production of exotic nuclear isomers in fragmentation and deep-inelastic reactions
Post on 23-Feb-2016
59 Views
Preview:
DESCRIPTION
TRANSCRIPT
Production of Exotic Nuclear Isomers in Fragmentation and Deep-Inelastic Reactions
Paddy ReganDept. of Physics, University of Surrey,
Guildford, GU2 7XH, UK e-mail: p.regan@surrey.ac.uk
Outline of Talk
• Where do you find (long-lived) isomers ?
• Restrictions ?
• Isomer predictions.
• Towards the neutron rich….
• Deep-inelastic reactions and results
• Projectile Fragmentation, effects of atomic stripping
What is an isomer ?
Metastable (long-lived) nuclear excited state.
‘Long-lived’ could mean
~10-19 seconds, shape isomers in alpha-clusters or
~1015 years 180Ta 9-->1+ decay.
Why/when do you get isomers?
If there is (i) large change in spin (‘spin-trap’)
(ii) small energy change
(iii) dramatic change in structure (shape, K-value)
What do isomers tell you ?
Isomers occur due to single particle structure.
Winnie the Pooh, (trapped by a potential barrier !)
A.A. Milne (1927)
Walker and Dracoulis, Physics World Feb. 1994
E0 (ec) decay
74Kr, shape isomer
High-spin, yrast-trap (E3) in 212Fr K-isomer in 178Hf
decay to states in 208Pb.
212Po, high-spin -decaying yrast trap. (also proton decaying isomers, e.g, 53Co PLB33 (1970) 281ff).T1/2=0.3s
Ex > 1 MeV, T1/2 > 1 ms (red), T1/2 > 1 hour (black)
From Walker and Dracoulis, Nature 399, p35 (1999)
Bohr and Mottelson, Phys. Rev. 90, 717 (1953)
NB. wrong spin for isomer !!! I>11 shown later to be K=8-, Korner et al. Phys. Rev. Letts. 27 , 1593 (1971) K-value and detailed spectroscopy very imporant in understanding isomers.
Search for long (>100ms) K-isomers in neutron-rich(ish) A~180 nuclei.
low-K high-K mid-K j
K
:rule sel. -K
Walker and Dracoulis Hyp. Int. 135 83 (2001)
(Stable beam) fusion limit makes high-K in neutronrich hard to synthesiseAlaga, Alder,
Bohr and Mottelson,Mat. Fys. Medd. 29 no 9 (1955)
‘Forbiddenness’ in K isomersCan use single particle (‘Weisskopf’) estimates for transitions rates for a given multipolarity.(E (keV) , T1/2(s), Firestone and Shirley, Table of Isotopes (1996). sAETM
sAETE
sETM
sAETE
EAT
W
W
W
W
/
83/2572/1
103/4562/1
13-352/1
153/2362/1
21
101.31010.32
103.0 1052.92
101.8 1020.21
106.1 1076.61
keV 500 180, for Estimates Weisskopf
Hindrance (F) (removing dependence on multipolarity and E is defined by
rates trans. Weisskopfand expt. of ratio 2/1
2/1
WT
TF
Reduced Hindrance ( f) gives yardstick for ‘goodness’ of K- quantum number andvalidity of K-selection rule (possibly a measure of axial symmetry?)
K
TTFf W ,
/1
2/1
2/1/1f~ 100 typical value for ‘good’ K isomer (see Lobner Phys. Lett. B26 (1968) p279)
Smith,Walker et al., in press Phys. Rev. C
‘Classic’ 31 yr 16+ isomer in178Hf , spin-trap + K-forbidden (NB. Idaho invention!)
Smith, Walker et al., in press Phys. Rev. C
170Dy, double mid-shell, best case yet for ‘pure’ K-isomer ? (see PHR, Oi et al. Phys. Rev. C65 (2002) 037302)
Ways to make 178Hf 31 yr isomer ?
