The concept of compound nuclear reaction:

1
We use compound nuclear reactions to study
nuclear structure. We study nuclear structure to
calculate compound reaction cross sections.
The concept of compound nuclear reaction
aB ? C ? dF
The particle transmission coefficients T are
usually known from cross sections of inverse
reactions (from optical model parameters).
Level densities and gamma-transmission
coefficients are most uncertain values !!!
2
How is nuclear level density estimated (current
status) ?
Traditionally, for most of the nuclei, the level
density is estimated on the basis of
experimental information from low-lying discrete
levels and neutron resonance spacing
Level density is known for most of the stable
nuclei
Level density is unknown for most of the nuclei
a, d -parameters
Level density
s f(a, d)
Bn
E
Excitation energy
3
Test of the formulas proposed in the work for the
LD at the neutron resonance energy. The dashed
lines mark a difference by a factor of 2 between
experimental and calculated values.
T.von Egidy, D.Bucurescu, Phys.Rev. C 72, 044311
(2005)
4
The Oslo method is based on the measurements of
particle-gamma coincidences from ( 3He, a?) and (
3He, 3He?) reactions
(M. Guttormsen et al)
?(E) ?(E)Aexp(BE)
A,B are uncertain
Level density
Bn
E
Excitation energy
5
The level density from particle spectra of
compound nuclear reactions
The concept

• The problem
• Make sure that the compound reaction mechanism
  dominates.
• Possible solutions
• Select appropriate reactions (beam species,
  energies, targets).
• Measure the outgoing particles at backward angles
• Compare reactions with different targets and
  incoming species
• leading to the same final nuclei

6
Early works on level densities from evaporation
spectra H. Vonach (Vienna, Austria) S.Grimes
(OU) B.Zhuravlev (Obninsk, Russia)
(n,p) (n,a) (a,n) (p,n)
Advantage The compound nuclear mechanism
dominates Drawback (for us) Negative Q-values of
reactions that require higher energy beams not
available from our tandem accelerator of Edwards
Lab.
Our options d, 3He, 12C, 6Li, 7Li beams
available from our tandem accelerator Q-reactions
are positive (5-15 MeV).
7
Swinger facility
d, 3He
neutrons
NE213
target
Flight path 8m
8
Si
Scheme of experimental set-up for charge-particle
spectra measurements Edwards Accelerator Lab,
Ohio University
Si
Si
Si
Si
Target
beam
2m flight path
Si
Si
Si
Si
Si
9
Experimental level densities from (d,n) reactions
measured at Edwards Lab.
Testing the level density with 27Al(d,n)28Si
10
55Mn(d,n)56Fe, Ed7.5 MeV
56Fe
11
55Mn(d,n)56Fe, Ed7.5 MeV
A.Voinov et al, PRC 74, 014314 (2006)
12
(No Transcript)
13
65Cu(d,n)66Zn, Ed7.5 MeV
14
Main results from (d,n) experiments 1.
Neutron spectra measured at backward angles are
suitable for level density determination. 2.
For many nuclei we got different level densities
(shape and absolute numbers) compared to
predictions from recent level density
systematics based on neutron resonance
spacings
15
Reactions with deuterons and He-3
59Co
3He
58Fe
d


61Ni
a
n
p
60Ni
57Fe
60Co
16
n
p
a
3He 58Fe
d 59Co
A.Voinov et al, PRC, accepted for publication
17
We also measured reactions with 12C, 6Li and 7Li
projectiles The following reactions have been
measured 6Li 55Mn 61Ni (d 59Co and 3He
58Fe) 6,7Li 58,57Fe 64Cu 12C 27Al
39K
Main result all of these reactions can be used
for the measurement of level densities of
residual nuclei.
The next steps 1. Determining level
density parameters from particle evaporation
spectra for more nuclei to build new level
density systematics which will be
different from that based on neutron resonance
spacing. Improve empirical formulas.
2. Investigate level density for nuclei off
stability line. 24Mg 58Ni experiment is
scheduled at Yale Lab. in one month.
18
B. Zhuravlev et al, Phys.Atomic Nuclei 69, 363
(2006)
Fig.6. Dependence of nuclear level density
parameter ã from (N-Z) for Sb
isotopes. o present work, ? - 12.
Curve calculation according
a?A/exp?(N-Z)2 with ? 0.154
and ? 0.00064 10.
19
? strength function in continuum
i
From (?,n) reactions
Particle separation threshold
Excitation energy
?- Energy (MeV)
0
20
Some results of ?-strength functions for
rare-earth nuclei
From Oslo Cyclotron Lab
21
?-strength function of iron isotopesLow energy
upbend phenomenon
- 56Fe
- 57Fe
E? (MeV)
A.Voinov et al, Phys.Rev. Lett., 93, 142504
(2004).
22
?-strength function of molybdenum isotopes
M. Guttormsen et al, Phys. Rev. C, 71, 044307
(2005).
23
Method of two-step ? cascades from neutron
capture reactions
History ProposedA.M. Hoogenboom, NIM
3,57(1958) Developed in Dubna(Russia)
(A. Sukhovoj) (since 1980)
PhD thesis A.Voinov (1994) F. Becvar
(Prague) (since 1992) A.Schiller, A.Voinov et
al, Los Alamos, 2001 A.Voinov, E. Algin et al
Budapest, 2002
Bn
E1
Intensity
E2
Intensity
Ground state
E1E2
E?
Problem level density is needed !!!
24
Measurement of gamma-strength function at
Edwards Lab.
(p,2?) (d,n)
To the same product nucleus
Strategy
1. We obtain a level density from neutron
evaporation spectra. 2. We obtain a ?-strength
function from 2?- spectra
The first candidate is 59Co(p,2?) 60Ni reaction
at Ep1.9 MeV
The level density of 60Ni has already been
measured from 59Co(d,n) 60Ni reaction
A.Voinov et al, PRC,
accepted for publication
25
First results from 59Co(p,2?)
26
We have unique opportunity to study level
densities and ?-strength function needed for
the basic physics and applications. Edwards
Lab. has unique facilities to do such kind of
research. Our strategy is based on
combinations of different experimental
techniques including particle evaporation spectra
and (p,2g) measurements at Edwards Lab,
measurements of level density and ?-strength
function in collaboration with Oslo Cyclotron
Lab. We also plan to start studying level
densities for nuclei off stability line (The
first experiment is scheduled next month at Yale
Lab. !!!)
27
Collaborators OU
S.Grimes, A.Schiller, C.Brune, T. Massey Oslo
University M. Guttormsen, S.Siem et al Livermore
Lab U. Agvaanluvsan,
28
Motivation

• Curiosity. We think that what we can measure has
  not
• been measured before. This will bring new
  knowledge about nuclei.
• Edwards Lab. has unique facilities to do
  such kind of research.
• The practical application. The new knowledge will
  allow us to calculate
• reaction cross sections more accurately.
• Astrophysics, reactor physics.