anisotropic neutron evaporation from spinning fission fragments
DESCRIPTION
Anisotropic neutron evaporation from spinning fission fragments. Kazimierz 2008. F. Gönnenwein, Univ. Tübingen for the collaboration. E. Chernysheva, Frank Lab Dubna O. Dorvaux, In2P3 Strasbourg F.-J. Hambsch, IRMM Geel F.Hanappe, ULB Bruxelles J. Itkis, Flerov Lab Dubna - PowerPoint PPT PresentationTRANSCRIPT
Anisotropic neutron evaporation from spinning fission fragments
Kazimierz 2008
2008 Kaz 01
E. Chernysheva, Frank Lab Dubna
O. Dorvaux, In2P3 Strasbourg
F.-J. Hambsch, IRMM Geel
F.Hanappe, ULB Bruxelles
J. Itkis, Flerov Lab Dubna
Y. Kopatch, Frank Lab Dubna
M. Mutterer, TU Darmstadt
L. Stuttgé, In2P3 Strasbourg
H.-J. Wollersheim, GSI Darmstadt
F. Gönnenwein, Univ. Tübingen
for the collaboration
Kinematical anisotropy of neutrons in lab system
Assume neutrons are evaporated from fully accelerated fission fragments but with the emission in their own cms being isotropic
θcmθlab
Centre of mass system of LF
Laboratory
system
velocity n velocitiesn velocities
of LF fragment
In the transformation from the cms to the lab system the neutrons are thrown into the forward hemisphere. In the lab the angular distribution of neutrons is no longer isotropic as shown here for LF moving to the leftLikewise the energy distribution of neutrons is transformed
fission axisLF HF
Experimental Result for 252Cf(sf)
Density of neutrons in velocity space:
ρ(V)d³V = ρ(V,θlab) V² dV dω
for FF masses
<ML> ~ 120 and <MH> ~ 132 H.R.Bowman et al. 1963
2008 Kaz 2
adiabatic
132 15610884
Mass
4
8
12
Fission Fragments carry large angular momenta
J.O.Rasmussen et al 1969
M.Zielinska-Pfabé, K. Dietrich 1974
Bending Model:
Coulomb + nuclear forces bring about a potential pocket
which aligns deformed fragments on the fission axis.
Angular vibrations are excited as zero point oscillations or
non-adiabatically at neck rupture or thermally.
235U(n,f) 238U(γ,f)
J.L.Durell 1997 D.DeFrenne 1984
<I p
rim>
/ ħ
<I p
rim>
/ ħ
Mass
12
8
4
80 120 160
Large angular momenta
Note:
Angular momenta are perpendicular to fission axis
● ●
Non-deformed (spherical) fragments acquire very large
angular momenta via single particle excitations
2008 Kaz 3
Neutron evaporation from rotating nuclei
Neutrons evaporated from a rotating nucleus will preferentially be emitted in the equatorial plane of spin
z
x
I
fission axis
n
n
cms n anisotropy ~ (1 + A sin² θ)
θ
Averaging over all possible orientations of spin ┴ fission axis, a
forward – backward preference along fission axis results in cmsAveraged over spin
cms n anisotropy ~ (1 + b cos² θcm)
For fixed spin
spin
where θ is polar angle relative to spin
where θcm is polar angle relative to fi-axis
Calculated anisotropy in cms of fragment
Isotropic evaporation for l = 0 neutrons.Anisotropic evaporation for l > 0.
Note that the yield of neutrons is rapidly decreasing for l increasing
From V. Bunakov, I. Guseva et al. 2004
2008 Kaz 4
non-zero n-anisotropy in cms observed from fit to n-spectrum
Ratio of n spectra with/without anisotropy
where cms anisotropy ~ ( 1 + b cos² θcm)
b = 0
n-spectrum from evaporation theory
J. Terrell 1959: “it is probably not possible to prove anything about anisotropy from the fission neutron spectrum alone”
235U(n,f) En = thermal
without cms anisotropy
From F.-J. Hambsch et al. 2003
2008 Kaz 5
ISSUE
Scission Neutrons and/or cms Anisotropy
The cms anisotropy will reinforce the kinematical anisotropy in the labHowever, the effect is very small, prohibitively small for experiments
From V. Bunakov , I.Guseva et al 2005
235U(n,f)
Isotropy in cms
No scission n
- - - - 15% scission n
50% from LF
35% from HF
235U(n,f)
Angular distribution of neutrons in lab as a function of angle θlab relative to LF. The intensities with both LF and HF contributing at 0° : 90° : 180° are 9 : 1 : 4
From K. Skarsvag et al 19632008 Kaz 6
z
x
I
fission axis
n
n
θ
spin
Concept of “CORA” experiment
How to disentangle in the lab the contributions of kinematical and cms anisotropy?
