Response function measurements of an NE102A organic scintillator using an 241 Am-Be source

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1 Nuclear Instruments and Methods m Physics Research A 345 (1994) North-Holland NCLEAR INSTRMENTS & METHODS IN PHYSICS RESEARCH Section A Response function measurements of an NE12A organic scintillator using an 241 Am-Be source A.A. Naqvi *, A. Aksoy, F.Z. Khiari, A. Coban, M.M. Nagadi, M.A. Al-Ohali, M.A. Al-Jalal ' Energy Research Laboratory, King Fahd nicersity of Petroleum and Minerals, Dhahran 31261, Saudi Arabia (Received 11 October 1993 ; revised form received 19 January 1994) The response function of a 125 mm diameter NE12A organic scintillation detector has been measured over the MeV neutron energy range. The detector response function was derived from the light output for monoenergetic neutrons and gamma rays. The light output of the detector for monoenergetic neutrons was measured by selecting narrow gates in the time-of-flight (TOF) spectrum for a 241Am-Be neutron source. In order to provide check points on the data, the detector light output was also measured for monoenergetic neutrons from the D(d, n) and T(d, n) reactions. The response function of the NE12A detector is in good agreement (within 1-5%) with the published data of Cecil et al. [Nucl. Instr. and Meth. 161 (1979) 439]. 1. Introduction Response functions of organic scintillators for neutrons are required to calculate their neutron detection efficiency. For an organic neutron detector, the response function is defined by the correlation of the proton and electron energies which yield the same amount of light in the detector. Generally, protons and electrons are produced by scattering of neutrons and gamma rays in the organic scintillator, respectively. The response function of a detector, which depends on its energy resolution, manufacturer and history, are generally measured using monoenergetic neutrons produced in accelerator based nuclear reactions [1]. Some measurements are also carried out using neutron sources like 252Cf and 241 Am-Be [2,3]. It is easier to carry out the scintillator response function measurements using neutron sources than accelerators. This is due to the fact that time-of-flight spectra of the neutron sources do not have the accelerator associated backgrounds. Additionally, the 252Cf source neutron energy spectrum, which has been precisely measured, is used in relative neutron detection efficiency measurements [2]. Recently, response function measurements of an NE213 detector have been carried out using a new technique in which a combination of a 24 'Am-Be neutron source and nuclear reactions like T(d, n) and D(d, n) were used to measure the light output of the * Corresponding author. i Present Address : Department of Physics, Massachusetts Institute of Technology, MA, SA. detector for neutrons [4]. Now, this technique has been used to measure the response function of a 125 mm diameter NE12A detector. In this paper the technique and the response function for the NE12A detector are described. 2. Response function measurements The NE12A detector, whose response function was measured in this study, had a 125 mm diameter and 5 mm thickness. The response function measurements consist of two parts, i.e., light output measurements for electrons and light output measurements for neutrons. The light output measurements for neutrons were carried out in two steps. In the first step the light output of the detector for neutron energies below 6.5 MeV was measured using a 41 Àm-Be neutron source while in the second step the detector light output was measured for 2.8, 9.8 and 14.8 MeV neutrons using the D(d, n), 12 C(n, ni )12C and T(d, n) reactions. Finally, the results of both steps were combined to generate a complete set of light output data for the detector covering the 2.7 to 14.8 MeV neutron energy range. The light output of the NE12A detector for electrons was measured by acquiring Compton recoil spectra of 22 Na, 132Cs and 65Zn monoenergetic gamma-ray sources. The measurements were carried out using gamma-gamma coincidence technique [5] and are described in detail elsewhere [6]. In these measurements the light output of the detector corresponding to the half-height of the Compton recoil edge, E, and the maximum energy of Compton electrons, E., were mea /94/$ Elsevier Science B.V. All rights reserved SSDI168-92(94)173-5

2 A.A. Naqvi et al. /Nucl. Instr. and Meth. in Phys. Res. A 345 (1994) ent TIme Zero Detector 1 Am-ße Neutron Source CFD NE12A1NE213 ra PA electrons [6]. For the response function measurements, the channel corresponding to the half height of the Compton recoil edge was assigned the maximum energy of Compton electrons E, Finally, the response functions were derived from the correlation of the electron and proton energy which yield the light output corresponding to the same pulse height. Since the light output of the NE12A detector for neutrons was measured in two steps utiliing a neutron source and accelerator based nuclear reactions, they involved different experimental setups. In the following they will be described separately. TOF T 2.1. Measurement of NE12A detector light output for neutrons using ar Am-Be neutron source L CAMAC Based VAX 11/785 Data Acquisition System Fig. 1. Block diagram of the electronics used for detector light output measurements for neutrons using a 24 'Am-Be neutron source. sured separately. It was observed that for the 125 mm diameter NE12A detector the deviation between Eh and E, varies from 13.3 to 22.8% for.341 to 1.1 MeV The light output of the NE12A detector for neutron energies below 6 MeV was measured by acquiring the detector pulse height for neutrons with preselected energies from a 24y-Be neutron source. The neutron energy was determined from the time-of-flight (TOF) spectrum of the 241Ám-Be neutrons, which was acquired using a 5 mm diameter and 5 mm thick NE213 detector as a start detector and the NE12A detector as a stop detector. The start detector was placed very close to the source while the stop detector was placed at a distance of cm from the source. The electronics circuit used in the present study is shown in Fig. 1. It consists of three branches, namely a time-of-flight (TOF) branch, a pulse shape discrimination (PSD) branch and a pulse height (PH) branch. The outputs of the three branches were connected to the three analogue to digital converters (ADC), namely TOF-ADC, PSD-ADC and PH-ADC. Data from the looo soo 1 r a CHANNEL NMBER Fig. 2. Time-of-flight (TOF) spectrum of the NE12A detector for 24 'Am-Be source neutrons along with the TOF gates.

