Project Investigation of Prompt Fission Neutron Emission in Fission (“ЕNGREN”)

 

Investigations of prompt fission neutron emission are of importance in understanding the fission process in general and the sharing of excitation energy among the fission fragments in particular. Experimental activities at JINR on prompt fission neutron (PFN) emission are underway for more than 20 years. Main focus lied on investigations of prompt neutron emission from the reactions 252Cf (sf) and 235U(n,f) [2-20,23-26] in the region of the resolved resonances. For the last reaction strong fluctuations of fission fragment mass and the mean total kinetic energy distributions have been observed as a function of incident neutron energy [16, 37]. In addition fluctuations of prompt neutron multiplicities were also observed in [44]. The goal of the present study is to verify the current knowledge of prompt neutron multiplicity fluctuations and to study correlations with fission fragment properties. Recent measurement of PFN multiplicity in resonance neutron induced fission of 235U(n,f) reaction [27] reveal surprising result, stimulated us to investigate the PFN multiplicity at IREN with new high efficiency  experimental setup.

 

Collaboration

Frank Laboratory of Neutron Physics, JINR, Dubna Moscow region

State University  «Dubna»

UniversityNovi-Sad,Scince Faculty,Physics Division,Novi-Sad,Serbia

Institute of Nuclear Physics and Enginering of Bulgarian Academy of Science (BAS), Sofia, Bulgaria

Project Lider: Zeynalov Sh.S.

Project Deputy: Mytsina L.V.

 

1. Introduction

The possible correlation between the variation of prompt fission neutron (PFN) multiplicity and the fission fragments (FF) total kinetic energy (TKE) variation measurement is the goal of the project. Investigations of PFN properties for more than six decades have reached considerable success due to efforts have been applied to modification of method with low geometric efficiency method (LGE) first suggested by H.R. Bowman et al in Phys. Rev. 126, 2120 (1962). C. Budtz-Jorgensen and H.H. Knitter, suggested twin back-to-back ionization chamber (TBIC) for correlated FF and PFN properties investigation. TBIC along with PFN detector located at ~0.5-0.7 m distance along the axis of the chamber was called method of PFN investigation with low geometric efficiency (LGE ~0.001) [1]. This method provides information on main parameters characterizing correlated FF: TKE, masses, PFN multiplicity, and PFN velocity measured by TOF method. Several experiments improved understanding of fission process with a new data obtained on PFN properties in reactions 252Cf(sf), 235U(nth,f) и 235U(nres,f) published in [2-20,23-26]. Interpretation of the experiments was done in the framework of multi modal random neck rupture (MM-RNR) model of low excitation energy fission. The model considers nuclei leaving the compound state by various paths to disintegration. These paths (fission modes) related to Bohr fission channels chosen in stochastic manner by the fissile system. Furman and J. Kliman found link between fission channels and fission modes, providing the way of evaluation the probabilities of fission mode realization using experimental data.

            Recently correction of PFN multiplicity dependence on FF mass and TKE for 235U(nth,f)  reaction was reported in [23] (see Fig. 1). Investigations carried out in IBR30 in 1999-2000 pulsed reactor in Dubna [16,17], confirmed existence of TKE variations in neutron resonances first reported by F.-J. Hambsch et al in IRMM [36].  PFN multiplicity measurement with LGE was carried out at GELINA in 2007-2008, using PFN detectors from DEMON collaboration and TBIC loaded by1 mg U235. However, statistical accuracy of data taken in resonances was not enough for reliable PFN multiplicity analysis. Therefore, we developed setup with chamber loaded by 230 mg 235U target (99.999%) and PFN detector composed of 32 fast neutron detection modules (76 mm diameter, 51 mm thickness) located at distance ~54 cm from the target (LEM =0.012). We expect improve the statistical accuracy of measurement at 9.2 flight path of IREN facility (full neutron beam intensity ~2*1011 sec/4π) at least by order of magnitude.

 

2. Current Status of PFN Investigations

 

First investigation of PFN multiplicity in resonance induced fission reveal multiplicity variations between 235U(n,f) reaction in resonance energy neutron region [11]. Later in experiments performed in EC-JRC-IRMM [36] variations of TKE in resonance region of neutron energy was observed as demonstrated in Fig. 1. Recently experiments, intended to investigate the

 


Fig. 1 TKE variations in 235U(nres,f) reaction measured in GELINA (left graph) and IBR30 (right graph)

 

correlations between FF TKE and PFN multiplicity variations was reported in [27, 33]. Authors used position-sensitive TBIC as FF detector and the array of 12 PFN detection modules. The aim of the experiment was simultaneous measurement of FF TKE and PFN


Fig. 2 Correction of PFN multiplicity dependence on mass&TKE measured in 235U(nth,f) in Dubna. Blue line taken from Ref. [40]. Black line is corrected NuBar(A) improves a mass resolution of FF obtained using 2E method.

 

multiplicity variation using LEM. However, simultaneous measurement of FF TKE and PFN has limitation on the target material weigh (should be as thin as possible). For PFN multiplicity variation investigations in JINR at IREN facility we divided experiment by two steps. At the first step PFN multiplicity measured with thick target of 230 mg. At this step we expect most possible PFN production/detection efficiency with available neutron beam intensity. During preparation

 

 

                                               Fig. 3. Experimental setup developed for use in ENGREN project

 

of the project test measurements were carried out. Fig. 4 demonstrates simplified setup used to estimate the time required for PFN variation measurement at IREN facility, having beam intensity ~2*1011 n/sec*4π. Distance between fission chamber and the neutron detector was 17.5 cm in order the PFN detecting efficiency to be close to the PFN detection efficiency of the setup shown in Fig. 3. Fig. 4 demonstrates data acquisition system setup, developed for digitization of PFN and fission detector pulses. Data acquisition software was partially ready, but can be used for test experiment. Only two out of 40 digitization channels were used in test experiment. Resonance neutron time of flight (tof) was measured using time stamps provided by CAEN N6730 waveform digitizer. To obtain correct tof value “T-Zero” pulse from the IREN generated just before neutron burst was used to reset the time stamps counter.

