Studies of neutron emission from relativistic nuclear interactions

Radiation Measurements

PERGAMON

Radiation Measurements 31 (1999) 555-558

STUDIES OF NEUTRON EMISSION FROM RELATMSTIC NUCLEAR INTERACTIONS S.-L. GUO *, L. LI *, Y.-L. WANG *, H-Y. GUO *, B.H. SA *, Y.M. ZHENG * R. BRANDT **, P. VATER **, J. S. WAN **, M. OCHS **, B.A. KULAKOV ***, A.N. SOSNIN***, M.I. KRIVOPUSTOV ***, V.S. BUTSEV *** AND V. BRADNOVA *** • China Institute of Atomic Energy, Beijing 102413, China • * Kernchemie, FB 15, Universitrtt Marburg, D-35032 Marburg, Germany • ** Joint Institute for Nuclear Research, Dubna, Russia

ABSTRACT Studies were carried out on the yields and spatial distributions of secondary neutrons produced in the relativistic nuclear interactions of 1.5 GeV to 14.4 GeV projectiles p, d and ot -particles with targets Pb and U/Pb. CR-39 track detectors were used to measure the neutrons. It shows that: (1) Secondary neutrons are produced in the whole length of Pb or U targets having a thiclo]ess of 20cm The neutron intensities produced by proton bombardments are reduced along the proton beam direction in the targets. The higher the energy of protons, the lower the reduction rate of the neutrons. The reduction rate of neutrons in U target is higher than in Pb target for the same energy of protons. (2) The radial intensities of neutrons decrease as the distance increases from the target central line. (3) The neutron yield in U target by proton bombardments is -55% higher than in Pb target. (4) The ratio of neutron yield by 14.4 GeV ot to 7.3 GeV d bombardment in Pb target is 1.74 + 0.20.

KEYWORDS Neutron emission; relativistic nuclear interactions; accelerator-driven subcritical nuclear reactor: CR-39 detectors.

INTRODUCTION The study of accelerator-driven subcritical nuclear reactors for electricity generation and disposal of long-life radioactive wastes has attracted attention of nuclear physicists and engineers in the world in recent years (Schapira, 1995). Enthusiasm in this field has also arisen in scientific community in China. One of the key components of this type of new installation is a particle accelerator, from which high energy projectiles impinge on a target in which high intensity of secondary neutrons are produced. These neutrons then initiate operation of a subcritical nuclear reactor. In the central part of the core of the reactor is the target. The optimization of the physical parameters of the target and the accelerated projectiles is of vital importance for the successful operation of the reactor. Our work in tiffs research aims at the determination of neutron parameters at the targets, that is, determination of neutron yield and spatial distribution within and around the target. This paper reports the results of our measurements on neutron parameters in the interactions of protons, deuterons and 0-particles with Pb and U targets.

EXPERIMENTS The experiments were carried out at the Synchrophasotron accelerator in the Joint Institute for Nuclear Research (JINR), Dubna, Russia. The experimental set-up is shown in Fig. 1. 1350-4487/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S 1350-4487(99)00195-X

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Fig. 1. Experimental set-up to study neutron yield and spatial distribution l~om relativistic nuclear interactions with CR-39 detectors. The beams of protons, deuterons or a -particles from the accelerator reached the energies of 1.5, 3.7 and 7.4 GeV for protons, 7.3 GeV for deuterons and 14.4 GeV for a~-particles and impinged on the target of Pb or U/Pb. The size of the Pb target was 8 cm in diameter and 20 cm in length (thickness). The size of the U target was 3.6 cm in diameter and 20 cm in length. Outside the U target there was a Pb cylinder, whose outer diameter was 8cm and the length was 20 cm. The inner diameter of the Pb cylinder was just against the outer surface of the U target. Outside the Pb or U/Pb target there was a cylindrical paraffin moderator having a thickness of 6 cm and length of 31 cm. Intensive secondary neutrons were produced in the Pb or U/Pb target on irradiation with particles. The neutrons were recorded with CR-39 detector strips of 20 cm in length and 1 c m in width, which were placed in the gap between the target Pb or U/Pb and the paraffin moderator as well as on the outer surface of the paraffin moderator and in distances of 15 cm and 19 cm from the target central line as shown in Fig, 1. The secondary neutrons from the targets produce recoil nuclei C, O and H and other charged reaction products in the CR-39 detectors whose composition is [C12H1807]n . The recoil nuclei and other products create tracks in the CR-39. The numbers of 3.5 GeV protons impinged on the Pb and U targets were 1.15 × 1013 and 1.20 × 1013, respectively. The numbers of 7.3 GeV d and 14.4 GeV o~-particles impinged on Pb targets were 1.14× 1013 and 5.83 × 1012, respectively. After irradiation with secondary neutrons in each run of the experiments, the CR-39 detectors were etched in 6.5N NaOH solution at 70°C for 45 minutes to develop the tracks in the detectors. The tracks at certain positions in each CR-39 detector were counted with track image analyser. The background tracks in the CR-39 detectors were deducted by using an unirradiated CR-39 detector. Detailed techniques of the measurements can be found in our previous paper (Guo et al., 1997). RESULTS Neutron distributions along targets

