New detection systems at U-120M cyclotron

Intensive neutron beams with energies up to 33 MeV are produced using cyclotron driven broadspectrum and quasi-monoenergetic neutron generators at the NPI CAS. The neutron beams are well characterized with the TOF and PRT measurements. The segmented Fe-CH2 collimator is used to collimate the neutron beam and two detection systems are being developed for the studies of the reactions with neutrons. The first one is the detection system of the charged particles at different angles from the (neutron, charged particle) reactions in a vacuum chamber, the second is an array of four HPGe detectors for the detection of direct and delayed gamma photons produced in reactions of the studied material with neutrons. The advanced bunching system is also being developed at the U120M cyclotron. It will allow to direct a short (few ns long) pulse of accelerated particles to the neutron converter with the 1 μs delay between bunches. Together with 7 m long flight path this extension will open new possibilities in the cross section measurements.


Introduction
Isochronous cyclotron U-120M can accelerate light-ion beams (H + , H − , D + , D − , 3 He +2 , 4 He +2 ). It provides protons in the energy range of 17-35 MeV and currents up to 20 µA and deuterons in the energy range of 11-20 MeV. In the fast neutron laboratory, two different fast neutron cyclotron-driven sources were developed and one is in the development: 1) Variable-energy broad-spectrum neutron generator. Proton and deuteron beams are suitable to produce a broad-spectrum neutron fields with variable mean energy in the range from 4 to 12 MeV using standard p/d+Be (thick target) source reactions. The integral flux is up to 10 11 n/cm 2 /s.
2) Quasi-monoenergetic neutron generator. The standard 7 Li(p,n) reaction on thin lithium target (with carbon beam stopper) induced by 17-35 MeV proton beam is used for the production of quasi-monoenergetic neutron field ( Figure 1). The neutron flux density of the monoenergetic peak is up to 10 9 n/cm 2 /s and peak energy is up to 33 MeV [1].
3) Variable-energy broad-spectrum neutron generator with high neutron flux density. A thick Be target will be irradiated by 24 MeV protons from the new high current beam cyclotron. The integral flux will be up to 10 12 n/cm 2 /s.
The fast neutron sources are used mainly for neutron cross section measurements, radiation hardness tests, material damage production, neutron activation analysis, detector tests.
Moreover, charged particle cross section measurements are also performed. The equipment is developed for irradiation of samples by protons and deuterons. The * e-mail: novak@ujf.cas.cz The neutron beam can be shaped using the collimator system and subsequently stopped in the neutron beam dump. The neutron beam spectrum is estimated by the simulation + the measurement of the charged particle beam current. In the case of the Li target station, the measurement of the γ activation after irradiation can serve for the spectrum estimation. The last methods for the neutron beam spectrum estimation are TOF and PRT.

Time of flight
The neutron time of flight measurement in the defined source -detector distance is based on the neutron detection by an organic scintillator ( Figure 2).
The scintillator is coupled to the photomultiplier. Its signal is sampled by a 1 GHz digitizer together with the cyclotron radiofrequency for the start time determination.
EPJ Web of Conferences 239, 17020 (2020) https://doi.org/10.1051/epjconf/202023917020 ND2019 The neutron-γ discrimination is performed by the pulseshape analysis. The dynamic threshold method is used for the calculation of the neutron energy spectrum from the measured pulse spectrum. The calculation is based on the counting of pulses with the amplitude higher than the threshold from the (p,n) scattering to angles between 0 deg. and some fixed angle ( Figure 3).  This approach removes the necessity to know the whole response function of the scintillator, the results are based only on the number of hydrogen atoms in the scintillator and on the cross-section of elastic scattering of neutrons on hydrogen, which are both accurately known. Besides the direct neutron spectra measurements using TOF, the neutron energy can become one of acquisition parameters, in the multichannel coincidence set up. From the two-dimensional measurement of the scintillator response vs. time/energy, the scintillator response function at given time/energy can be extracted ( Figure 5).
Currently, a new TOF set up for the significantly longer base and the significantly longer time range is in construction thanks to the bunching system for the U-120M cyclotron development.

