Experimental Nuclear StructurePart I
Filip G. Kondev
kondev@anl.gov
Workshop on “Nuclear Structure and Decay
Data: Theory and Evaluation”, Trieste, Italy
April 28th-May 9th, 2008
Outline
I)
Lecture I: Experimental nuclear structure physics
 Introduction
 Nuclear Reactions – production of excited nuclear states
 Techniques for lifetime measurements
 Coulomb excitation, electronic, activity, indirect
II) Lecture II: Contemporary Nuclear Structure Physics at the Extreme
 Spectroscopy of nuclear K-Isomers
 Physics with large g-ray arrays
 Gamma-ray tracking – the future of the g-ray spectroscopy
Have attempted to avoid formulas and jargon, and material
covered by other lecturers – will give many examples
Please feel free to interrupt at any time!
2
Some Useful Books
“Handbook of nuclear spectroscopy”, J. Kantele,1995
“Radiation detection and measurements”, G.F. Knoll, 1989
“In-beam gamma-ray spectroscopy”, H. Morinaga and T. Yamazaki,
1976
“Gamma-ray and electron spectroscopy in Nuclear Physics”, H. Ejiri
and M.J.A. de Voigt, 1989
“Techniques in Nuclear Structure Physics”, J.B.A. England, 1964
“Techniques for Nuclear and Particle Physics Experiments”, W.R. Leo,
1987
“Nuclear Spectroscopy and Reactions”, Ed. J. Cerny, Vol. A-C
“Alpha-, Beta- and Gamma-ray Spectroscopy”, Ed. K. Siegbahn, 1965
“The Electromagnetic Interaction in Nuclear Spectroscopy”, Ed. W.D.
Hamilton, 1975
Plenty of information on the Web
3
Input from many colleagues
R.V.F. Janssens, C.J. Lister and I. Ahmad, Argonne National Laboratory, USA
M.A. Riley, Florida State University, USA
I.Y. Lee, Lawrence Berkeley National Laboratory, USA
D. Radford, Oak Ridge National Laboratory, USA
A. Heinz, Yale University, USA
C. Svensson, University of Guelph, Canada
G.D. Dracoulis and T. Kibedi, Australian National University, Australia
J. Simpson, Daresbury Laboratory, UK
E. Paul, University of Liverpool, UK
P. Regan, University of Surrey, UK
and many others …
4
~6000 nuclei
are predicted
to exist
~3000
~3000 the
knowledge is very
limited!
5
Introduction
 The nucleus is one of the most interesting
quantum few-body systems in Nature
 It brings together many types of behaviour almost all of which are found in other systems
 The major elementary excitations in nuclei can be
associated with single-particle and collective
degrees of freedom. While these modes can exist
in isolation, it is the interaction between them that
gives nuclear spectroscopy its rich diversity
6
The Nucleus is Unique!
 Its uniqueness arises on the one hand because all
forces of nature are presented - strong, weak,
electromagnetic and even gravity if one considered
condensed stellar objects as a huge number of nuclei
held together by the gravitational attraction, in
