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In-beam gamma-ray spectroscopy with fast beams of rare isotopes Experiments to elucidate the structure of very exotic nuclei Thomas Glasmacher National Superconducting Cyclotron Laboratory Michigan State University. Examples of nuclear structure observables with fast beams - PowerPoint PPT Presentation
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INPC 2007 –Slide 1
In-beam gamma-ray spectroscopy withfast beams of rare isotopes
Experiments to elucidate the structure of very exotic nuclei
Thomas GlasmacherNational Superconducting Cyclotron Laboratory
Michigan State University
1. Examples of nuclear structure observables with fast beams2. Discovery of new isotopes3. The boundaries of the Island of Inversion4. Collectivity in silicon and sulfur towards N=28
INPC 2007 –Slide 2
Goal: Develop predictive theory of atomic nuclei and their reactions
• Stable nuclei are well-described with closed-shell (so-called “magic”) nuclei as key benchmarks
• Rare isotopes show evidence for– disappearance of canonical shell
gaps,– appearance of new shell gaps,– reduction of spin-orbit force
• The most exotic nuclei accessible (largest N/Z ratio) are light nuclei and they have low beam rates
• New precision techniques have been developed in past decade to enable spectroscopy of these most exotic nuclei
• Experimental task: Quantify changes encountered in rare isotopes and measure with defined certainty observables that are calculable and can be used to discriminate between theories
INPC 2007 –Slide 3
In-flight production of rare isotopes
Driver accelerator (v = 0.25-0.6 c) Fragment separator (A1900 NSCL, FRS GSI, BigRIPS RIKEN, ALPHA spectrometer/LISE GANIL) Identification and beam transport
Stopped beam experiments, reaccelerated beam experiments Fast beam experiments
Secondary reaction Reaction product identification (S800 spectrograph, CATE/Aladin, Silicon telescopes/TOF wall, SPEG)
Fragment separator A1900 fragment separator
Identification and beam transport
Reaction product identification S800 spectrograph
INPC 2007 –Slide 4
In-flight production of rare isotopesExample: 78Ni from 86Kr at NSCL
Driver accelerator Fragment separator (A1900 NSCL, FRS GSI, BigRIPS RIKEN, ALPHA/LISE GANIL) Identification and beam transport
Stopped beam experiments Fast beam experiments
Secondary reaction Reaction product identification (S800 spectrograph, CATE/Aladin, Silicon telescopes/TOF wall, SPEG)
Fragments at focal plane
Fragments at production target
Fragments at achromatic degrader
65% of theproduced 78Nitransmitted
INPC 2007 –Slide 5
Examples of observables in experiments within-flight produced exotic beams
Rates are within 1-2 orders of magnitude depending on implementation and structure of nucleus
Stopped beam experiments• Possible experiments and quality Possible experiments and quality
of observables depend on number of observables depend on number of beam particlesof beam particles
• * Discovery (few particles total)
• Half live(few ten particles)
• Mass (few hundred particles)
Fast beam experiments• Possible experiments and quality Possible experiments and quality
of observables depend on of observables depend on luminosityluminosity
• Interaction radii(few particles per hour)
• g-factors(hundreds of particles per second)
Fast beam experiments (cont’d)• Excited state energies
– * Multi-nucleon removal(hundreds of particles per second)
• Excited state energies, transition matrix elements
– * Inelastic scattering: Coulomb excitation, p-, scattering(ten particles per second)
– Lifetimes from -ray line shapes(hundreds of particles per second)
• Occupation numbers– * Single nucleon removal
(particles per second)– Two-nucleon removal
(hundreds of particles per second)– * Nucleon addition
(hundreds of particles per second)
INPC 2007 –Slide 6
Fast-beam -ray spectroscopy at rates of a few nuclei per second
• NRNT × NB × – NR reaction rate (yields
observable)– NT atomic density of target– NB beam rate– cross section (given by
nature)
• Fast exotic beams allow for– event-by-event identification and
trigger– efficient detection of recoiling beam
particles due to kinematic focusing– thick secondary targets (100-1000
thicker than at low energy)• Strategy: Use of photons to indicate
inelastic scattering, measure particle- coincidences
• Example– = 100 mbarn– NT = 1021 cm-2
– NB = 3 Hz
– NR = 26/day = 3×10-4 Hz
beam targetscatteredbeam
INPC 2007 –Slide 7
Direct reactions with heavy ions and coincident -ray spectroscopy
M. Ishihara, Nucl. Phys. A 400 (1983) 153c
INPC 2007 –Slide 8
Direct reactions with heavy ions and coincident -ray spectroscopy
M. Ishihara, Nucl. Phys. A 400 (1983) 153c
INPC 2007 –Slide 9
Intermediate-energy Coulomb excitation of 32MgLarge transition matrix element indicates breakdown of the N=20 shell gap
Experiment:
Max. determines min. impact param.
