235U Prompt Fission Neutrn Spectra Conflict of the Irfu235U裂变中子谱型IRFU冲突

1、the “scission neutron emission” is last or first stage of nuclear fission?nikolay kornilovexperiments microscopic (differential) experiments (as a rule tof) macroscopic experiments integral experiments benchmark experiments keff= 1 thermal neutrons235uactivated samples deeen)()(irmm-2007problem duri

2、ng more than 50 years of the experimental efforts!pfns from microscopic experiments at thermal energy do not agree with integral data and can not reproduce keff for benchmark experiments1972, islam and knitter motivated new experiment though the results of the above measurements, characterized by th

3、e average fission neutron energy, , agree reasonably with each other, a number of integral experiments shows higher values for indicating a harder fission neutron spectrum than that given by differential measurements.2003, madland no calculated thermal spectrum has been found that simultaneously rep

4、roduces either of the two modern thermal differential measurement and the set of measured integral cross-sections to within an acceptable level” endf/b-7235u: average energy of fission neutronsincident neutron energy, mev012345, mev1.92.02.12.2evaluation 1999 (kornilov et al) evaluation 2004 (endf/b

5、-7)average for eth (exp i)ippe+ri; 1990-95 (exp i)johansson, 1977 (exp ii)evaluation 2002 (hambsch et al)keff=1?available differential data 235u thermal point: starostov et al (1983), 3 spectra, lajtai et al (1985), yufeng et al (1989), kornilov et al (2008), 3 spectra; 0.5 mev: trufanov (1994), sta

6、ples (1995), irmm (2006-08) 8 spectra; 1-5mev: boykov (1991, 2.9 mev), trufanov (1994, 5mev), staples (1995, 1.5, 2.5, 3.0mev)sources of information024681012 -1,0-0,50,00,51,01n(e,)e, mev0.00.20.40.60.81.00.91.01.11.21.31.41.51.6 r()angular distribution of scn relative to ff -1.0-0.50.00.51.00.00.10

7、.20.30.40.50.60.7 yscn(),1/d -1.0-0.50.00.51.00.040.060.080.100.120.140.160.180.20 yscn(), 1/ de1-e2=0.5 - 6 mevenergy spectrum of scn in ls02468101e-30.010.1 yscn(e), 1/meve, mevnon “direct” method for scn estimation shape of the pfns at thermal point requires the scn incorporation (problem #1).024

8、6810120.60.81.01.2 starostov, 1982 irmm, 2008 lanl, a=a/10.2 r(e), =1.988meve (mev)2=3.5 3 sources modelin case of scn emission, the resulting spectrum is composed of three sources (3 sources model): )()()()(1enenenenscnaa (1) 1. neutrons from fragments after fission of the compound nucleus a+1 )()1

9、()(11ewenaa (2) where is the share of scission neutrons and wa+1 is the spectrum which describes the neutron emission from accelerated fragments; 2. neutrons from accelerated fragments after fission of the nucleus a, which is formed due to the emission of one scn: /)()1()(ewenaa. (3) 3. scission neu

10、trons themselves: 222121exp1exp)(tetteteenscn, (4) where is the share of the low energy component and is the neutron multiplicity. “3 sources model” spectrum and thermal data 0246810120.00.20.40.60.81.01.21.4 this work eq. 1 eq. 2 eq. 3 eq. 4 r(e), =1.988meve (mev)the residual chi-square is 2=0.64 e

11、xperimental results for 235u and 252cfvorobev (2007) et al estimated 10% for 235u at thermal pointparameter 252cf, bowman, 1962 252cf, budtz-jorgensen, 1988 235u skarsvag, 1963 235u irmm-2008 , 1/f 0.460.02 0.3820.003 0.3780.006 0.41 1 0.410.03 0.6790.012 0.6570.041 0.26 t1, v 0.350.04 0.4780.008 0.

12、490.02 0.34 t2, v 1.770.07 1.540.04 1.370.07 1.31 , v 2.380.13 1.640.04 1.580.09 2.12 what is happened at 0.5mev? = 2.031mev ; = 2.045mev irmm(th) (2008) and johansson (1977) 0.5mev data0246810120.60.81.01.2 e0=thermal e0=0.5mev r(e)e (mev)the difference of the pfns shape between thermal and 0.5 mev

13、 input energy can not be predicted with existing theoretical model (problem #2).0246810120.60.81.01.21.4 johansson irmm endf/b-vii, a=a/11 r(e), =1.988meve (mev)what is happened at 0.5mev (cont)?0246810120.60.81.01.2 irmm, 2006, 0.5mev staples, 1995, 0.5mev endf/b-7, a=a/11 irmm, 2008, thermal irmm,

14、 2007, 0.5mev r(e)e (mev)b)what does mean angular effect? problem #302468100.70.80.91.01.11.2 jan08-r90 jan08-l90 jan08-r150 apr07-r90 apr07-l120 apr07-r150 jul06-r90 jul06-l120 endf/b-vii r(e), =2.002meve (mev)what does mean left-right asymmetry?sampledetector 1detector 2detector 3proton beampb bar

15、pb barmonitor2468100.900.951.001.051.10 r90/l90 l03 000 r03 e (mev)angular effect can be reproduced.0246810120.60.81.01.2=0.0 r(e), =2.002meve (mev)=0.45why share of scn is changing?there is no any idea to explain the difference between differential and integral data! (problem#4)024681012140.80.91.0

16、1.11.2 endf/b-vii experimental pfns, eq. 1 r(e)=c/e (mev)0.010.11100.60.81.01.21.4 irmm-2008 lajtai starostov 3 source model endf/b-vii r(e), =1.988meve (mev)integral datapossible explanation ?one may conclude that a factor exists which has a rather strong influence on the pfns shape and asymmetry e

17、ffects but was not fixed in experimental investigations at 0.5mev input neutron energy all experiments which results were used in the report were made with 7li(p,n) reaction as a neutron source and pulsed mode. one may assume that this factor is the neutron polarization. we should take into account

18、the possible proton polarization also due to pulsed mode of the accelerators (chopper, bunching high voltages, analyzing and switching magnets). in the preparation stage of any pfns experiment it was assumed that this factor is not important or by definition should be equal to zero. if this explanat

19、ion is true, the transmission mechanism of the information from the incident neutron to the secondary fission neutron should be found. the only possibility might be scission neutron emission, a fast process without formation of the compound nucleus. this may provide the link between the incident neu

20、tron and the secondary fission neutron. so, for real clarification of this effect we need new experiments with polarized thermal neutron beam. when we will confirm and verify this effect new theoretical model should be developed. the most difficult for understanding is the problem #4. there are not

21、any realistic ideas for its solving. may be they will come after new experimental efforts mentioned above. neutron decay from excited states in the second minimum?bn0bniconclusionnew experimental and theoretical efforts are necessary to answer the following very important questions: what is the mechanism of neutron emission in fission and fission process itself. 40% of fission should be happened without compound nucleus formation; why the shape of the prompt fission neutron spectrum may change so drastically. thermal-0.5 mev data