contrast to all other known physical objects.
 On the other hand, the small number of nucleons
leads to specific finite-system effects, where even a
few particles can change the “face” of the whole
system.
7
The Nucleus and other many-body systems
 The physics of the nucleus is not completely secluded
from the other many-body systems known in nature
A variety of nuclear properties can be described by
the shell model, where nucleons move independently
in their average potential, in close analogy with the
atomic shell model.
The nucleus often behaves collectively, like a fluid even a superfluid, in fact the smallest superfluid
object known in nature and there are close analogies
both to condensed matter physics and to familiar
macroscopic systems, such as the liquid drop.
8
To summarize …
NUCLEAR PHYSICS IS A BIG CHALLENGE
because of complicated forces, energy scale and sizes involved
The challenge is to understand how nucleon-nucleon interactions
build to create the mean field or how single-particle motions build
collective effects like pairing, vibrations and shapes
NUCLEAR PHYSICS IS IMPORTANT
(intellectually, astrophysics, energy production and security)
IT IS A GREAT TIME IN NUCLEAR PHYSICS
with new (RIB) facilities just around the corner we have a chance
to make major contributions to the knowledge - with advances in
theory we have a great chance to understand it all - by compiling
& evaluating data we support various applications, assist
scientific discoveries and preserve the knowledge for future
generations
9
To learn many of the
secrets of the nucleus we have to put it at
extreme conditions and
study how it survives
such a stress!
10
11
Nuclear Reactions – very schematic!
Target
Residual
nucleus
Projectile
CN
a multi-step process
 Gamma-rays & neutron induced
 no Coulomb barrier
 low-spin states
 Light charged particles - p, d, t, a
 Coulomb barrier
 low-spin states
 Heavy Ions (1970 - ????)
 high-spin phenomena
 nuclei away from the line of stability
12
HI Reactions – Classical Picture
R, Rt, Rp – half-density radii
b – impact parameter
“SOFT” GRAZING
INELASTIC SCATTERING
DIRECT REACTIONS
COMPOUND NUCLEUS
Rp
b<R=Rp+Rt
FUSION
b
R
“HARD” GRAZING
Rt
FRAGMENTATION
DEEP INELASTC REACTIONS
DISTANT COLLISION
b>R
E<Vc
ELASTIC SCATTERING
COULOMB EXCITATION
13
Distant Collision
Soft grazing
Fusion
Hard grazing
14
Heavy Ions reactions at the Coulomb barrier
 properties of the collision can be quite well estimated by just
using conservation of momentum and energy
Ecm = Mt / (Mb + Mt) Elab
 energy scale is determined by Coulomb barrier, Vcb
Vcb = (4pe)-1 ZbZte2 / R = 1.44 ZbZt / 1.16 [(Ab 1/3+At 1/3) + 2] MeV
Lmax = 0.22 R [ m (Ecm – Vcb) ]1/2

