The neutron production in thick targets irradiated with 1 GeV protons was studied experimentally, and results are well understood with model calculations, including MCNPX 2.7a. However, one observes very large neutron production rates in the interaction of 44 GeV 12C onto thick Cu-, Pb-, and U-targets beyond calculated rates. The experimental spallation product yield curve in a 20 cm thick Cu target irradiated with 72 GeV 40Ar also cannot be reproduced by several model codes, including MCNPX 2.7a. This may be due to secondary fragments produced in high energy (
Recent investigations on the interaction of relativistic ions (called primaries in this paper) above a total energy Spallation mass-yield curves in any thin target are completely understood with conventional reaction models like “limiting fragmentation” and “factorisation” [ The neutron emission from thick targets irradiated with ions having
Experimental results published for thick targets irradiated with relativistic ions are scarce and sometimes contradictory. In the experiment on “neutron yields from 1 GeV/nucleon 238U ion beams on Fe target” by Yordanov et al. [ workers in the laboratories, materials close to the beam line and target areas, and—not the least—the surrounding environment.
The aim of this paper is to concentrate on the experimentally known facts which may serve as benchmarks for any radiation protection model. Two major topics will be considered. The neutron emission from thick targets irradiated with ions in the energy range of 1 GeV ≤ The experimental spallation mass-yields produced in a 20 cm thick Cu target in the irradiation with 72 GeV 40Ar at the LBNL in Berkeley (USA). Calculations with modern code MCNPX 2.7a [
A key question in all investigations is the determination of the total number of neutrons produced in a thick target by a single ion with a well-defined primary energy. A target is considered as being thick when a large fraction of secondary particles induce additional interactions within this target.
In an early experiment, a very consistent measurement of the numbers of neutrons emitted from a thick Pb target (
The direct measurement of neutron production in THICK Pb-targets irradiated at the Synchrophasotron [
The total number of neutrons
Ion | Mass | Number of neutrons | Number of neutrons | |
H | 1 | |||
H | 2 | |||
4 | ||||
C | 12 |
The total number of neutrons
Ion | Mass | Number of neutrons | Number of neutrons | |
H | 1 | 23.5 | 73.4 | 3.12 |
H | 2 | 48.0 | 118.9 | 2.48 |
4 | 77.5 | 201.6 | 2.60 | |
C | 12 | 134.8 | 494.3 |
A recent publication by Yurevich et al. [
A recent publication [
The spallation sources used in JINR in Dubna, Russia: (a) “Gamma-2” target, (b) “Energy plus Transmutation” (
The target system “Gamma-2” (for details see Figure
The GAMMA-2 target (Since 2007, GAMMA-2 has been an IAEA benchmark target for transmutation studies, see text and [
The Pb core in the “Energy plus Transmutation” (
The neutron dose measurements were carried out with solid state nuclear track detectors (SSNTD’s) by Fragopoulou et al. [ low-energy neutrons with epithermal neutrons with 1 eV < intermediate-fast neutrons with 0.3 MeV <
The actual neutron ambient dose equivalent in units of Sievert (Sv) was calculated using experimental conversion factors. The results are shown in Table
Experimental neutron ambient dose equivalent from the irradiation of a Pb target in “Gamma-2” and “Energy plus Transmutation” (
Target and position of SSNTD | Thermal-epithermal neutron dose | Intermediate-fast neutron dose | ||
Experiment | Calculation | Experiment | calculation | |
Gamma-2, (at 1 GeV) close to target | — | — | ||
Gamma-2, behind 1 m concrete | 0.375 mSv | <0.05 mSv | 0.020 mSv | |
— | — | |||
<0.0015 mSv | 0.0013 mSv | <0.05 mSv | 0.020 mSv |
The agreement between experiment and calculation is fine within uncertainties for these two target systems and in this energy range. The calculation for the “Gamma-2” target gave 15 neutrons per 1.0 GeV proton which is smaller than the corresponding number in Table
A further detailed analysis of the fission rate inside the massive uranium blanket for the (
The intermediate-fast neutron dose around the (
The “Gamma-2” target produced a considerable thermal neutron dose behind the concrete wall. The irradiation lasted 11 hours with a total fluence of 1013 protons of 1 GeV on target, corresponding to an average of
Further experiments measuring the neutron dose equivalent under well-defined conditions during the irradiation with heavy ions onto thick targets are necessary with relativistic ions like 2H, 4He, and 12C. Such irradiations had been carried out a decade ago using the “Gamma-2” target at the Synchrophasotron, however, without any quantitative measurements of the neutron ambient neutron dose equivalent outside the concrete shielding. In this paper, a “postfactum” estimate of the corresponding neutron ambient dose equivalent outside the concrete shielding is presented.
