Abstract

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, Email: vinogradov@expd.vniief.ru • 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 252Cf source, has shown that the bottom range of measurements of neutron spectra decreases till 60-80 keV. Reference list: • Kuvshinov M. I., Shvetsov A. M., Egorov V. P. et al. Measurement of neutron spectra for spherical critical assemblies of 235U(90 %), 235U(36 %), and 239Pu(98 %) // Proceedings International Conference on the Physics of Reactors PHYSOR96, Mito, Ibaraki, Japan, September 16–20, 1996 / Japan Atomic Energy Research Institute Inc., Japan. 1996. Vol. 2. P.338‑346. • Shvetsov A. M., Egorov V. P., Fomushkin E. F. et al. Measurements of leakage neutrons spectra for spherical assemblies of 235U(90 %), 235U(36 %), and 239Pu(98 %) // Proceedings International Conference on Nuclear Data for Science and Technology, Trieste, Italy, May 18–23, 1997 / Italian Phis. Soc. Inc., Bologna, Italy. 1997. Part 2. P.1359–1361. • 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.1748-1753. Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

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 (CH4 + some 3He 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, kouriakine@mail.ru

3. Principle of energy calibration Thermal peak N,Counts k0 Δk Channel, k • Gas counter contains some 3He for calibration • Use 3He(n,p)T reaction with peak E=0.764 MeV Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

4. Why need n-γdiscrimination n+γ N,counts neutron counter spectra n (clean) E,MeV • 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 Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

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

6. n-γ discrimination criteria Preliminary study show that: • tfront criteria is not good for n-γ • (dE/dt)max/Emax criteria is good neutrons γ-quantum N,counts Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

7. n-γ discrimination efficiency and range • Fit [E,E+ΔE] slice spectra with 2 log-normal distributions • Find Sγ, Sn and cross (mutual overlap) ΔS areas • Criteria for n-γ low range isΔS/Sn·100% < 5% • This range depends on Nγ/Nn Nγ/Nn= 2.5 [70÷80] keV slice spectra Nγ/Nn= 0.7 Nγ/Nn= 0.3 Nγ/Nn= 0.2 Low energy n-γrange (by 5% level) depends on Nγ/Nn ratio. Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

8. Low energy n-γ range vs Nγ/Nn En-γ(0.05),MeV Low energy n-γrange (by 5% level) depends on Nγ/Nn ratio. Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

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 Nγ/Nn) n+γ– source (252Cf) γ– source (СОСГИ-М ) neutrons γ-quantum γ-quantum E,MeV E,MeV (dE/dt)max/Emax, µs-1 Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

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, kouriakine@mail.ru

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 (dE/dX) max n γ E max Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru

12. Offline software • Preliminary spectrum processing • Energy calibration by 3He(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 1st 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. Alexey Kuryakin, RFNC-VNIIEF, Sarov, Russia, kouriakine@mail.ru