Angular and energy distributions of the prompt fission neutrons from thermal neutron-induced fission of 239 Pu

Transcription

1 Angular and energy distributions of the prompt fission neutrons from thermal neutron-induced fission of 239 Pu Vorobyev AS, Shcherbakov OA, Gagarski AM, Val ski GV, Petrov GA National Research Center Kurchatov Institute BP Konstantinov Petersburg Nuclear Physics Institute , Gatchina, Leningrad district, Russia

2 Motivation The main purpose is experimental investigation of the prompt neutron emission mechanism by the multi-parameter coincidence measurement of angular and energy distributions of neutrons and fission fragments It is known from previous experimental works: The main source of prompt fission neutrons (PFNs) is accelerated fission fragments The angular anisotropy of neutron emission in the center-ofmass system of fission fragment is not established The contribution of neutrons due to other emission mechanism ( additional neutrons) to the total yield of PFNs ranges from 1% to 20% The yield of additional neutrons can be obtained only by comparison of the experimental fragment-neutron angular correlations with model calculations based on the assumption of neutron emission from accelerated fragments 2

3 Motivation The experimental data needed for such investigation, ideally, should be obtained using the same set-up and data processing for many fissioning nuclei Therefore, using the same experimental set-up and data processing a series of experiments has been carried out to measure the angular and energy distributions of prompt neutrons from spontaneous fission of 252 Cf and thermal neutron-induced fission of 233,235 U and 239 Pu In this presentation the results of the measurements for 239 Pu carried out at the PNPI research reactor WWR-M are presented and discussed 3

4 Motivation Experimental works for neutron-induced fission of 239 Pu where the yield of additional neutron was deduced Authors, References Exp Set-up 239 Pu(n,f) Investigation of (n,f)-angular correlation < SCN >/ < tot > <E SCN >, MeV 1+AP 2 (cos cm ), Anisotropy, A JS Fraser et al (1965) Two plastic scint for FFs spectroscopy (TOF with base 125cm and 99cm); Four neutron detectors (plastic scint) were used, TOF(106cm) The neutron spectra measurements were done simultaneously at 10 0, 25 0, 45 0 and 80 0 relative to FFs direction YuS Zamyatnin et al (1979) IC with collimator used for FFs spectroscopy; One neutron detector (plastic scint) was placed interchangeably at 0 0 and 90 0 relative to FFs direction, TOF (40cm) Investigation of (n,n)-angular correlation VE Sokolov et al (PNPI, 2010) Two stilbene neutron detectors, n/ pulse shape discrimination IS Guseva et al (PNPI, 2016) Two stilbene neutron detectors, n/ pulse shape discrimination 30% 2 0 all results are consistent with A = % Not investigated 14 3 % 4 15 % 18 MeV weak Sensitive 4

5 Schematic view of the experimental set-up 235 U Stop MWPDs Start MWPD Reaction Chamber: Actinides target (Ø15mm) onto 70 μg/сm 2 Ti backing; start MWPD (68 x 92 mm 2 ) located within 7 mm range from the target; stop MWPD (72 x 38 mm 2 ) located at a distance of 140 mm from the chamber axis Neutron detectors: stilbene crystals (50 x 50 mm 2 and 40 x 60 mm 2 mounted on the Hamamatsu - R6091) neutron registration threshold kev; double-discrimination method pulse shape and time-of-flight criteria time-of-flight distance from target ~ 50 cm timing resolution ns

6 Electronic set-up Delay TFA 2 Delay ND 1 TFA 2 CFD Delay TFA 2 Delay ND 2 TFA 2 CFD STOP ND1 STOP ND2 AND AND PH 1 PS 1 PH 2 PS 2 PC QDC QDC Stop MWPDs Arc N1, T11 TFA 2 CFD Stop MWPDs Arc N1, T12 TFA 2 CFD Stop MWPDs Arc N2, T21 Stop MWPDs Arc N2, T22 Start MWPD TFA 2 CFD TFA 2 CFD TFA 2 CFD GATE STOP1 FF1 STOP2 FF1 STOP1 FF2 STOP2 FF2 AND AND PF 1 PF 2 PF 3 QDC Input Timer Counter Clear TDC START C A M A C ND1, ND2 1-st and 2-nd neutron detectors; PA - Preamplifier; TFA - Timing Filter Amplifier; CFD - Constant Fraction Discriminator; TDC - Single-Event Time Digital Converter; QDC - Charge Digital Converter; Input - Coincidence Register; Linux embedded PC 6

7 Raw experimental data: count rate of fission fragments from different stop fragment detectors Number of recorded fission events as a function of the difference in arrival time of the stop MWPC pulses at both ends of the delay line (digits from 1 to 8 were used to mark the peaks corresponding to the stop MWPCs) 7