T. Motobayashi et al., Phys. Lett. B 346 (1995) 9
T.G. Ann. Rev. Nucl. Part. Sci. 48 (1998) 1
Impact parameter
Scattering angle
Coul = 87 mb B(E2↑) = 454(78) e2fm4 = <0+||O(2)||2+>
Shell model
Valence nucleonmodel
20181614N 12
INPC 2007 –Slide 10
Intermediate-energy Coulomb excitation
• Active programs at GANIL, GSI, MSU and RIKEN
• Published measurements to elucidate N=8, 20, 28, 32, 34, N=Z=36, light tin isotopes, …
• Single-step process• Sensitive to E1, E2, E3
excitations• Results agree with prior
measurements where available
-50
-25
0
25
50
75
100
125
150
175
200
225
250
40Ar 36
Ar24
Mg30
S78
Kr 58Ni
76Ge
26Mg
Intermediate-energyCoulomb excitationAdopted value
Adopted and measured B(E2) values for stable nuclei
Adopted valueCoulomb excitationResonance fluorescenceDoppler shift attenuationRecoil distributionElectron scatteringIntermediate-energyCoulomb excitation
Mg26
B(E2) values fromdifferent methods for 26Mg
J. Cook et al., Phys. Rev. C 73 (2006) 024315.
Diff
. b
tw.
me
asu
red
an
d a
do
pte
d B
(E2)
(%
)
INPC 2007 –Slide 11
Wavefunction spectroscopy with knock-out reactions
• Shape of recoil momentum distribution of the A-1 fragment is measured and sensitive to l-value of knocked out nucleon
• Individual final states are tagged by -rays
• Spectroscopic factors are determined from the population of excited states in the A-1 fragment.
),(),(),(
),(),()( 2
ndiffrspn
stripspnsp
jnsp
BjBjBj
BjInjSCnI
P.G. Hansen and J. A. Tostevin, Ann. Rev. Nucl. Sci. 53 (2003) 219 P4-1 (Wed)
INPC 2007 –Slide 12
Breakdown of the N=8 shell gap in 12Be12Be ground state only 32% (1s1p)8 and 68% (1s1p)6-(2s,1d)2
1/2+
1/2-
5/2+ d5/2
p1/2 17.5(26) mb
s1/2 32.0(47) mb
12Be 11Be+n+
INPC 2007 –Slide 13
Exploring the boundaries of Island of Inversion
• C. Thibault et al: PRC 12 (1975) 64631,32Na, local increase of S2n N=20 shell gap decreased
• X. Campi et al: NPA 251(1975) 193explained by deformation via filling of (f7/2)
• C. Detraz et al: PRC19 (1979) 16432Na 32Mg decay, low 2+ in 32Mg
• E. Warburton et al: PRC 41 (1990) 1147Z=10-12 and N=20-22 have ground state wavefunctions dominated by (sd)-2(fp)+2 configurations
• T. Motobayashi et al: PLB 346 (1995) 9large B(E2;0+ 2+) in 32Mg
See also • V. Tripathi et al. Phys. Rev. Lett. 94 (2005)
162501 (2005)• G. Neyens et al. Phys. Rev. Lett. 94 (2005)
022501 (2005)• A. Obertelli et al., Phys. Lett B 633 (2006) 33 • H. Iwasaki et al., Phys.Lett. B 620 (2005) 118