3 516 A A Naqut et al. I Nucl. Instr. and Meth. in Phys. Res. A 345 (1994) H 1 Fig. 3. Typical sorted pulse height spectrum for one of the TOF gates of 241 Am-Be source neutrons three ADCs was acquired event by event in List mode using a Multiparameter Data Acquisition Program running on the VAX 11/785 data acquisition system of ERL [2]. The data was written on magnetic tapes. For each event detected by the 125 mm detector, three parameters, namely TOF, PSD and PH can be stored. For the NE12A detector only two parameters namely, TOF and PH, were acquired. The experiment was continued over a period of about 37 h, using a.35 Ci Am-Be neutron source. In total, 8 X 1 6 events were stored and were subsequently analyed off-line. The time calibration of the TOF time-to-amplitude converters (TAC) for a 1 ns range, which was carried out several times during the experiment, was measured to be.13 ns/channel. In order to accommodate different energy ranges within the pulse height spectrum, the pulse height spectra were acquired at three different gain settings of the double delay line amplifier. In the offline analysis, the pulse height corresponding to a 3-4 channel wide gate 1 r- c Fig 4 Detector pulse height spectrum for 14.8 MeV neutrons.

4 A.A. Naque et al. /Nucl. Instr and Meth. in Phys. Res.A 345 (1994) iodo_ 8 m r Fig. 5. Time-of-flight spectrum of the NE12A detector for elastically and inelastically scattered neutrons from a graphite scatterer at 45 lab angle along with the gate chosen for the inelastic peak. on the TOF spectrum was sorted out. From the pulse height spectra, the channel corresponding to the half height of the proton recoil edge was determined and it was assumed that it corresponds to the mean neutron energy of the 3-4 channel wide neutron TOF gate. The neutron energy was calculated using the flight path length, the TOF channel and the location of the gamma peak in the TOF spectrum. The finite width of the gates resulted in an uncertainty of.5-.7% in the mean neutron energies over 1.6 to 6. MeV, respectively. Fig. 2 shows the TOF spectrum of the NE12A detector for 241 Am-Be neutrons along with the TOF gates for a few neutron energies, while Fig. 3 shows a typical sorted pulse height spectrum for one of the 2 í5l y 1[ 5L Fig. 6. Sorted pulse height spectrum of the NE12A detector for the gate of the inelastic peak with 9.8 MeV neutrons.

5 51 8 A.A. Naqui et al. /Nucl. Instr. and Meth. in Phys. Res. A 345 (1994) TOF gates. The calibration of pulse height channels in Table 1 terms of electron energy was carried out periodically using the procedure described in ref. [5]. Response function detector data of the 125 mm diameter NE12A 2.2. Detector light output measurements for neutrons using D(d, n), T(d, n) and 12 C(n, n t ) In the second step, the detector light output was measured for 2.82, 9.75 and 14.8 MeV neutrons using the D(d, n), 12C(n, n t ) and T(d, n) reactions. The experiment was carried out at the 45 degree beam line of the ERL 35 kev accelerator [7], using a pulsed deuteron beam with a pulse width less than 1 ns. The tritium target had a tritium activity of 3 Ci/in. 2, while the deuterium target had a deuterium loading of 1.2 cm 3/in. 2. In order to measure the light output of the detector for 2.8 and 14.8 MeV neutrons, the NE12A detector was placed at ero degree angle and at 4 m target-detector distance. Fig. 4 shows the pulse height spectrum for 14.8 MeV neutrons. The detector light output was also measured for 9.75 MeV neutrons which were produced by inelastic scattering of the 14.8 MeV neutrons from a cylindrical graphite scatterer. In the scattering experiment, the detector was placed inside a paraffin-lithiumcarbonate shield at an angle of 45 degrees with respect to the beam axis. The scatterer-detector distance was 4 m, while the target-scatterer distance was 15 cm. The source neutrons from the target were stopped from reaching the detector by inserting copper shadow bars between the neutron target and the entrance of the detector collimator. spread of the scattered neutrons In order to minimie the energy caused by multiple scattering inside the scatterer, a cylindrical graphite scatterer of 2 mm X 2 mm (height X diameter) was used. Fig. 5 shows the TOF spectrum of the scattered neutrons with the elastic peak on the right side and the inelastic peak on the left side. During the experiment, a gate was set on the inelastic peak and the corresponding pulse height was sorted out. Fig. 6 shows the sorted pulse height spectrum corresponding to the inelastic neutron peak. The channel corresponding to the half-height of the proton recoil spectrum for the inelastic peak was converted into electron energy. Proton energy [MeV] MeV neutrons from the D(d, n), 12 C(n, ni) and T(d, n) reactions, was equal to the light output for.93, 4.93 and 8.65 MeV electrons. For these measurements the uncertainty in proton energy was 1-2% while the uncertainty in the determination of electron energies was less than 1%. The response function data of the NE12A detector measured in the present study is d en d w w " cecil o Electron energy [MeV] Present work _ _ _ _ _ _ _ _ _ _ _ _ _ _ ERL Cecil et al. [8] 2-3. Results and discussion The light output of the NE12A detector for 2.64, 3.2, 3.31, 3.4, 3.78, 4.14 and 4.55 MeV neutrons from a 241 Àm-Be source was equal to the light output for.81,.97, 1.14, 1.22, 1.43, 1.59 and 1.77 MeV electrons, respectively. The uncertainties in proton energies were.5-.7% while the uncertainty in the determination of electron energies was typically less than 1%. The light output for 2.82, 9.75 and i 1. Proton Energy (MeV) Fig. 7. NE12A detector response function data (ERL data) along with data from Cecil et al. [8]. The solid line represents a second order polynomial fit to the ERLdata points.

