Exotic high neutron multiplicity modes in 252Cf(sf).
Yu.V. Pyatkov1,2, A.N. Tjukavkin2, D.V. Kamanin2, O.V. Falomkina3, J.E.
Lavrova1, E.A. Sokol2, E.A. Kuznetsova2
1
Moscow Engineering Physics Institute
2
Joint Institute for Nuclear Research
3
Moscow State University
neutron counters
n
Ni
Ni
Standard
FOBOS
modules
(PSAC + BIC)
Start detector
(252Cf +PAC)
ν= ?
Neutron belt at FOBOS spectrometer
140 3He- counters
Geometrical efficiency – 19%
Preg of “normal” FF neutrons -3.6%
3He
+ n → 3H + p + 780 keV:
cross-talk is absent
Model of neutron registration channel
Components taken into consideration:
- neutrons from conventional binary fission;
- random coincidences ;
- background in the experimental hall (50n/sec)
D.V. Kamanin et al., Phys. Atomic Nuclei, 66,
(2003) 1655
Yu.V. Pyatkov et al., preprint JINR E15-2004-65
Parameters of the neutron registration channel.
- registration efficiency for the neutrons emitted from the FF ε=3.6%;
- geometrical efficiency of all the neutron belt
η=19%;
- “ capture efficiency” (probability that a neutron which achieved
the neutron belt will be detected)
f=60%;
- time-gate for registration of neutrons
128 μs;
- activity of the 252Cf source used (fissions)
n=330 sec-1
- neutron background in the experimental hall
b=50 sec-1
- const of neutron deceleration
λ=0.0384
- registration efficiency for the neutrons emitted from the isotropic
source to be in rest (δ= η* f)
δ=11.4%;
- probability ratio (isotropic/emitted from fragment)
δ/ε ~3.
Comparison with the experimental data
experiment
model
number of detected neutrons
“bump”
Structures at neutron gating
Y~4*10-3
Fine structure in the FF mass-mass distribution
n≥2
Well - ordered sets of
points – linear
structures.
It is hard to believe
that a background
exceeds 20% in the
region of interest
Ms=const
Fine structure in the FF mass- mass distribution
85As
n≥2 & w5
68Ni
128Sn
140Xe
Fine structure in the FF mass- mass distribution
n≥3
Fine structure in the FF mass- mass distribution
n≥3 & w3
80, 82Ge
132Sn
144Ba
Fine structure in the FF mass-mass distribution
What is the statistical
n≥2
reliability of the structures,
for instance, the blue line?
Formal algorithm of
structure revealing
(Hough transformation)
Searching for the line of
the same length in the set
of random matrices
reliability
Hough transformation, definition
One to one correspondence:
line in (X, Y) plane - (R,θ) point
Demonstration of Hough transformation algorithm
Y
θ
X
Number of points “captured” by
the line having coordinates in the
range (ΔR, Δθ)
R
Demonstration of Hough transformation algorithm
http://homepages.inf.ed.ac.uk/amos/hough.html
Fine structure in the FF mass-mass distribution
n≥2
Hough - simulation:
reliability ~98%
Neutron mode which gives rise to the fine structures under discussion 1
n≥2
CCT
n≥3
High neutron multiplicity –
possible reasons:
1. Conventional binary “hot”
fission (large ν);
2. “soft” n-spectrum (larger
registration efficiency);
3. The source to be almost in
rest.
Isotropic neutron sours to be in rest, multiplicities 3 ÷ 12
Neutron mode which gives rise to the fine structures under discussion
- Isotropic sours
- ν~6
- YCCT~10-3/ bin. fiss.
ν=6 curve shows the slope very close to the experimental one (Δ);
even ν=5 curve is essentially worser
2
Neutron mode which gives rise to the fine structures under discussion 3
Binary
fission
50%
CCT
20%
10%
background
YCCT:
region_4/region3 ~8/3
Effect / background 4/1
4/3
Normal binary events (scattered) Æ represent a
background for the CCT mode. I suppose that at n=2
Ybackgr ~0.2YCCT
Resultant model and partial contributions
exp
random
FF
ν=12
η=40%
model
Shadow cones…, but for neutrons
θmin (α) ≥ 450
n
Isotropic neutron source
to be in rest
E. Piasecki, L. Nowicki, “Polar emission in fission”, IAEA-SM-241/ F11
Conclusions
1. Simulation based on Hough transformation let us estimate quantitatively a
reliability of the linear structures (manifestation of the CCT) observed in the
FF mass-mass distribution gated by neutrons. Even the most simple linier
graphic cluster observed can be reproduced by chance in 2% of samples
only.
2. Basing on the model of the neutron registration channel we came to
conclusion that as neutron emitter one of the CCT modes constituted the
“bump” in the FF mass-mass distribution represents isotropic neutron source
of multiplicity ν ~ 6 to be in rest. The yield of the mode is about 10-3/ binary
fission in good agreement with the yield of the “bump” estimated
independently earlier (4*10-3).
3. Results obtained prove to be strong confirmation of the physical nature of the
“bump” under discussion.