The area track densities ( p 7) which are ~ 3 × 107 c m -2 to ~ 7 X 107 cm 2 were measured in the CR-39 detectors which were placed in the gap between the Pb or U/Pb target and the paraffin moderator. The normalized results are shown in Fig. 2. The abscissa X = 0 and 20 are at the up stream end and down stream end of the targets to the projectile beam as indicated in Fig. 1. All the track densities are normalized to 1 at the X = 0. In this way, we can easily make comparision between results for different incident energies and different targets. If we suppose that the energy spectra of neutrons from different targets and different energies of protons at different detector positions have only small differences for creating tracks in the CR-39 detectors, then the track densities in the CR-39 detectors are proportional to the fluxes of the secondary

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neutrons at the places. This is approximately true for 44 and 18 GeV ~=C + Cu interactions. From Fig. 2 one can see that the secondary neutrons which create tracks in the CR-39 detectors are produced in the whole length of Pb and U/Pb targets by bombardments of 1.5, 3.7 and 7.4 GeV protons. The neutron intensities decrease along the proton beam direction in the targets. The higher the energy of protons, the lower the reduction rate of the neutron intensities in the targets. The reduction rate of neutron intensities in U/Pb target is higher than in Pb target for the same energy of protons. 10-s •

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Radial neutron distribation around targets

The areal track densities(PT) in the CR-39 detectors which were placed at different distances from the central line of the Pb or U/Pb targets were measured. The track yield (Y) from single proton, which is the ratio of PT to the number of the incident particles, are shown in Fig. 3. The track yields in the CR39 detectors reflect the neutron fluxes at the places of CR-39 strips. From Fig. 3 one can see that the neutron fluxes decrease as the distances increase. The fluxes of neutrons from U/Pb target is higher than that from Pb target. Neutron yield versus target nuclear mass.

From Fig. 3 one can also see that U/Pb as target can produce more neutrons than Pb target for 3.7 GeV proton beam. The ratio of the neutron yields of U/Pb to Pb is 1.55+0.29. It means that one of the advantages of U target is that it produces ~ 5 5 % more neutrons than Pb target by 3.7 GeV proton acclerator. Neutron yield versus incident particle mass

The areal track densities at different positions in the CR-39 detectors on the surfaces o f the Pb targets were measured for 14.4 GeV cr + Pb and 7.3 GeV d + Pb interactions. The results are shown in Fig. 4. The track yield ratios o f 14.4 GeV a + Pb to 7.3 GeV d + Pb interactions are shown in Fig. 5, whose

S. -L. Guo et at,./Radiation Measurements 31 (1999) 555-558

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average is 1.74___0.20, ingoring the difference of neutron spectra from the above interactions. The value of the ratio implies that less neutrons are produced by a -particle than by deuteron in Pb target if the consumption of energy is the same for the two projectiles.

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Fig. 4. Areal track densities in CR-39 detectors on Pb target in irradiations with 14.4 GeV ot- particle and 7.3 GeV deuteron.

Fig. 5. Track yield ratio of 14.4 GeV a + Pb to 7.3 GeV d + Pb interactions.

CONCLUSIONS From the above measurements we can draw the following conclusions: (1) Higher energy of proton beam allows thicker target to be used in the nuclear reactors. (2) The radial neutron flux distribution for Pb and U/Pb target extends to 15 - 20 cm while the neutron flux decreases about one order of magnitude compared to that on the beam central line. (3) U/Pb target can produce ~ 5 5 % more neutrons than Pb target. Therefore, U/Pb target is better than Pb target from the view-point of strong neutron source. (4) The neutron yield may not be proportional to the number of nucleons in the projectiles for the same incident energy per nucleon. In the case of incident a -particles and deuterons, a -particles consume more energy than deuteron to produce the same number of secondary neutrons. (5) CR-39 detectors of very thin and small strips can be inserted into narrow and small gaps in the target structures and surrounding substances in the core region of the new type of nuclear reactors to carry out the measurements of neutrons without interference with the neutron field in the reactor assembly.

Acknowledgements--The authors thank the operating crew of the synchrophasotron accelerator in Dubna, Russia for successful irradiation of our detectors with ion beams. This work was supported by the Science Foundation of Nuclear Industry of China. REFERENCES Schapira J.P. (1995) Possibility o f using accelerators to burn nuclear wastes. Nuclear physics-At

the Frontiers of Knowledge. Proceedings of the International Nuclear Physics conference-INPC " 95, Beijing, 1995. pp.753-775. Guo S.-L., Wang Y.-L., Tu C.-Q., Brandt R., Vater P., Sa B.H., Zheng Y.M., Liu Y.-L., Zheng W.-J., Hu X.-H., Kulakov B.A., Krivopustov M.I., Butsev V.S. and Bradnova V. (1997) Study o f neutron generation from relativistic heavy ion interactions. Radiat. Meas. 28, 273-276.