• Single-neutron removal to look for negative parity states, indicating the presence of f7/2 intruder configurations in the ground state of the parent: (30Mg,29Mg+) (26Ne,25Ne+) (28Ne,27Ne+)
• (38Si,36Mg+) two-proton removal
INPC 2007 –Slide 14
Single-neutron removal from 26,28Ne and 30Mg
25Ne level structure known• A.T. Reed et al., PRC 60 (1999) 024311• S. Padgett et al., PRC 72 (2005) 064330
• 25Ne is well-described in sd shell with no evidence for intruder states below 3 MeV
→ No evidence for intruder configurations in the ground state wave function of 26Ne
J.R. Terry et al., Phys. Lett. B 640 (2006) 86
→ Island of inversion is growing
INPC 2007 –Slide 15
Proton scattering in inverse kinematics
• Many experiments at GANIL (MUST detector)
• Few experiments at MSU• Thin target to avoid angular and
energy straggeling of low-energy protons
p(56Ni,p’) at GSI G. Kraus et al., PRL 73 (1994) 1773
Normal kinematics
Inverse kinematics
F. Maréchal et al., PRC 60 (1999) 034615
INPC 2007 –Slide 16
Thick-target -tagged inelastic proton scattering Pioneering experiment at RIKEN p(30Ne,30Neg)
Y. Yanagisawa et al. PLB 566 (2003) 84
10
11
12
13
14
15
16
17
18
19
16 20 24 28
36Si setting
40Si setting
•
Inelastic p scattering using two cocktail beam settings to study chains of isotopes accessible in a consistent approach
Determine excited state energies and collectivity towards N=28 Univ. Tokyo, RIKEN, Rikkyo Univ., Michigan State Univ.
collaboration
40Si
N
Z
36Si
Sulfur
SiliconRIKEN/Kyushu/Rikkyo LH2 target:H. Ryuto et al., NIM A 555 (2005) 1
30 mm diameter 9 mm thick, nominal Negligible contaminant reactions
C.M. Campbell Ph.D. thesis (2007) Michigan State Univ.
INPC 2007 –Slide 17
Evolution of 2+ energies towards N=28
Above Z=20Ti, Cr and Fe
Below Z=20Si, S and Ar
40Si986(6) keV
• 1.5 million 40Si beam particles
• p,p’ = 20(3) mb
• ~30 counts in the photopeak• Low energy indicative of neutron
collectivity in 42Si
C.M. Campbell et al., PRL 97 (2006) 112501
INPC 2007 –Slide 18
Evolution of 2+ energies towards N=28
Above Z=20Ti, Cr and Fe
Below Z=20Si, S and Ar
40Si986(6) keV
• 1.5 million 40Si beam particles
• p,p’ = 20(3) mb
• ~30 counts in the photopeak• Low energy indicative of neutron
collectivity in 42Si
C.M. Campbell et al., PRL 97 (2006) 112501 Discovery of low-lying 2+ state in 42Si at GANIL B. Bastin, S. Grévy et al. arXiv 0705.4526 F9-2 (Wed)
INPC 2007 –Slide 19
Higher excited states in 40Si
• Two additional -rays in fragmentation channel 42P-p-n
– States at 1.62(1) MeV and 1.82(1) MeV?