 
Fig. 4 Diagram of the electronic apparatus of data acquisition system

 

             

Fig. 5 Simplified setup

 
Pulse from fission chamber, connected to fission detection initiates time stamp write operation  into waveforms memory buffer. Each fission chamber pulse caused recording of event, consisted of time stamp and two waveforms, recorded from fission chamber and PFN detector.  Data analysis performed offline, creating two types of tof-distributions. In first one each fission event was counted forming tof-distribution. In second one only events where fission events coincided

 

 

Fig. 6 Tof-distributions with demand of coincidence between fission chamber (red curves) and without demand of coincidence (blue curves). Graphs plotted for data collected in two days (~45 hours). Right graph demonstrates thermal neutron energy range of tof-distribution)

 

 

Fig. 7 Fission chamber and PFN detector coincidence waveforms (left graph). Neutron gamma separation graph (central graph). PFN separated from gammas (right graph demonstrates that share of PFN with demand of coincidence is only ~0.13 part of full FF counts, meaning that PFN share is 0.015 of FF counts

 

with PFN detector pulse within 80 ns formed conditional tof-distribution as demonstrated on Fig. 6.  Coincidence between fission event and PFN registration can be caused by prompt gammas.  Hence the coincidence events should be refined from gammas using pulse shape analysis as demonstrated in Fig. 6. The share of PFN is only 0.13 of data collected with coincidence between fission chamber and PFN detector and it is only 0.016 of FF counts. From data analysis of measurement with simplified setup we concluded that for strongest resonances 45 hour measurement of PFN multiplicity provides ~3% of statistical accuracy. 

 

3. Data analysis procedure

 

Data analysis procedure was performed off-line as was described above. Data collected with demand of coincidence with PFN detector first refined from prompt gammas by pulse shape analysis. Then using thermal part of tof-distribution one can obtain average PFN detecting efficiency ξ by normalizing the total number of refined PFN counts to total number of FF. Assuming validity of the formula:

Where we use the following definitions: PFN and FF – are the numbers of PFN and FF respectively, detected in the thermal part of tof-distribution. ξ– is the PFN detection efficiency and 2.41 – is the NuBar value for 235U(nth,f) reaction known from the literature. Then for resonances and resonance groups we can use the following formula to calculate the  dependence on neutron energy:

 

                                                .

 

4. Conclusions

The share of PFN is only 0.13 of data collected with coincidence between fission chamber, henсe the share of PFN counts is only 0.016 of FF counts. From analysis of data measured with simplified setup (target of 230 mg) we concluded that for strongest resonances 45 hour measurement of PFN multiplicity would give ~3% of statistical accuracy. For statistical accuracy of ~3% in mass&TKE variation with 1 mg target we will need at least 50 weeks of measurement.  Additionally 50 weeks would be necessary for development apparatus and data acquisition software and preparations at the channel 2 of IREN.  

 

5. Outlook

We are planning to get new data on PFN properties in resonance neutron energy range of 235U(n,f) reaction as calibration of our method. The next step we will perform measurement of PFN emission in  fission of 237Np, 239Pu in the resonances.

 

References

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Collaboration

Shakir Zeynalov – PhD in Physics.

Has competence and good experience in realization of national and international projects (IAEA, 1997-1999, EC-JRC-IRMM, 2004-2009). He has experience in the following research areas: Heavy ion reaction, alpha spectroscopy, fission investigation of heavy ion production reactions, spontaneous fission, neutron induced fission, Multiple neutron and gamma emission detectors, design of modern experimental facilities for investigation of nuclear fission, nuclear electronics system design, design of digitization system and digital signal processing, computer programming using modern software and operating systems.

 

Sidorova Olga – PhD in mathematics.

Has good experience in solution of partial derivative equations of mathematical physics, has good experience in using specialized packages of programs (ORIGIN, MathLab) for data analysis in scientific investigations. She is professional in applications of digital signal processing for analysis of pulses from nuclear detectors. In addition she is specialist in Monte Karlo simulation of detector system response to particle registration  

 

Mytsina Ludmila –PhD in Physics.

Has good experience in data analysis of experimental data.

 

Suhovoy Analoly –PhD in Physics.

Has good experience in data analysis of experimental data.

 

Grigorian RolandKamyshnikov Denis - students from Dubna  University

 

Yovancheich Nicola – PhD in physyics, from the University Novi-Sad, Serbia

 

Semkova Valentina – PhD in physyics,  from Institute of Nuclear Physics and Nucler Energy of Bulgarian Academy of Science, Bulgaria.

 

Kuznetsov Alexey – Head of FLNP Workshop

 

Lebedev Artem – engineer

 

 

Contacts

Project Lesader Shakir Zeynalov- This email address is being protected from spambots. You need JavaScript enabled to view it.

Project deputy Mytsina Ludmila This email address is being protected from spambots. You need JavaScript enabled to view it.

Adressс: Frank Laboratory of Neutron Physics (FLNPH),  Joint Institute for Nuclear Research (JINR), Joliot-Curie  6, 141980 Dubna, Moscow region, Russian Federation