Analyse triple coincidences between 1 fission fragment and 2 fission neutrons
z
x
yI
O●●
●●
Assume - for the sake of argument - extreme cms anisotropy: all n in a plane spin
Project fission axis and
all n events on a plane
perpendicular to fi axis.All neutron events will
be aligned on a single
line, e.g. the x-axis.
as observed in the
projection plane
Extreme cms ANISOTROPY Perfect ISOTROPY
triple coincidences
Assume - for the sake of argument - orientation of fragment spin is fixed
x
y
●O
● ●● ●●●x
y
O●
●
●
●
●
●●
Φ1Φ2
all n-events on a line n-events distributed isotropically in plane
Due to kinematical focussing
density of events is enhanced
near origin O
Plane of projectionPlane of
projection
Fission axis
2008 Kaz 7
Setup of “CORA” experiment
252Cf(sf) fission source
CODIS back-to-back
Ionisation Chamber
with 5-sector cathode
DEMON
neutron detectors
Up to 100 units
●
Fission axis
x
yO
Projection plane for evaluation
Only fission events in cone ± 15° are considered
VIEW FROM TOP
2008 Kaz 8
252Cf spont. fission source
2008 Kaz 9
Layout of experiment CORA II (June 2008)
Fragment detector CODIS
In green: modular neutron detectors DEMON
Predictions from Theory
0 10 20 30 40 50 60 70 80 900
2
4
6
8
10
12
14
16
heavy fragment, <Ii>=8.2 h
light fragment, <Ii>=7.5 h
A*sin2()
W(
)/W
(0o)-
1, %
, deg.
0 30 60 90 120 150 180 210 240 270 300 330 360
0,98
0,99
1,00
1,01
1,02
1,03
n-n
co
un
t ra
te, a
rb.u
.
, deg.
wn() = 1 + 0.9 x sin2(
n-anisotropy in FF cm system
A sin² θcm
heavy fragment
light fragment
Distribution of ΔΦ
with ΔΦ the difference in Φ-angle
for 2 neutrons from same fi event
relative to fragment spin
I. Guseva 2007
2008 Kaz 10
MC simulation for isotropic neutron distribution
0 30 60 90 120 150 180 210 240 270 300 330 3600
200
400
600
800
1000
Efficiency
Cou
nts
n [o]
Due to the modular pattern of the neutron detectors of DEMON the counting efficiency can not be expected to be perfectly flat
Neutron counting efficiency versus detection angle Φ
Difference ΔΦ = Φ2 – Φ1 in Φ- angle for two neutrons detected per fission
Contrary to the Φ-distribution the ΔΦ- distribution is rather smooth
2008 Kaz 11
0 30 60 90 120 150 180 210 240 270 300 330 3600
250
500
750
1000
1250
1500
1750
Co
un
ts
Summary
● WELL KNOWN: fission neutrons being emitted from moving fragments exhibit a kinematical anisotropy in lab
● ISSUE: are neutrons evaporated isotropically or anisotropically in their own cms system ?
● SUGGESTION: cms anisotropy may be attributed to large angular momenta of fission fragments
● PROBLEM: how to disentangle in experiment kinematical and cms anisotropy ?
● CORA: is a project to directly observe the cms anisotropy based on triple correlation data (fragment, n, n)
● EVALUATION: particular scheme of evaluation allows to have model-free access to cm anisotropy
with kinematical anisotropy being “switched off”
● EXPERIMENT: underway since July 2008
● FINAL AIM: find from experimental ΔΦ distribution the cms anisotropy (1 + A sin² θ) relative to FF spin
and by averaging over spin the cms anisotropy (1 + b cos² θcm) relative to fission axis.
Calculate n-spectra taking into account anisotropy b and compare to experiment
2008 Kaz 12