 excitation energy of residues is usually lowered by the Q-value
and K.E. of “evaporated” particles
E* = Ecm + Q – K.E.
15
HI Fusion-Evaporation Reactions
 R  p
l max
2
 (2l  1)T
l 0
l
 p 2 (l max 1) 2
A1 A2
   / 2mECM , m 
A1  A2
2
lmax
 2mR 2 
  2 ECM - VC )
  
16
Decay of the Compound Nucleus
 In HI fusion-evaporation
reactions the final nucleus is
often left with L~60-80 hbar
and E*~30-50 MeV
 The excited nucleus cools off by
emitting g-rays - the typical number
is quite large, usually 30-40, with the
average energy is ~1-2 MeV. It is not a
trivial task to detect all of them - the
big advantage comes with the large
g-ray arrays!
17
Channel Selection for g-ray spectroscopy
Detection of Light Charged Particles (a,p,n)
PROS: efficient, flexible, powerful ... inexpensive
CONS: count-rate limited, contaminants (Carbon etc, isotopic impurities)
makes absolute identification of new nuclei difficult
CROSS SECTION LOWER LIMIT: ~100 mb
that is,
Detection of Residues in Vacuum Mass Separator
~10-4
PROS: true M/q, even true mass measurements. With suitable focal plane
detector can be ULTRA sensitive. Suppresses contaminants
CONS: low efficiency
CROSS SECTION LOWER LIMIT: ~100 nb
that is
~10-7
Detection of Residues in Gas Filled Separator
PROS: better efficiency (compared to vacuum separators)
CONS: loss of mass information and cleanliness. In some cases (heavy nuclei)
focal plane counters clean up the data with a good sensitivity
18
Some Channel Selection Detectors
Argonne FMA
USA
Light charged-particle
detector Microball – 96 CsI
with photo diodes
USA
Jyvaskyla RITU
Europe
19
Calculate Reaction Rates
Y = Ib x N t x 
Reaction Yield:
[nuclei/s]
Ib = i/(eq); with i - electric current [A], q - charge state, e = 1.6 10-19 [C]
Nt = [Na /A] rx; with Na = Avagadros #, A = mass, r = density [g/cm3] & x =
thickness [cm] of the target
- reaction cross section [cm2] .... note 1 [barn] is 10-24 [cm2]
Accumulated data:
D = Y x Time x Efficiency
[counts]
A typical “close to the line of stability” experiment may have:
i=100 [nA], q=10, A=100, rx =10-3 [g/cm2], =1 [barn] & efficiency of 10 % produces
~3.8x104 [counts/sec], BUT
A typical “far from the line of stability” experiment may have:
=100 [nb], so the accumulated data is ~14 [counts/hour];
10 [pb] gives ~2 counts every 10 weeks!!!.....the present situation for producing the
heaviest elements
20
The basic knowledge
What we want to know?
Jp,K
 Excitation energy
Ex
excited state
a,b-,b,
EC,p,
fission,
g, ICC
 Quantum numbers
and their projections
 Lifetime
 Branching ratios
T1/2,es
Jp,K
How?
T1/2,gs
Eg
00
ground state
stable or,
a, b-, b, EC, p, cluster, fission
 By measuring properties of
signature radiations
21
What is Stable?
A surprisingly difficult question with a somewhat arbitrary answer!
CAN’T Decay to something else, BUT
CAN’T Decay is a Philosophical Issue
 Violation of some quantity which we believe is conserved, such
as Energy, Spin, Parity, Charge, Baryon or Lepton number, etc.
DOESN’T Decay is an Experimental Issue that backs up the beliefs
Activity: A=dN/dt = lN
Activity of 1 mole of material (6.02 x 1023 atoms) with T1/2=109 y (l2.2 x
10-17 s-1) is ~0.4 mCi (1 Ci=3.7 x 1010 dps) .... a blazing source!
 Current limit on proton half-life, based on just counting a tank of water
is T1/2> 1.5 x 1025 yr.
22
Stable Nuclei: Segre’s Chart
~280 Nuclei have Half-lives >106 years
So they are (quite) stable against:
Decay of their constituents (p,n)
Weak Decay (b+, b– and EC)
N
a decay
More complex cluster emission
Fission
(Mostly because of energy conservation)
Atomic Number, Z
23
Mean Lifetime
f decay (t ) 
Ae - lt
 le

- lt
- lt
Ae
dt

t
P n (t )   le -lt 'dt '
0
0
Probability for decay of a nuclear state
(normalized distribution function); l decay constant
t
1 - P n (t )  1 -  le
Probability that a nucleus will decay within time t

-lt '
dt ' e
-lt
0
Probability that a nucleus will remain at time t
 t     te dt 
0
-lt
1
l
The average survival time (mean lifetime - ) is then the
mean value of this probability
24
Half-life & Decay Width
T1/2: the time required for half the
atoms in a radioactive substance
to disintegrate
relation between , T1/2 and l

T 1/ 2 1

ln 2 l
6.58 10-16
 

 [ s]