6 A.A. Nagvi et al /Nucl. Instr. and Meth. in Phys. Res. A345 (1994) Table 2 Coefficients of the polynomial fit to the response function data. For comparison the coefficients of the polynomial fit of a 125 mm diameter NE213 detector's response function data are also listed in the table (taken from ref. [4]) Coefficients NE12A NE213 detector detector [4] A B C.1.8 listed in Table 1. For comparison, the Cecil et al [8] data have also been included in Table 1. The response function data taken in this study agree within 1-5% with those reported by Cecil et al. Fig. 7 shows the response function of the NE12A detector along with the data of Cecil et al. Finally the response function of the NE12A detector was fitted with a second order polynomial of the form : Ee(MeV) =A +BEP (MeV) +C[EP(MeV)l, where A, B and C are coefficients determined from the fit to the response function data. Fig. 7 also shows the second order polynomial fit to the response function data. The values of the coefficients A, B and C are listed in Table 2. For comparison, the coefficient values for a 125 mm diameter NE213 detector [4] are also listed in Table 2. For both detectors the coefficient C has a value close to ero. The coefficient B of the fit for both detectors, which mainly determines the slope of the response function curve, agree with each other within 5%. The present technique has proved to be successful in measuring the response function of organic detectors like NE12A using a 24'Am-Be source. Moreover, the present study has also provided useful data on the response function of the 125 mm diameter NE12A detector of the Energy Research Laboratory, King Fahd niversity of Petroleum and Minerals, Dhahran Saudi Arabia. 4. Conclusion The response function of an NE12A detector was measured over the MeV neutron energy range. The detector response function was derived from the light output for monoenergetic neutrons and gamma rays. For neutron energies below 6.5 MeV, the light output of the detector for monoenergetic neutrons was selected by choosing narrow gates in the time-of-flight spectrum using a 41 Am-Be neutron source. In order to provide check points on the data, the detector light output was also measured for monoenergetic neutrons from the D(d, n) and T(d, n) reactions. The measured response function of the NE12A detector shows good agreement with the Cecil et al data. Acknowledgement This work is part of a KFPM/RI project at ERL supported by the Research Institute of King Fahd niversity of Petroleum and Minerals. References [1] A. Aksoy, A. Coban, A.A. Naqvi, F.Z. Khiari, J.M. Hanly, C.R. Howell, W. Tornow, P.D. Felscher,M.A. AI-Ohali and R.L. Walter, Nucl. Instr. Meth.A 337 (1994) 486. [2] K. Gul, A.A. Naqvi and H.A. Al Juwair, Nucl. Instr. and Meth. A 278 (1989) 47. [3] V.V. Filchenkov, A.D. Konin and A.I. Rudenko, Nucl. Instr. and Meth.A 291 (199) 54. [4] A.A. Naqvi, F.Z. Khiari, A. Aksoy, A. Coban and M.A. A]-Jalal, Nucl. Instr. and Meth. A 325 (1993) 574. [5] A.A. Naqvi, H.A. Al-Juwair and K. Gul, Nucl. Instr. and Meth. A 36 (1991) 267. [6] A.A. Naqvi, F.Z. Khiari, A. Aksoy, A. Coban and M.A. A]-Jalal, Nucl. Instr. and Meth. A 324 (1993) 223. [71 H. Al Juwair, G. Blume, R.J. Jarasma, C.R. Meitler, and K.H. Purser, Nucl. Instr. and Meth., B 24/25 (1987) 81. [8] R.A. Cecil, B.D. Anderson and R. Madey, Nucl. Instr. and Meth. 161 (1979) 439.