Novel Approach to Prompt Fission Neutron Investigation

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1 Novel Approach to Prompt ission Neutron Investigation Sh. Zenalov, O. Zenalova, P.Sedshev, V. Shvetsov JIN-Joint Institute for Nuclear esearch, Dubna, ussia ISINN, Alushta Ukraine, Ma -5, 3

2 Motivation Scission neutrons were predicted in 939 in classic paper of N. Bohr and J. Wheeler. irst eperiments made in 96-ies b Manhattan Group concluded eistence of scission neutrons for 5 Cfsf. Soon after scission neutrons were discovered for 35 Un,f b K. Skarsvag and I. Singstad. C. Budtz-Jǿrgensen and H.-H. Knitter investigated 5 Cfsf in simultaneous measurement of, kinetic energies, mass and the emission angle. Less than<% of scission neutrons was concluded from the PN angular distribution analsis in centre-of-mass reference sstem in that work. According to the theor models linear dependence of average PN multiplicit on TKE in fission is epected, but the eperiment did not confirm that. The results obtained using DPP for PN investigation in 5 Cfsf reaction in EC-JC- IMM during 5-7 for the first time confirm results of C. Budtz-Jǿrgensen and H.- H. Knitter, at the same time revealing linear dependence of PN number on TKE release in fission. We further developed fission detector and mathematical data analsis, which we believe opens the new perspectives in stud nuclei at low ecitation energies, undergoing the nuclear superfluid to normal liquid phase transition. The scission configuration offers the unique possibilit to investigate, how two different nuclei at constant temperature share the available intrinsic ecitation when the are in thermal contact. ISINN, Alushta Ukraine, Ma -5, 3

3 Pre-scission nuclear shape parameterization in MM-N and PN emission PN emission mechanism in MM-N is considered as N, leading to the ecited s. PN emission is the first stage of s de-ecitation, following b γ-ra emission. At scission configuration the temperatures of the nascent fragments remain different in spite of the flow of ecitation energ from the hot to cold fragment. ISINN, Alushta Ukraine, Ma -5, 3

4 PN emission kinematics The detailed information on the intrinsic state of the fragments can be obtained b measurement of dependence of the average number of PN emitted b the with mass number A and TKE release of two fission fragments. To do this one needs reconstruction of PN emission kinematics as illustrated in the figure above. A A, TKE Y A, TKE dtke Y A, TKE dtke or TKE A, TKE Y A, TKE da Y A, TKE da A, TKE Y A, TKE dtkeda, Y A, TKE dtkeda ISINN, Alushta Ukraine, Ma -5, 3

5 Twin risch-grid ionization chamber Signals from the TGIC anodes arise because of the motion of charge carriers after the are formed b the The time evolution of the signal is of fundamental importance in understanding the timing properties of pulses as well as in predicting the effects of changes in the location of the radiation interactions on the shape of the pulse. The Poisson equation is the starting point for these calculations, where, Electric field grad ISINN, Alushta Ukraine, Ma -5, 3

6 The Signal ormation Q q To find this weighting potential ϕ as a function of position,one must solve the Laplace equation for the geometr of the detector, but with some artificial boundar conditions:. The voltage on the electrode for which the induced charge is to be calculated is set equal to unit.. The voltages on all other electrodes are set to zero. 3. Even if a trapped charge is present within the detector volume, it is ignored in the calculation. ISINN, Alushta Ukraine, Ma -5, 3

7 inite difference approimation,, equation Laplace difference - finite, o u p q v o u p o q v puqv qv pu u p u u p p q v q q v v pu u p u u p p qv v q q q v v ISINN, Alushta Ukraine, Ma -5, 3

8 Ycoord. [arb] Averaged potential Weighting potential 57, Weighting potential for the anode,8 a 5 a,6 468,4,, Grid position Xcoord. [arb.] Distance from the cathode arb. Q cos D for D l D for D D cos dq d l D D l q Q E, D d L e E, cos D d N E Xˆ E cos D e T Q,cos D W l D X Q cos D ISINN, Alushta Ukraine, Ma -5, 3

9 Eperimental setup for PN investigation ISINN, Alushta Ukraine, Ma -5, 3

10 Digital pulse processing steps ISINN, Alushta Ukraine, Ma -5, 3

- 74 6 58 eal measurement of angular distribution 74 6 58 4 - -4 8,5 99, D plot of pulse

Partial derivative plotted along with the respective dp Q, T DT profiles right.