• Not enough statistics to test if the two transitions are in coincidence
• Independent of the sequence, the second excited state is low in energy
• In the shell model second excited state is ph-excitation across N=28
• Low energy supports narrowing of the N=28 gap in 40Si
C.M. Campbell et al., PRL 97 (2006) 112501
INPC 2007 –Slide 20
1.5
2
2.5
3
3.5
20 22 24 26 28N
E(4
+1)
/ E(2
+1)
Sulfur
0.1
0.2
0.3
0.4
0.5
16 20 24 28
2| (p
,p')
Evolution and character of collectivity
Prior data from F. Maréchal et al, PRC 60, 034615 (1999)P.D. Cottle et al., PRL 88, 172502 (2002)
Silicon
0.1
0.2
0.3
0.4
0.5
16 20 24 28
2|
(p,p
')
NN
Rotational limit
Vibrational limit
Sulfur
Silicon
• Enhanced collectivity towards N=28 in silicon
• Sulfur transitions from vibrational mid-shell to rotational towards N=28
• Silicon vibrational mid-shell
C.M. Campbell Ph.D. thesis (2007) MSU
INPC 2007 –Slide 21
Single-proton addition
• Traditionally performed in normal kinematics at lower beam energies, e.g. (d,n), (3He,d), (,t), …
• Few mb cross sections observed at intermediate energies
• Proton well-bound in 9BeSp(9Be) = 16.9 MeV
• 8Li has only one bound excited state
Beam identification Reaction product identification
INPC 2007 –Slide 22
Measure coincident -rays and particle momenta
INPC 2007 –Slide 23
Resulting strength distribution compared to normal kinematics measurement
A.M. Mukhamedzhanov et al. Phys. Rev. C 73 (2006) 035806
A. Gade et al. in preparation (2007)
INPC 2007 –Slide 24
Structure of 23F: Multiple reactions in one experiment~35 MeV/nucl,100 mg/cm2 liquid helium target at RIKEN
A. Michimasa et al. Phys. Lett. B 638 (2006) 146
4He(22O,23F) 2000/sProton addition
4He(23F,23F) 550/sInelastic scattering
4He(24F,23F) 470/sneutron-knockout
4He(25Ne,23F) 1200/stwo-proton knockout
INPC 2007 –Slide 25
Summary and outlook• Discovery of new isotopes 40Mg, 42Al, 43Al, 44Si at NSCL• In-beam gamma-ray spectroscopy with fast beams
– Enables further scientific reach through luminosity gains of 100-1000 for a given driver accelerator
– Has in past decade transitioned into a worldwide effort with a set of robust experimental techniques that have contributed to many discoveries
• As new powerful drivers have come into operation in Japan, are under construction in Europe, and are being considered in United States (→ Prof. Tribble J4-4 Wed) this field will gain 1-2 orders of scientific reach from new detectors such as GRETA and AGATA
•
GRETA, I.Y. Lee, Lawrence Berkeley Lab GRETINA in front of S800 www.nscl.msu.edu/isf
INPC 2007 –Slide 26
Collaborators
P. Adricha), N. Aoic), D. Bazina), M. D. Bowena,b), B. A. Browna,b), C. Campbella,b),J. M. Cooka,b), D.-C. Dincaa,b), A. Gadea,b), M. Horoid), S. Kannoe), K. Kemperk), T. Motobayashic), A. Obertellia), T. Otsukaa,c,h), L. A. Rileyf), H. Sagawag), H. Sakuraic), K. Starostaa,b), H. Suzukih), S. Takeuchic), J. R. Terrya,b), J. Tostevini), Y. Utsunoij), D. Weisshaara), K. Yonedac), H. Zwahlena,b)
a) National Superconducting Cyclotron Laboratory, Michigan State Universityb) Department of Physics and Astronomy, Michigan State Universityc) RIKEN Nishina Center for Accelerator-Based Scienced) Physics Department, Central Michigan Universitye) Department of Physics, Rikkyo Universityf) Department of Physics and Astronomy, Ursinus Collegeg) Center for Mathematical Sciences, University of Aizuh) Department of Physics, University of Tokyoi) Department of Physics, University of Surreyj) Japan Atomic Energy Research Institutek) Physics Department, Florida State University