8 Raw experimental data: Specific loss spectra for fission fragments of 235 U(n, f) in a cathode - anode gas gap of the n-th MWPC (a) start MWPC (PF1) (b) stop MWPCs (PF3) of Arc N2 Numbers 2, 3 and 4 correspond to stop MWPCs N2, N3 and N4 (see fig 1), respectively 8

9 Raw experimental data: The time-of-flight spectra of fission fragments detected by MWPC N3 of Arc N2 The right part of the figure shows the spectrum for 235 U(n, f); The left part shows the spectrum for 252 Cf(sf) Bs indicates the separation boundary between the light- (LF) and heavy-fragment (HF) groups 9

10 Partial Integral [arb units] Raw experimental data: neutron - - quanta separation method 300 Neutrons quanta Total Integral [arb units] Both integrals were measured for pulse of neutron detector in a time window of 300 nsec, while the partial integral window with a delay ~30 nsec 10

11 Applied correction for: Time uncertainties in TOF measurement: Pulse-height dependent time walk in neutron and fission fragment channels Different fission fragments TOF to start MWPD Neutron detector background : a double-discrimination suppression factor ~200) method (TOF and pulse shape with gamma true coincidence due to the neutron and fission fragment belonging to different fission event registered was subtracted the linear approximation of the remaining part of background was used Fission fragment detector efficiency Complementary fission fragment contribution Angular and neutron energy resolution (timing resolution : ns) Bin-width correction Analysis of the data Normalization correction arising from the fact that experimental angular histograms were used in the measurements instead of continuous distributions Neutron detector efficiency determined as the ratio of the measured total neutron spectrum of 252 Cf(sf) to the reference standard spectrum of Mannhart 11

12 Analysis of the data Calculation procedure Firstly, using energy spectra in the laboratory system measured for 89 0, and angles relative to the fission direction, the neutron energy spectra for light and heavy fragments are obtained in the center-of-mass system; Used equations (two fragments approximation): The neutron spectra in the laboratory system: n lab (E n, lab ) = (E n / E cm ) 1/2 φ(e cm, cm ) n cm (E cm ) The neutron energy and spectra in the cms of the fission fragments: E cm = E n + E f - 2 cos( lab ) (E n E f ) 1/2 φ(e cm, cm ) = 1 + A 2 (E cm ) (3 cos 2 ( cm ) - 1) / 2, where anisotropy A 2 (E cm ) = E cm 2b/(3+b) and b = ( φ(1,0 0 ) / φ(1,90 0 ) -1) 0 Two fragments approximation gives a very good result, since, as it was shown for 252 Cf(sf) **, it has a minor influence on the total neutron energy spectrum **DG Madland, IAEA Report INDC(NDS) 251, Vienna, 1991, p

13 13 Calculation procedure Further, the spectra obtained in the center-of-mass system are used for calculation of neutron angular and energy distributions in the laboratory system exp 2 ) (1 exp 4 ),,, ( ) ( T E T E T E T E E T T Fit E n m c n m c n m c n m c n m c n m c n m c Analysis of the data

14 Analysis of the data Advantages of the calculation The prompt fission neutron distributions can be calculated without any assumption about the features of neutron emission mechanism such as redistribution of neutrons between light and heavy fission fragments, a shape of neutron spectra and their dependence on fission fragments characteristics This calculation is performed using only the experimental data obtained from measurements of angular and energy distributions of the prompt fission neutrons A good agreement is achieved between experimental data and description done in a framework of the model assuming that all prompt neutrons are emitted from fully accelerated fragments This gives the possibility to determine the main characteristics of the prompt neutrons and to use them in any other model calculation and a verification of existed and codes widely used for calculation of properties of neutron emission 14

15 Analysis of the data Reliability of the calculation Assuming that all prompt neutrons are emitted from fully accelerated fragments, neutron distributions have been calculated by means of different methods and using the same input parameters PbP (Point by Point) - deterministic method 235 U developed by the University of Bucharest and JRC-IRRM team and is an extended version of LAM (Los-Alamos or Madland-Nix model) FREYA (Fission Reaction Event Yield Algorithm) Monte-Carlo fission model developed through a collaboration between LLNL and LBNL (USA) CGMF Monte-Carlo code developed at LANL (USA) FIFRELIN (FIssion FRagment Evaporation Leading to an Investigation of Nuclear data) - Monte-Carlo code developed at CEA-Cadarache (France) with the aim of calculating the main fission observables Comparison of the total prompt fission neutrons spectrum for thermal-neutron induced fission demonstrates that the existing calculation methods used in practice do not provide necessary accuracy The method realized in this work gives the result in agreement with experimental data 15