[eV]
 |  1 | M | 2 |2
Determine the matrix element
describing the mode of decay
between the initial and final
state
25
log ft values in b-decay
log ft  log f  log t
t  T1b/ 2i 
T1/ 2
BRi
partial half-life of a given b(b+,EC) decay branch
f  f b  f n , n  0,1,2...
statistical rate function (phase-space
factor): the energy & nuclear structure
dependences of the decay transition
Decay
Mode
Type
log f
b-
allowed
b-
1st-forb
log f 0-  log( f1- / f 0- )
EC+b
allowed
log( f 0EC  f 0 )
f 0- , f1- , etc.
log f 0-
N.B. Gove and M. Martin, Nuclear Data Tables 10 (1971) 205
26
Hindrance Factor in a-decay
| I i - I f | L | I i  I f |
p ip f  (-1) L
Strong dependence on L
L=0 - unhindered decay (fast)
Exp
T1Exp
(
a
)
T
/ BR
HFi  / 2Theoryi  1/ 2 Theory i
T1/ 2
T1/ 2
T1Theory
/2
M.A. Preston, Phys. Rev. 71 (1947) 865
I. Ahmad et al., Phys. Rev. C68 (2003) 044306
27
g-ray decay
I ip i
| I i - I f | L | I i  I f |
p ( EL)  (-1) L
p ( ML)  (-1) L 1
electric multipole
magnetic multipole
Ei
L
pf
If
Ef
Eg  Ei - E f
dipole
quadrupole
octupole
E1:L=1,yes
E2:L=2,no
E3:L=3,yes
E4:L=4,no
E5:L=5,yes
M1:L=1,no
M2:L=2,yes
M3:L=3,no
M4:L=4,yes
M5:L=5,no
2-
0
L  0!
0
E0
1
hexadecapole
8
L  1,2 & 3
E1( M 2, E 3)
6
1
L  2...14
E 2( M 3, E 4...)
L 1
0
M1
28
Partial lifetime & Transition Probability
exp
T1g/ 2  T1exp
/
BR

T
/2
g
1/ 2 
 Ig i  (1  aTi )
T1exp
/2
Ei
Ig 1
Ig
Ig 2
Ig 3
partial half-life
ln 2
8p ( L  1)  Eg 
 
Pg ( XL : I i  I f )  g 
2 
T1/ 2 L2 L  1)!!  c 
Partial g-ray Transition Probability
B ( XL : I i  I f ) 
2 L 1
B( XL : I i  I f )
Reduced Transition Probability
I i M ( XL) I f
2
2Ii  1
contains the nuclear structure information
29
Hindrance Factor in g-ray decay
FW ( N ) 
B( XL)Theory
B( XL) Exp

T1g/ 2 ( XL) Exp
T1g/ 2 ( XL)Theory
Hindrance Factor: Weisskopf (W): based on spherical
shell model potential
Nilsson (N): based on deformed
Nilsson model potential
… usually an upper limit, but …
T1g/ 2 ( EL)W , sec
B ( ML )W , m N2 fm2 L - 2
T1g/ 2 ( ML )W , sec
EL
B ( EL)W , e 2 fm2 L
E1
0.06446 A2 / 3
6.762 A-2 / 3 Eg-3 10-15 M1
1.7905
E2
0.0594 A4 / 3
9.523 A-4 / 3 Eg-5 10-9 M2
1.6501A2 / 3
3.100 A-2 / 3 Eg-5 10-8
E3
0.0594 A2
2.044 A-2 Eg-7 10-2
M3
1.6501A4 / 3
6.655 A-4 / 3 Eg-7 10-2
E4
0.06285 A8 / 3
6.499 A-8 / 3 Eg-9 104
M4
1.7458 A2
2.116 A-2 Eg-9 105
E5
0.06929 A10 / 3
2.893 A-10/ 3 Eg-11 1011 M5
1.9247 A8 / 3
9.419 A-8 / 3 Eg-11 1011
ML
2.202Eg-3 10-14
30
Quadrupole Deformation
12+
deformed nucleus
10+
I
8+
6+
4+
I=Jw
2+
0+
156Dy
EI=2/2J I(I+1)
B(E2) ~ 200 W.U.
8.16  1013
B ( E 2)  5
[e 2b 2 ]
Eg [keV ] g [ ps ]
5
B( E 2; KI i  KI f ) 
Q02 I i K 20 I f K
16p
(from collective models)
b 2  -7
p
80

