Average fast neutron flux in three energy ranges in the Quinta assembly irradiated by two types of beams

This work was performed within the international project "Energy plus Transmutation of Radioactive Wastes" (E&T RAW) for investigations of energy production and transmutation of radioactive waste of the nuclear power industry. 89Y (Yttrium 89) samples were located in the Quinta assembly in order to measure an average high neutron flux density in three different energy ranges using deuteron and proton beams from Dubna accelerators. Our analysis showed that the neutron density flux for the neutron energy range 20.8 32.7 MeV is higher than for the neutron energy range 11.5 20.8 MeV both for protons with an energy of 0.66 GeV and deuterons with an energy of 2 GeV, while for deuteron beams of 4 and 6 GeV we did not observe this.


Introduction -Motivation
Up to now, we have been concentrated on measuring the neutron flux distribution in the deeply subcritical Quinta assembly versus the axis and radius of the assembly for the neutron energy above 10 MeV applying proton and deuteron beams of an energy from 1 GeV to 8 GeV extracted from the NUCLOTRON accelerator.We have applied a proton beam of an energy of 0.66 GeV recently extracted from the PHASOTRON accelerator.Nuclear data handling at the experiment session have turned our attention to the neutron flux density measured on five foil plates in terms of three different neutron energies: 11.5 -20.8 MeV, 20.8 -32.7 MeV and 32.7 -100 MeV.The neutron density flux for the neutron energy range 20.8 -32.7 MeV being higher than for the range 11. 5 -20.8 MeV has appeared an unexpected feature of the measurement.Repeating in the same way data handling for the other planes as in collecting them in one figure it can be noticed that the same effect is observed for planes 2 -5 but except plane 1, which describes the average neutron flux density in the first section where process of spallation begins.This unexpected feature of the measurement is presented in figure 1.
Finding this unexpected feature of an average neutron flux in the experiment with the proton beam of an energy of 0.66 GeV motivated us to make an overview of our experiments performed earlier for the deuteron beam of an energy of 1 -8 GeV.

Principles of the measurement
To study the spectrum of high-energy neutrons, we have used threshold 89 Y detectors which have the following advantages: one stable isotope, several threshold reaction channels, several resulting isotopes with a half life time long enough -longer than 12 hours.The first threshold energy of the reaction (n, 2n) giving 88 Y has been for an energy of neutrons equal to 11.5 MeV.The next possible threshold energies 20.8, 32.7, 42.1 and 54. 4 MeV have been for the reactions (n, 3n) 87 Y, (n, 4n) 86 Y, (n, 5n) 85 Y and (n, 6n) 84 Y respectively [1].The neutron field has been determined with a certain number of 89 Y foils placed in specified positions (given by the radial and axial distance) inside the experimental facility (figure 2).After neutron irradiation, the gamma activity of 89 Y foils has been measured with an HPGe spectrometer.Taking into account necessary corrections [1] and spectra analysis by the DEIMOS program [2], we have determined isotope production per one gram of a sample and per one beam deuteron/proton at specified positions of the Quinta assembly.
Having isotope production determined inside of the experimental facility for the three isotopes 88 Y, 87 Y and 86 Y, we have been able to evaluate three average high-energy neutron fluxes in each 89 Y foil location for certain energy ranges [3]

Overview of experiments for the deuteron beam with an energy of 2 -6 GeV
Repeating data handling in the same way as in figure 1 for the deuteron beam with an energy of 2 GeV shows (figure 3) that the same effect is observed for planes 2, 4 and 5 and is not clear for planes 1 and 3 which describes the average neutron flux density in the first and third sections.with an energy of 4 GeV behaves as expected (figure 4).In this case, the explanation like for the experimental data in figure 1

Discussion
Such an unexpected behavior of the average neutron flux density versus three different energy ranges can be explained to some extent by the dependence of the micro cross section in the considered neutron energy range shown in figure 1 and figure 3. The threshold neutron energies of yttrium (n,xn) reactions determine the considered energy ranges.
In the first energy range 11.5-20.8MeV, the area under the cross section curve of 89 Y(n,2n) 88 Y is smaller than in the second energy range of the same reaction.Moreover there is also an area under the curve of 89 Y(n,3n) 87 Y (figure 6).Since the measured amount of neutrons is proportional to the area under the cross section curves, the results in figure 1 and figure 3 are understandable.The above explanation for the experimental data presented in figure 4 and figure 5 is not sufficient.Calculations using the Monte Carlo method [1] show that in the energy range (20.8-32MeV) the neutron fluence is higher than in the range 11.5-20.8MeV for the energy of the deuteron beam equal to 1 GeV (figure 7).Follow the same article [7].Calculations using the Monte Carlo method show that in the three energy ranges (11.5-20.8MeV, 20.8-32 MeV and 32-100 MeV) the neutron fluence is continuously decreasing with the neutron energy for the energy of the deuteron beam equal to 4 GeV (figure 8).This explains the behavior of the neutron flux density presented in figure 4 and figure 5.

Conclusions
This brief analysis carried out above let us to infer that the neutron flux density in the range above 10 MeV should be our concern in the future experimental research.
Theoretical analysis carried out by the authors [7] using MC method explains to some extent our experimental observation.
The tendencies presented above are vague and need to be checked in next experiments because we do not see clearly this effect in all our experimental results.
Though the measured amount of neutrons proportional to the area under the cross section curves in the energy range 11.5 -20.8 MeV explains the results for proton or deuteron beams in the range from 0.66 to 2 GeV, it does not explain the behavior of the average neutron flux for an energy of different beams higher than 2 GeV.
The measurement of the high neutron flux density and explanation of the results still need to be developed by an additional experiments and calculation.

Figure 2 .
Figure 2. Scheme of the Quinta assembly.On the left, there is a view of the uranium target with supporting structures and plastics used for sample placing (foils plates), on the right, there is a view of the lead shielding enfolding the target.

Figure 7 .
Figure 7. Neutron spectrum as a function of energy (black solid line) for the deuteron beam with an energy of 1 GeV [7].

Figure 8 .
Figure 8. Neutron spectrum as a function of energy (black solid line) for the deuteron beam with an energy of 4 GeV [7].