• Neutron capture • 176Yb(,2n)178Hf • 176Yb(9Be,3n)178Hf (see Dracoulis talk)• Coulomb excitation (Hayes et al., PRL (2002))• Deep-inelastic heavy-ion binary reactions• Projectile Fragmentation
Astrophysical Consequences of Isomers
Ta is ‘stable’ in its isomeric state, but its ground state decays in hours!
Longstanding problem as to how the isomeric state is created in nature (via eg. S-process). Possible mechanism via heavier nuclei spallation or K-mixing of higher states in 180Ta.
(from Wiescher, Regan and Aprahamian Physics World, Feb 2002).
K=9- isomer might be de-excited to 1+ ground state through intermediate path with states of K=5+ (see Walker, Dracoulis and Carroll Phys. Rev. C64 061302(R) (2001))
Towards the Neutron-Rich ?Fusion-evap. great for high-spins, BUT….stable beams/targets create predominantly neutron-deficient nuclei.
Z (p
roto
ns)
N (neutrons)
A
B
C=A+B
Locus of -stablenuclei
Projectile Fragmentation Reactions
hotspot
Excited pre-fragment
Finalfragment
projectile
target
Energy (velocity) of beam > Fermi velocity inside nucleus ~30 MeV/uCan ‘shear off’ different combinations of protons and neutrons.Large variety of exotic nuclear species created, all at forward angleswith ~beam velocity. Some of these final fragments can get trapped in isomeric states.
Problem 1: Isotopic identification. Problem 2: Isomeric identification.
primary beamPb @ 1GeV/u
Production target
Central focus, S2Final focus, S4
E(Z2)
cueB
QA
FTO
catcher
degraderdegrader
dipole, B
scintscint
MW=x,y
scint(veto)Use FRS@GSI or LISE3@GANIL to ID nuclei.
Transport some in isomeric states (TOF~ x00ns).Stop and correlate isomeric decays with nuclei id.
eg. R. Grzywacz et al. Phys. Rev. C55 (1997) p1126 -> LISE C.Chandler et al. Phys. Rev. C61 (2000) 044309 -> LISE M. Pfutzner et al. Phys. Lett. B444 (1998) p32 -> FRS Zs. Podolyak et al. Phys. Lett. B491 (2000) p225 -> FRS M. Pfutzner et al. Phys Rev. C65 (2002) 064604 -> FRS
In-Flight Technique Using Projectile Fragmentation
C. Chandler et al. Phys. Rev. C61 (2000) 044309
67Ge
69Se
76Rb
92Mo fragmentation on natNi target
74Kr isomer from 92Mo fragmentationat GANIL. 456 keV 2+->0+ transitions decays (a) too fast (500 ns flight time) & (b) too slow for measured value of 2+ state (~25 ps) ?
undressing (to fiddle the decay probability)
0+
2+
0+
456 keV gamma
eEEEtot ,2,20
E0, 0+->0+
e- conversiondecay
Ex=509 keV, T1/2~20 ns
Fully stripping the nucleus of its atomic electrons (in-flight) ‘switches off’ the electron conversion decay branches.Result is that the bare nuclear isomeric lifetime is increased compared to ‘atomic’ value. (important in explosive stellar scenarios).
from Bouchez et al., Phys. Rev. Lett. 90 082502 (2003)
208Pb beam at 1 GeV/u allows production of (a) neutron-rich heavy (A>160) and (b) high-spin isomers, Schlegel et al.Physica Scripta T88 (2000) p72
High spins (>35/2) populated
Gamma-gamma analysis on 200Pt isomer (21 ns!), M. Caamano et al. Nucl. Phys. A682 (2001) p223c; Acta Phys. Pol. B32 (2001) p763 stripping effect again (a la 74Kr).
M. Pfutzner, PHR et al. Phys Rev. C65, 064604 (2002)Higher spins for greater A.
Can not use fusion-evaporation reactions to study high-spin states in beta-stable and neutron-rich systems.
Use deep-inelastic reactions.