7pQ0
49p

80 6eZr02 A2 / 3
 g [ ps ]  (1.58  0.28) 1014E2-4 [keV ]Z -2A2 / 3

1
b2 
466  41
A  E2 [keV ]
1
31
2
Octupole Deformation
 E 3[ s]  0.012264  E3-7  [ B( E 3) ]-1
1
E3- [ MeV ]; B( E 3)  [e 2 fm6 ]
1
b3 
4p
ZR 3
B( E 3) 
e2
32
K-forbidden decay
w
Deformed, axially symmetric nuclei
K is approximately a good quantum number
each state has not only
Jp but also K
W1
W2 K=W1+W2
 The solid line shows the dependence of FW on
ΔK for some E1 transitions according to an
empirical rule: log FW = 2{|ΔK| - λ} = 2ν
 i.e. FW values increase approximately by a
factor of 100 per degree of K forbiddenness
 fn=(Fw)1/n – reduced hindrance per degree of K
forbiddenness
33
Experimental techniques
Others:
 Direct width measurements
 Inelastic electron scattering
 Blocking technique
 Mossbauer technique
34
Activity measurements
Time Range: a few seconds up to several years
A  dN / dt  lN  A0 e
- t /
 Statistical uncertainties are usually small
 Systematic uncertainties (dead time, geometry, etc.)
dominate
usually want to follow at least 5 x T1/2
Tag on specific signature radiations (a, b, ce or g) in a “singles” mode
Clock
Detector
Source
35
Activity Measurements: Example 1
198
Hg (g , n)197Hg
 More than 270 spectra were measured
 Followed 4 x T1/2
191.4
36
Activity measurements: Example 2
Si det
C foil
1 GeV pulsed proton beam on 51 g/cm2 ThCx target
on-line mass separation (ISOLDE)/CERN
H. De Witte et al., EPJ A23 (2005) 243
37
Very long-lived cases – Example 1
Time Range: longer than 102 yr
A  lN
T1/ 2  ln 2
N
A
the number of atoms estimated by other
means, e.g. mass spectrometry
38
Very long-lived cases – Example 2
Ap (t )
Ad (t )

ld
ld - l p
(1 - e
- ( ld - l p ) t
T1/2(250Cf)=13.05 (9) y
Ap , l p
)
daughter
250Cf
a
13 y
246Cm
parent
Ad , ld
a
4747 y
242Pu
a
5105 y
mass-separated source
alpha-decay counting technique
T1/2 = 4706 (40) years / Compared to values ranging from
T1/2=2300 up to 6620 years
39
Electronic techniques
Time Range: tens of ps up to a few seconds
The “Clock” - TAC, TDC (START/STOP); Digital Clock

“singles” – Eg-time, Ea-time

“coincidence” – Eg1-Eg2-t ; Ea1-Eg1-t
Ea1-Ea2-t
Difficulties at the boundaries: e.g. for very short– and very long-lived cases!
40
Prompt Response Function
 all detectors and auxiliary electronics show statistical fluctuations in the
time necessary to develop an appropriate pulse for the “clock”
 depend on the characteristics of the detectors: e.g. light output for
scintillators, bias voltage, detector geometry, etc.
 instrumental imperfections in the electronics – e.g. noise in the
preamplifiers
Some typical values
Detector
FWHM, ps
plastic scintillators
~100
BaF2
~100
Si
~200
Na(I)
~500
Ge
0.6-9 ns
41
Prompt Response Function: Ge detectors
F ( x, l ) 