11 eal measurement of angular distribution ,5 99, D plot of pulse height vs drift time PQ,Talong with profiles to Q-,Taesleft. Partial derivative plotted along with the respective dp Q, T DT profiles right. Minimum and maimum on the T-profile corresponds to the and 9 degree angle between and the cathode normal respectivel ISINN, Alushta Ukraine, Ma -5, 3

12 Counts Numerical simulation d - - Angular distribution Derivative = Derivative = Derivative =4 ep -,5,,5,,5 COSQ Numerical simulation of rectangular cosine distribution measured with Gaussian response function black curve. esult of differentiation using Eq. : d d d d d ISINN, Alushta Ukraine, Ma -5, 3

13 X/D eal measurement of angular distribution 8 6 Chamber Chamber Pulse height [arb] The pulse height dependence of T min Q~X the centre of charge distribution along the trace is plotted in for both halves of the TGIC. or fied Q pulse height the minimum value of drift time realized for moving along the normal to the cathode plane. The drift time is maimum for at grazing angles. ISINN, Alushta Ukraine, Ma -5, 3

14 Counts Comparison of angular distributions 75 5 Cfsf 5 5 Laer side Backing side -,5,,5,5,75,,5 COSQ Angular distribution measured from the target laer side demonstrates rather better accurac, almost independent on the angle in respect to the laer surface. The backing side accurac degrades at angle close to 9. This fact makes measurement of PN emission for targets like 35 U and 39 Pu ver difficult task. To overcome that we developed a position sensitive TIC, allowing measurement with an arbitrar allocated multiple ND ISINN, Alushta Ukraine, Ma -5, 3

15 Application: The Stripped Detector The anode of modified TIC was made of strips and the grid was removed. Strips divided along the diogonal forming two isolated Δ-electrodes. Separate electrical contacts generall are made to each Δ-electrode. No space charges are present in the chamber volume. Dimensions of the chamber in the and у directions are large compared with the thickness of strips, so that edge effects can be neglected. e ISINN, Alushta Ukraine, Ma -5, 3

,,8,6,4,, Weighting potential for the single strip,,8,6,4,, 4 6 8 The weighting potential in the TIC volume was calculated for one strip potential raised to leaving other electrodes grounded.

16 ,,8,6,4,, Weighting potential for the single strip,,8,6,4,, The weighting potential in the TIC volume was calculated for one strip potential raised to leaving other electrodes grounded. If the strips are operated at a positive potential relative to the cathode surface, then electrons will be attracted along field lines that are parallel to each other and perpendicular to the anode surfaces. It is ver important to notice that the onl electrons that collected b the selected strip will contribute to the pulse formed on the strip circuit. ISINN, Alushta Ukraine, Ma -5, 3

17 Potential [V] Averaged potential Comparison with TGIC, ,,8 Weighting potential for the anode,5 a,6,4 Grid position, 5 5 Distance from anode plane [arb. units],, Distance from the cathode arb. Left graph demonstrates the weighting potential dependence on the ratio of the anode strip width to the anode-cathode distance. On the right graph the weighting potential for the TGIC is shown. Comparison demonstrates that if anode-cathode distance is properl chosen, the there is no need in risch-grid at all. This has influence on the pulse shape of individual strip and possibl could facilitate determination of ionization densit along the fission fragment deceleration path. This effect was not investigated et. Another feature of new design is improve of pulse rise time, which makes the TIC more attractive for eperiments with targets with high intrinsic alpha-radiation. ISINN, Alushta Ukraine, Ma -5, 3