Proton recoil telescope
The proton recoil telescope (PRT) was used [2] to determine the neutron spectrum at short distances from the source where TOF method is not applicable. The PRT is based on the detection of protons from the (n,p) scattering in a thin hydrogenated target. By correcting for the energy dependent efficiency, the energy spectrum for the neutrons emitted by the source is determined from the detected proton spectrum, on the kinematics relations basis. Protons are detected at defined angle by the telescope of Si detectors, Figure 6. Protons are distinguished from other particles by Bethe formula E.∆E ∼ A.Z 2 . The multiplied response functions at different neutron energies are superposed to fit the measured proton-recoil spectrum. Multiplication factors pertaining to response functions create the resulting neutron spectrum. The response functions were calculated analytically using the full geometry arrangement of the target-radiator-detector setup. The MCNPX simulated neutron spectrum at the radiator position, the neutron spectrum measured by TOF data by Uwamino [3] extrapolated to the radiator position and the neutron spectrum deconvoluted from the experimental net-effect proton spectrum (full squares) [2] are compared in Figure 7. The measured spectra from PRT-data deconvolution agree with MCNPX simulation [4] within 10%. Protonrecoil-telescope method was found suitable for validation of MCNPX-simulated neutron flux at foil position. Moreover, the MCNPX simulation reproduces well the measured data from proton-recoil-telescope method.

Detection systems at U-120M cyclotron
For the reaction product measurement, one detection system is used and two systems are in construction.

Measurement of γ activation after irradiation
The targets from target stations, as well as samples irradiated by fast neutrons, are transported by pneumatic transport system to the HPGe detector station in tens of seconds after irradiation (Figure 8). The residual products are also measureable after irradiation with charged particles.

System for the direct detection of charged particles, which are produced in reaction with neutrons
The system is in construction. It consists of a large vacuum chamber with a target in the center (Figure 9). The electronically controlled manipulator allows to change up to 4 targets without opening the chamber. The dE-E telescopes for the charged particle detection are composed of thin and thick silicon detectors to distinguish the type of the charged particle based on Bethe's formula. The telescopes are placed on a rotating table at different angles, outside the reach of the neutron beam. The rotation is remotely controlled by electronics. The above-mentioned bunching system for the U-120 M cyclotron as well as the new TOF set up will make the detection system usable for direct cross-section measurements of (neutron, charged particle) reactions. By measuring angular distributions of the charged particles, a more detailed information on the reaction mechanism will be obtained. This will extend the laboratory know-how from measurements to nuclear reaction modeling.

System for the direct detection of γ photons, produced in reaction with neutrons
The system is in construction. Four positionable HPGe detectors ( Figure 10) are placed around the irradiated sample at different angles in respect to the beam direction. The samples are irradiated by fast neutron beam, whereas the γ detector position is chosen to be outside the reach of the direct neutrons. The system is usable also for the measurement with charged particles. So far, the γ measurement can start some tens of seconds after irradiation.
The new system will allow to measure the residuals with decay times down to ms. Moreover, the bunching system will make the online gamma spectrometer usable for the direct cross-section measurement of the (neutron, xγ) reactions. In combination with the neutron generators, the detection system will be most efficient in the neutron energy range of 10-33 MeV.

Conclusions
The variable-energy broad-spectrum neutron generator and the quasi-monoenergetic neutron generator are successfully used for neutron experiments. The accuracy of the produced fast neutron spectra determination is continually increasing by the use of improved measurement techniques. The experience of using the current generators is used during construction of a new broad-spectrum target station. The neutron TOF method has been successfully used. The largely improved TOF set up system is in construction. The system of the off-beam γ detection after neutron or charged particle irradiation is currently commonly used. Moreover, two new detection systems are in construction. First of them is the system for the direct detection of charged particles, which are produced in reaction with neutrons. The second one is the system for the direct detection of γ photons, which are produced in reaction with neutrons.