The comparison of the neutron emission from the “Gamma-2” target irradiated with heavy ions at the Synchrophasotron accelerator and with protons at the Nuclotron accelerator will allow the inter-calibration of experimental results from both accelerators. Figure
The “Gamma-2” target allows two-parameter experiments: In the irradiation with relativistic ions onto the metallic core, all kinds of spallation products are produced inside the metallic disks. These spallation products can be determined with standard radiochemical techniques after the irradiation. Spallation neutrons are simultaneously produced, which enter the paraffin. These neutrons are partially moderated, with many neutrons even reaching the thermal regime. All neutrons induce (
Proton energy + Target | Experiment: | Calculation: neutrons/proton ( | ||
MCNPX 2.7a | LAHET [ | DCM [ | ||
1.0 GeV + Pb | 15.0 | 15.3 | 17.4 | |
2.0 GeV + Pb | 26.4 | — | — | |
3.7 GeV + Pb | 43.0 | 38.7 | 43.9 | |
Ratio 2.0 GeV/1.0 GeV | 1.76 | — | — | |
Ratio 3.7 GeV/1.0 GeV | 2.87 | 2.52 | 2.52 |
Neutron fluences measured as
Target system | B(140La) | ||
Experiment | Model: DCM/CEM | ||
3.0 GeV 2H + Pb | 1.69 | ||
7.4 GeV 2H + Pb | |||
6 GeV 4He + Pb | 1.90 | ||
14.7 GeV 4He + Pb | |||
18 GeV 12C + Pb | 1.99 | ||
44 GeV 12C + Pb |
Table The recent experimental values of The
During the last decade of the operation of the Synchrophasotron until about the year of 2000, extended irradiations of “Gamma-2” targets with 2H-, 4He-, and 12C-beams in the range of total energies from 3 GeV up to 44 GeV were carried out [
Breeding rates
Breeding rates
The resulting distributions for 2H-, 4He-, and 18 GeV 12C-irradiations are surprisingly similar, irrespective of the projectile element and energy. In the 44 GeV 12C irradiation, however, one observes a drastic increase in the production of 140La and 239Np.
Similar results are observed in experiments using a Cu-core in “Gamma-2.” The results are shown in Figure
Neutron induced interactions were studied by this collaboration over a wide energy range: Wang et al. [
A summary of ( (
The large
All evidences demonstrate that one needs additional experiments to study the neutron production in thick targets using heavy ions at high energies. In these future experiments, one may be confronted with nontrivial radiation protection problems. 1 GeV protons on the “Gamma-2” Pb-target lead to a large neutron dose behind the concrete shielding; therefore, one might expect much more severe radiation protection problems in irradiations with 44 GeV 12C beams.
If assuming that one uses the same experimental setup as shown in Figure the “neutron ambient dose” behind the concrete shielding increases linearly with
