Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 1 Abstract NEUTRON SPECTROMETER BASED ON PROPORTIONAL RECOIL PROTONS COUNTER WITH POSSIBILITY OF γ-BACKGROUND DISCRIMINATION Y. I. Vinogradov, A.M. Shvetsov, O. P. Vikhlyantsev, A.V. Kuryakin Russian Federal Nuclear Center, All-Russian Research Institute of Experimental Physics, Sarov, Russia, The neutron spectrometer with proportional recoil protons counter was developed in VNIIEF and used for a number of years [1, 2]. The bottom range of measurements of the spectrometer in mixed n-γ fields has been limited by energy 0.5 MeV The carried out researches [3] have shown possibility of discrimination of impulses of the counter from neutrons and γ -quanta by their shape and expansions of a range of measurement of neutron spectra in the field of low energies to tens keV. Then it has been developed hardware and the spectrometer software, allowing to register amplitude of an impulse of the proportional counter and a maximum growth rate of an impulse on forward front and to write down them in ListMode (a mode of consecutive record of parametres of impulses). The program part includes a number of the service possibilities allowing to adjust registration equipment during measurements (in on-line mode), to estimate visually the quality of discrimination of impulses from neutrons and γ ‑ quanta and to receive hardware spectra. The complex of the programs is used for off-line mode analysis of the received data, which provides processing of the list of the received data with discrimination of impulses, caused by neutrons and γ ‑ quanta, construction of corresponding hardware spectra, calculation of response matrix of the proportional gas counter and reconstruction of spectra. Research of possibilities of the spectrometer with n-g discrimination, spent on 252 Cf source, has shown that the bottom range of measurements of neutron spectra decreases till keV. Reference list: 1.Kuvshinov M. I., Shvetsov A. M., Egorov V. P. et al. Measurement of neutron spectra for spherical critical assemblies of 235 U(90 %), 235 U(36 %), and 239 Pu(98 %) // Proceedings International Conference on the Physics of Reactors PHYSOR96, Mito, Ibaraki, Japan, September 16–20, 1996 / Japan Atomic Energy Research Institute Inc., Japan Vol. 2. P.338 ‑ Shvetsov A. M., Egorov V. P., Fomushkin E. F. et al. Measurements of leakage neutrons spectra for spherical assemblies of 235 U(90 %), 235 U(36 %), and 239 Pu(98 %) // Proceedings International Conference on Nuclear Data for Science and Technology, Trieste, Italy, May 18–23, 1997 / Italian Phis. Soc. Inc., Bologna, Italy Part 2. P.1359– A. M. Shvetsov, Yu. I. Vinogradov, O. P. Vikhlyantsev, A. V. Kuryakin. Investigation of the possibility of γ-background discrimination in a neutron spectrometer with a proportional counter of recoil protons. // Bulletin of the Russian Academy of Sciences: Physics, 2007, т.71, № 12, с

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 2 Neutron detector Designed and used in VNIIEF for number of years Proportional gas counter, ~14% efficiency, ΔE/E~3.7% Range: 0.5÷8 MeV without, or 0.05÷8 MeV with n-γ discrimination ~ 3 kV voltage, 3 bar methane (CH 4 + some 3 He uses for calibration) CAMAC based data acquisition, n spectra reconstruction software Construction of proportional counter: 1 – glass isolation; 2 - cathode, Ø 75 mm × 1 m; 3 – anode, Ø40 µm; 4 – guard pipes; 5 – spring for anode; 6 - flanges, 7 – fluoroplastic gaskets; 8 – details from kovar (Fe+Co+Ni alloy)

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 3 Principle of energy calibration Gas counter contains some 3 He for calibration Use 3 He(n,p)T reaction with peak E=0.764 MeV Thermal peak Channel, k N,Counts k0k0ΔkΔk

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 4 Why need n-γ discrimination γ influent to low range spectra (E < 0.5 MeV) γ influence is difficult to calculate theoretically Low energy range limited to 0.5 MeV due to n+γ Some kind of n-γ discrimination required for E < 0.5 MeV n+γ n (clean) neutron counter spectra E,MeV N,counts

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 5 Principle of n-γ discrimination Neutrons produce recoil protons γ–quanta produce electrons Hardware p, e pulses have different shapes (front timing) t front, (dE/dt) max /E max criteria can be used for n-γ separation E max (dE/dt) max time Hardware pulse from counter channel Typical hardware signal from gas detector t front Typical shape of pulse front registered with TDS-3000, 50 MHz

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 6 n-γ discrimination criteria Preliminary study show that: t front criteria is not good for n-γ (dE/dt) max /E max criteria is good γ-quantum neutrons N,counts

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 7 n-γ discrimination efficiency and range Fit [E,E+ Δ E] slice spectra with 2 log-normal distributions Find S γ, S n and cross (mutual overlap) ΔS areas Criteria for n-γ low range is Δ S/S n ·100% < 5% This range depends on N γ /N n [70÷80] keV slice spectra Low energy n-γ range (by 5% level) depends on N γ /N n ratio. N γ /N n = 2.5 N γ /N n = 0.7 N γ /N n = 0.3 N γ /N n = 0.2

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 8 Low energy n-γ range vs N γ /N n E n-γ(0.05),MeV Low energy n-γ range (by 5% level) depends on N γ /N n ratio.

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 9 n-γ discrimination tests n-γ discrimination tested with mixed n-γ and clear γ sources Low energy range without n-γ discrimination ~ 0.5 MeV Low energy range with n-γ discrimination ~ 60÷80 keV (depend on N γ /N n ) γ-quantum neutrons γ-quantum (dE/dt) max /E max, µs -1 E,MeV n+γ – source ( 252 Cf)γ – source ( СОСГИ-М )

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 10 n-γ discrimination hardware CAMAC based data acquisition system designed HV – high voltage unit (Canberra 3106D), PA – charge sensitive 10 ns preamplifier (POLON 1105A), SA – spectrometry 2 µs amplifier (Canberra 2025), DU – discriminator unit (Canberra 2035A), SD – logic shaper and delay (Canberra 2055), FA – fast amplifier (Canberra 2111), ST – stretcher (ORTEC 242), ADC – 13 bit analog to digital converter (ORTEC AD 413A), C1,C2 – counter (POLON 420A), OR – output register (POLON OR350), CC – CAMAC controller (PKK4, VNIIEF designed), PC – personal computer for data acquisition with CRW-DAQ control software by VNIIEF.

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 11 Online software (data acquisition) CRW-DAQ package designed in VNIIEF, Sarov Registered in Russian Federation registry  Data acquisition in List mode, 2-parameter events CAMAC based data acquisition, up to 60 kHz rate γ n (dE/dX) max E max

Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, 12 Offline software Preliminary spectrum processing –Energy calibration by 3 He(n,p)T reaction –n-γ discrimination, background subtraction Response matrix calculation (SUPREMAC) –Uses NDX, ENDF/B-VI nuclear constants database –Uses Monte-Carlo for response matrix calculation –Optimized for performance Neutron spectrum reconstruction (UFONIM) –Ill-posed problem ( Fredholm equations of 1 st kind ) –Uses iteration method by Tarasko –Randomization method to calculate stat. errors Designed in VNIIEF G - response matrix, U – n-spectrum to find, S – hardware spectrum.