Measurements of neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets bombarded by 0.5 and 1.0 GeV protons

Measurements have been carried out on neutron yields and spatial distributions in U/Pb, Pb and Hg thick targets and the surrounding paraffin moderators bombarded by 0.533 and 1.0 GeV protons. CR-39 detectors were deployed on the surfaces of targets and moderators to record the neutrons produced in the targets. The measurements show that:

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Article Outline

1. Introduction

2. Experiments

3. Results

3.1. Neutron yield of Hg target

3.2. Neutron distributions along targets

3.3. Energy of secondary neutrons

4. Conclusions

Acknowledgements

References

1. Introduction

The study of accelerator-driven subcritical nuclear reactor has attracted the attention of nuclear physicists and engineers in the world. The main aim of the facility to be built is to generate electricity, dispose long-life radioactive wastes and find a way not to produce or produce much less radioactive wastes. One of the key components of this type of new facility is a particle accelerator, from which high energy particles, most possibly protons, 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 neutron parameters in the target and the surrounding moderating materials is very important for successful operation of the reactor. This work aims to determine neutron yield, spatial distribution and energy characteristics in the target region of the accelerator. This paper reports the new results obtained by proton bombardments on U/Pb, Pb and Hg targets.

2. Experiments

The experiments were carried out at the Synchrophasotron accelerator in the Joint Institute for Nuclear Research (JINR), Dubna, Russia. The beams of protons with 0.533 or 1.0 GeV from the accelerator impinge on a target U/Pb, Pb or Hg. The size of the Pb target was 8 cm in diameter and 20 cm in length (thickness). The U/Pb target was composed of a U column in the central part and a Pb sleeve (cylinder) on the column. The size of the U column was 3.6 cm in diameter and 20 cm in length. The outer diameter of the sleeve was 8 cm and the length was 20 cm. The inner diameter of the sleeve was just in contact with the outer surface of the U column. The size of Hg target was the same as the Pb target. Outside the U/Pb, Pb, or Hg target there was a cylindrical paraffin moderator with a thickness of 6 cm and length of 31 cm. Intensive secondary neutrons were produced in the U/Pb, Pb or Hg target on irradiation with protons. The neutrons were recorded with CR-39 detector strips of 20 cm in length and 1 cm in width, which were placed in the gap between the target U/Pb, Pb, or Hg and the paraffin moderator (here, we call inner CR-39 detector) as well as on the outer surface of the paraffin moderator (we call outer CR-39 detector). The length of the CR-39 strips was the same as the targets and the starting point (X=0) of the CR-39 detector was just the starting point of the targets at the upper stream end for the incident beams. The ending point (X=20 cm) of the CR-39 detectors was just the ending point of the targets at downstream for the beam. 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 [C₁₂H₁₈O₇]_n. The recoil nuclei and other products create tracks in the CR-39. The numbers of 0.533 GeV protons impinged on Pb and U/Pb targets were 1.52×10¹³ and 1.502×10¹³, respectively. The numbers of 1.0 GeV protons impinged on Pb, U/Pb and Hg targets were 1.91×10¹³, 2.03×10¹³ and 2.236×10¹³, respectively. After irradiation with secondary neutrons in each run of the experiments, the CR-39 detectors were etched in 6.5 N NaOH solution at 70°C for 45 min to develop the tracks in the detectors. The tracks at certain positions (X=0,5,10,15 and 20 cm) in each CR-39 detector were counted with track image analyzer.

3. Results

3.1. Neutron yield of Hg target

The areal track densities (ρ_T) were measured at different positions (X) along the CR-39 detectors on the surfaces of the U/Pb and Hg targets and the moderator for 1.0 GeV p bombardments. Track yield (Y) from a single proton is defined to be the ratio of ρ_T to the number of the incident protons. The track yields in the outer CR-39 detectors are shown in Fig. 1. The track yield ratio R of U/Pb to Hg targets from the above outer CR-39 detectors are shown in Fig. 2. The average value of the ratios at different places (X) is 2.00±0.11. Similar measurements with inner CR-39 detectors on the surfaces of U/Pb and Hg targets were also carried out. The track yield ratio from the inner CR-39 is 2.11±0.33, which is in agreement with the value of the outer CR-39. The final result of the track yield ratio of U/Pb to Hg target is 2.01±0.10. If we suppose that the energy spectra of neutrons from different targets and different energies of protons at different detector positions are similar and their differences would not produce a great influence on track formation in CR-39 detectors, then the track densities in the CR-39 detectors are proportional to the fluences of the secondary neutrons at the place. The track yield is approximately proportional to the neutron yield in the corresponding place. This supposition is approximately true for 44 and 18 GeV ¹²C+Cu interactions (Bisplinghoff et al., 1995) and further evidence is shown in this paper in the following paragraph 3.3. From Fig. 1 and Fig. 2 we can see that the neutron yield in Hg target is less than in U/Pb target. Only about half of secondary neutrons are produced in Hg target compared with U/Pb target. Similar measurements were also performed with outer CR-39 detectors for Pb and Hg targets. The track yield ratio of Pb to Hg target is 1.76±0.33 at 1.0 GeV of protons. It means that Hg target produces lesser neutrons than even a Pb target. Hg target is less preferable in order to be used as a target in the reactor from the point of view to get more secondary neutrons.