16 Analysis of the data Reliability of the calculation The presented spectra were calculated according to scheme described above The calculation performed by Brosa et al and in the work of Batenkov et al were performed using a complete set of fission fragments for mass and energies unlike to this work where the approximation of two fragments with average mass and kinetic energies was used The results obtained by the different groups using different experimental data and data processing but the same calculation methods agree with each other The calculation method realized in this work (two fragments approximation) gives an accuracy not worse than those used in practice 16

17 n() [neutron/fission/sr] n() [neutron/fission/sr] n() [neutron/fission/sr] Results yield of prompt neutrons as a function of angle relative to the direction of light fission fragment in the lab system Bowman etal data (1962) Our data (2010) calculated with A 2 = K Skarsvag and K Bergheim (1963) Our data (2010) calculated with A 2 = Our data (2010) calculated with A 2 = Cf U U n() exp / n() calc [degree] [degree] [degree] 12 calculated with A 2 = calculated with A 2 = calculated with A 2 = 004 calculated with A 2 = 0 calculated with A 2 = 0 calculated with A 2 = 0 error corridor due to uncertainty of error corridor due to uncertainty of error corridor due to uncertainty of neutron cms spectra neutron cms spectra neutron cms spectra 10 n() exp / n() calc 10 n() exp / n() calc Cf U U [degree] [degree] [degree] Including of anisotropy with A 2 = 004 into the calculation improves agreement with obtained experimental data At that, there is some surplus of measured yield over calculated at angles near

18 Results main parameters of prompt neutrons as a function of angle relative to the direction of light fission fragment in the lab system Anisotropy of the prompt neutron emission in the centre-of-mass system of fission fragment (A 2 = 004) was taken into account in the calculation There is a general agreement between experimental data and calculation performed in the assumption that prompt neutrons are emitted from accelerated fragments 18

19 Results Prompt fission neutron spectra of 239 Pu for fixed angles The prompt fission neutrons spectra obtained from measurement and those calculated are consistent both in the shape and the average multiplicity 19

20 Results Prompt fission neutron spectrum of 239 Pu for angle 90 o The systematic difference of calculated yield of prompt fission neutrons spectrum from measured one for angle 90 o relative to fission fragment direction is visible This difference may be interpreted as a manifestation of additional neutrons and their yield can be estimated 20

21 Results Total prompt fission neutrons spectrum of 239 Pu There is an agreement between different experimental data within experimental uncertainties The systematic difference of calculated spectrum from measured one is visible in the neutron energy range lower than 06 MeV The observed difference may be interpreted as a manifestation of additional neutrons and, therefore, the average energy of these neutrons and their yield can be estimated GMA generalized least square fit of prompt fission neutron spectra measured by different experimental groups (non-model evaluation) taken from R Capote et al, Nuclear Data Sheet 131 (2016) 1 21

22 Results spectrum of additional neutrons for 239 Pu Circles the difference spectrum obtained using spectra measured at 72 o, 90 o and 108 o relative to the light fission fragment and the corresponding ones calculated in the assumption that all prompt neutrons are emitted from accelerated fragments Blue line fit of experimental data marked with circles by following equation: p 0 E E) 4 T S ( 2 S exp Red line the difference between total PFNS obtained by experiment (estimated data and it s errors) and calculated assuming that all prompt neutrons are evaporated from accelerated fragments To compare with partial difference spectrum presented by circle, a difference spectrum (red line) is reduced to the unit of solid angle (divided by 4) There is a good agreement within experimental uncertainties between spectra of additional neutrons obtained by two different ways Probably, the angular distribution of these neutrons is isotropic in the laboratory system 22 p E T S p o = 36 ± 05 % <E s > = 2T s = 09 ± 019 MeV

23 Conclusion The angular and energy distribution of the prompt neutrons for 239 Pu have been measured Up to now these data were absent A comparative analysis of the obtained angular and energy distributions of prompt neutrons from 239 Pu and calculated ones established: the angular anisotropy of the neutron emission in the fragment centerof mass system is alike to 1 + (006 ± 002) E cm cos 2 ( cm ) should be taken into account; there are some surplus of measured neutron yield above calculated one in low energy range for the total neutron spectrum as well as for neutron spectra at fixed angles near 90 o (relative to fission fragments direction); the yield of this low energy component of additional neutrons is not dependent on the angle in the laboratory system and is equal to 36 ± 05 % of total neutron yield per fission event; the maximal contribution of additional neutrons as not to exceed 7% of total neutron yield 23

24 Thank you very much for your attention 24