation and Measurement of Large NE213 Neutron Counter (Commemoration Issue Dedicated to Professor Sakae Shimizu on the Occasion of his Retirement)

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