then one obtains estimated neutron ambient doses as given in Table
Estimated neutron ambient doses (for
Beam + target | Neutron dose in | Calculated neutrons ( | ||
1 GeV | ||||
44 GeV 12C + Pb | 2070 (estimated) | |||
44 GeV 12C + Cu | 840 (estimated) | |||
44 GeV 12C+ U/Pb | 500+ | 3890 (estimated) | 726** | 5.5 |
72 GeV 40Ar + Cu | Not measured | Very large | 250** | — |
+
*Based on DCM/CEM, LAHET ([
**Based on MCNPX 2.7a (this work).
***
The measured ambient neutron dose (
Thick-target experiments at Lawrence Berkeley National Laboratory (LBNL) started around 1980 with the irradiation of two Cu disks in contact, with each disk having a thickness of 1 cm and a diameter of 8 cm. The aim of such studies was the investigation of possible differences in nuclear interactions of relativistic secondary fragments in comparison with the relativistic primary ions as reviewed in [
The original “two Cu disk experiments” in Berkeley showing the first evidence for
The original “two Cu disk experiments” in Berkeley showing the first evidence for
The ratio of nuclear interactions induced by primaries to those induced by secondaries is larger in the first Cu disk than in the second Cu disk. The study of experimental yield ratios of individual spallation products
The ratio
Figure One observes for product masses above
The comparison of experimental results (see Figure
Comparison of the experimental and theoretical
Some details of this comparison shall be emphasized For mass The experimental production rate of the isotope 24Na (as well as 22Na and 28Mg) is Above
The results for
It may be useful to carry out further experiments to learn more about these phenomena from additional experimental approaches: One should measure Additionally, one should carry out neutron counting experiments using thick Cu targets having thicknesses 2 cm ≤
The experimental
Comparison of the experimental
The last issue of this paper is concerned with the radiochemical aspects in the study of a 20 cm thick Cu target (i.e., 20 Cu disks of 1 cm thickness in contact) irradiated with 72 GeV 40Ar. In this irradiation at LBNL, a very strong neutron dose was registered even outside the experimental area of the Bevalac accelerator. However, quantitative neutron data about this event have never been released.
The 20 cm thick Cu target was designed as a two-parameter experiment: The determination of neutron production was Nuclear reactions inside the Cu disks were actually studied, that is, spallation product yields in several 1 cm Cu disks were determined, yielding information about nuclear interactions of relativistic ions inside the entire thick Cu target.
Both sets of information (neutron production rates
Some detailed experimental yield ratios
The following Figures 24Na is produced more abundantly than calculated in every disk, just as in the “2 cm Cu disk” experiment. 57Ni is produced a little less abundantly in this experiment as compared with calculation. This deficiency is observed in all Cu disks in the 20 cm Cu target.
Comparison of the experimental
Comparison of the experimental
Repeating the argumentation presented for Figure
Another question to be asked is: what amount of “enhanced neutron production” can one expect in the reaction (20 cm Cu + 72 GeV 40Ar) as compared with the well-investigated reaction (20 cm Cu + 44 GeV 12C)? There exists only one indirect measure for the “enhanced nuclear destruction” ability of secondary fragments in thick copper targets, which is the measurement of 1.5 GeV 1H onto two Cu disks yield 44 GeV 12C onto two Cu disks yield 72 GeV 40Ar onto two Cu disks yield
The ratio
The increase in
Recent calculations using the MCNPX code indicate that the simulated mass yields are model dependent. Conclusions presented here on the 72 GeV Ar + Cu results (Figures
Neutron ambient dose equivalents have been measured in the irradiation of a 20 cm thick Pb target with 1 GeV protons close to the target and at larger distances in the experimental hall. Experiments and calculations based on DCM/CEM code agree within uncertainties. Based on experiments where breeding rates
Detailed calculations carried out with the MCNPX 2.7a code have shown that radiochemical spallation yield distributions in thick Cu targets irradiated with 72 GeV 40Ar cannot be reproduced by calculations. Secondary fragments destroy Cu nuclei stronger than primary 72 GeV 40Ar ions, thus confirming observations reported in [ to design the radiation protection shielding for heavy ion accelerators producing high energy heavy ions with large intensities, and to learn more about the physical reason connected with these “unresolved problems.”
The authors wish to thank the operators of the Nuclotron accelerator in the LHEP for their continuous efforts to provide first class and stable beams. Considerable part of the work was made in the framework of an International Collaboration (GAMMA-2, Energy plus Transmutation) experimenting at the Nuclotron accelerator in JINR (Dubna, Russia).