 f (t , l ) P( x - t )dt
-
f (t , l )  le -lt (t  0)or  0(t  0)
PRF
decay
1
- ( z /  )2
1
P( z ) 
e 2
 2p
a schematic illustration
PRF depends on:
 the size of the detector
 the energy of the g-ray
42
Recoil-shadow technique
the shortest lifetime that can be measured is limited by the TOF
thin production target
43
One example: 140Dy experiment at ANL
54Fe
+ 92Mo @ 245 MeV
a2n channel, mass 140 only 5% from the total CS
44
Some of the equipments used …
1 70% Gammasphere HpGe detector
4 25% Golf-club style HPGe detectors
2 LEPS detectors
1 2"x2" Large Area Si detector
2"x2" Si Detector
45
140Dy:
Experimental Results
D.M. Cullen et al., Phys. Lett. B529 (2002)
T1/2=7.3 (15) ms
Similar results by the ORNL group, Krolas et al., PRC 65, 2002
46
Recoil-decay tagging
47
The Heart of RDT: DSSD
208Pb(48Ca,2n)254No
80 x 80 detector 300 mm strips,
Each with high, low, and delay line
amplifiers, for implant, decay, and
fast-decay recognition.
Data from DSSD showing implant
pattern 40 cm beyond the focal plane
48
a-a (parent-daughter) correlations
177Au
Implantation->Decay 1->Decay 2
within a single pixel
Implant
Timescale of Events
a1: 6.12 MeV
0
T1st
a2: 5.7 MeV
Time
T2nd decay
decay
F.G. Kondev et al. Phys. Lett. B528 (2002) 221
49
Neutron deficient nuclei
50
Odd-Z Au (Z=79) isotopes
F.G. Kondev et al. Phys. Lett. B528 (2002) 221
51
Odd-Z Au (Z=79) isotopes –sample spectra
52
a-g correlations
I. Ahmad et al., Phys. Rev. C68 (2003) 044306
53
Pulsed beam technique
the shortest lifetime that can be measured is limited
by the width of the pulsed beam
 Less effective, but …
 well defined “clock”
sensitive to in-beam and
decay events
“singles”: g-time
 coincidence: g-g-time
the longest lifetime that can be measured is limited
by the time interval between the beam pulses
54
Pulsed beam: g-time
F.G. Kondev et al. Phys. Rev. C54 (1996) R459
178Ta
176Yb(7Li,5n)
reveals the time history of levels above the
58 ms isomer !
55
Pulsed beam: g-time (short-lived)
The importance of PRF
depends on Eg for a single transition
175Ta
170Er(10B,5n)
F.G. Kondev et al. Nucl. Phys. A601 (1996) 195
56
Limitations: Pulsed beam g-time
 Complicated when more than one isomer is presented
 Complicated because of contaminations
 Limited time range – e.g. TAC (Ortec 567) – 3 ms
 Rate dependent time distortions
2
prompt
1
Gate 220 keV
1
Gate 350 keV
1
Gate 580 keV
prompt
 2  1
1
I sf
2
Gate 600 keV
57
Pulsed beam: g-g-time technique

“coincidence” – Eg1-Eg2-t ; Ea1-Eg1-t
Ea1-Ea2-t
58
g-g-time: decay of the 9/2- isomer in 175Ta
175Ta
170Er(10B,5n)
F.G. Kondev et al. Nucl. Phys. A601 (1996) 195
59
g-g-time: decay of the 21/2- isomer in 179Ta
179Ta
176Yb(7Li,4n)
Why there is a prompt component?
290
F.G. Kondev et al. Nucl. Phys. A617 (1997) 91
60
Centroid-shift technique: g-time
Time Range: near PRF
F ( x, l ) 


 f (t , l ) P( x - t )dt
-
M r ( F (t )) 
t
r
F (t )dt
  M1 ( F (t )) - M1 ( P(t ))
-
Introduced by Z. Bay, Phys. Rev. 77 (1950) 419
6 ch x 92 ps = 552 ps
179W
P.M. Walker et al. Nucl. Phys. A (1996)
61
Centroid-shift technique: g-g-time

Must be more careful!
The PRF depends on both Eg1 and Eg2
F.G. Kondev et al. Nucl. Phys. A632 (1998) 473
62
Coulomb excitations
Time Range: up to hundreds of ps
E  Vcb
Z1 Z 2 e 2

 1
v
19F(1
MeV) on 238U ~1.6
40Ar(152
MeV) on 238U ~130
63
Intermediate energy Coulomb excitations
  1.5(3) ps
64