18 Non point target Point target investigation was realized so far onl with 5 Cf nucleus. or the majorit of interesting nuclei point target option is not available for different reasons, but instead large area thin targets - sq. cm with thickness of 4-6 μg/sq. cm can be manufactured. PN investigation is difficult due to small cross section of fission reactions. One of the wa to overcome that difficult is to use large area target and as much as possible the number of ND. Then the task of reaction kinematics reconstruction can be solved b the position sensitive TIC measuring angles in 3D Cartesian coordinates ISINN, Alushta Ukraine, Ma -5, 3

19 angle determination in 3D Cartesian coordinates ISINN, Alushta Ukraine, Ma -5, 3 ; COSY ; COSY ; COSX ; X COS

20 Counts Counts Unfolding of angular distributions 5 COSQ COSX COSY 5 COSQ COSX COSZ 5 -, -, -,8 -,6 -,4 -,,,,4,6,8,, COSQ -, -, -,8 -,6 -,4 -,,,,4,6,8,, COSQ Unfolding procedure is illustrated b two figures above. irst graph illustrates COSX and COSY values Calculated from the row data truncated according to the following formula: if COSQ> then COSQ=. On the right side graph the unfolded angular distribution functions are presented. The idea of unfolding came from the Shannon sampling theorem. According to the theorem: continuous function can be represented b its sampled form periodicall epanded over the argument. Using this the unfolding of angular distribution can be reduced to the following simple formula: ifcosq > then D=-COSQ COSQunf= -D; esult of such unfolding procedure is presented on the right side graph.

21 Average PNM Average PN number versus mass in comparison with previous measurements 4 Digital data B-K Nifenecker et al A, TKE Y A, TKE dtke A, A, TKE Y A, TKE dtkeda, Y A, TKE dtkeda Y A, TKE dtke MASS [amu] ISINN, Alushta Ukraine, Ma -5, 3

22 Average total prompt neutron multiplicit Average PN number versus TKE in comparison with literature CfS Budtz-Jorgensen, Knitter, 989 EXO Bowman USABK 63 using YA of IMM EXO Zacharova USKU 98 EXO Bowman USABK 63 Zenalov et al. IMM 9 ussian PNPI PbP calc. using ussian PNPI ep.ya PbP Annals Nucl.Energ TKE MeV A, TKE Y A, TKE da TKE, A, TKE Y A, TKE dtkeda, Y A, TKE dtkeda Y A, TKE da ISINN, Alushta Ukraine, Ma -5, 3

23 Average PN number versus TKE for selected mass numbers Average PN number Average PN bumber A=5+6 A=7+8 A= A=+ 8 A=3+4 8 A= TKE [MeV] 6 8 TKE [MeV] 6 8 TKE [MeV] A, TKE Y A, TKE TKE A, A, TKE Y A, TKE dtkeda, Y A, TKE dtkeda Y A, TKE ISINN, Alushta Ukraine, Ma -5, 3

24 Neutron imaging with n-γ converter and CCD camera ISINN, Alushta Ukraine, Ma -5, 3

25 Summar and conclusions Significant modifications of the eperimental methods were done in our work thanks to fleibilit and power provided b the digital signal processing. Man conclusions of previous investigations have been confirmed in our work. At the same time we managed to resolve the longstanding contradiction between eperimental results and theoretical calculations. With higher confidence than in previous investigations we proved the statement on isotropic PN emission in s centre-of-mass reference frame. or the first time it was shown that dependence of average PN multiplicit is linear as was epected from the theor. This made obsolete eperiments, where search of pre-scission neutrons was epected to resolve the problem of PN deficit at low TKE values. We developed new fission detector and advanced mathematical data analsis, which we believe opens a new perspective in stud of PN emission in low energ fission. A new detector can be used in neutron imaging applications as a competitive option to eisting solutions. ISINN, Alushta Ukraine, Ma -5, 3

PROMPT FISSION NEUTRON MULTIPLICITY INVESTIGATION. Belgium

PROMPT FISSION NEUTRON MULTIPLICITY INVESTIGATION Sh. Zeynalov a, O. Zeynalova a, F.-J. Hambsch b, S. Oberstedt b a) JINR-Joint Institute for Nuclear Research, Dubna Moscow region, Russia b) EC JRC Institute