Z
N
Ebeam ~15-20% above Coulomb barrier
beam
target
(i) (ii) (iii)
-1
cos1
by calculated then is correctionDoppler The
coscoscoscossinsinsinsin)cos( where
)cos(r.r by given is angleray -fragment/ the
k )cos( , j )sin()sin( ,i )cos()sin(
k, and j i, rsunit vectoCartesian For
2
2,1'
2121212112
122121
1,2
EE
rr
rzryrx
z
x
y
Simon et al., Nucl. Inst. Meth. A452, 205 (2000)
BLF
TLF
beam tlftlf
blfblf
Ge
TOF ~5-10 ns.ns-s isomers can de-excite in bestopped by CHICO position detector. Delayeds can still be viewedby GAMMASPHERE.
Rochester Group
198Pt +136Xe @ 850 MeV, Dobon, Wheldon, PHR et al.,
33 ns isomer in 195Os (last stable 192Os), useful test of structure in prolate/oblate shape coexistence region. 194Os Wheldon et al. Phys. Rev. C63 (2001) 011304(R)
First id of ‘doubly mid-shell’ nucleus, 170Dy (N=104, Z=66). K=6+ isomers predicted for well deformed N=104 nuclei. TRS calcs (F.Xu) predict a very ‘stiff’, highly deformed prolate nucleus. Could be the best K-isomer?
Data from M.Caamano et al.
Target-like fragment isomers from 136Xe+198Pt DIC, Valiente-Dobon et al., (Surrey/Rochester/Berkeley/Manchester/bPaisley/Daresbury collaboration)
Target-like fragment isomers from 136Xe+198Pt DIC, Valiente-Dobon et al., (Surrey/Rochester/Berkeley/Manchester/bPaisley/Daresbury collaboration)
• Isomers in Nature, nuclear astrophysics aspects– 26Al in r-p processed path, inversion of states – 180Ta, nature’s only ‘stable’ isomer (nuclear battery ?)– 176Lu, cosmic chronometer and thermometer– r-process path and structure of odd-odd nuclei
• Production and identification of isomers ? – Fusion-evap, projectile frag. Deep-inelastics, spallation,
neutron capture…– electronic timing, proj. frag. – Mass separation for long-lived isomers
• Cheating with isomer half-lives….undressing!– 74Kr (GANIL) bare, 201,200Pt (GSI) H-like
• Summary of some ‘special’, exotic cases!– 178Hf K-isomer with many branches….e.g., E5 decays.– 176Lu, cosmothermoter for two phases in s-process.– 26Al decay seen from space as example of nucleosynthesis,
rp-process ‘by-pass’.– Nuclear batteries/gamma-ray lasers, can we de-excite the
isomers ? (180Ta paper by PMW, GDD, JJC; 178Hf 31 yrs state?).
– Lengthing the half-life…stripping of 74Kr, 201Pt etc.
Thanks!• Bertram Blank (Bordeaux) et al., GANIL• Zsolt Podolyak (Surrey) et al. GSI • Carl Wheldon (Surrey/GSI) Berkeley expts.• Surrey PhD students, Katie Chandler, Jose Javier Valiente-
Dobon, Monica Camaano, Arata Yamamoto, Sareh Al-Garni for hours and hours of analysis etc.
• Physics comments/help from Phil Walker, Bill Gelletly (Surrey), Dave Warner (Daresbury) + many others!
• Money from EPSRC (UK)
100Mo + 136Xe @ 750 MeV GAMMASPHERE + CHICO,PHR et al. Submitted to Phys. Rev. C. (Surrey, Rochester, Berkeley, Manchester)
TLFs
BLFs
elastics
0
10
20
30
40
50
%>Ecoul
Ltlf (roll)
v/c graz tlf
Linear(%>Ecoul)
0102030405060
620 648 677 705 733 761 790E_beam (MeV)
blf_graz
tlf_graz
lmax/10
Kinematics and angular mom. input calcs (assumes ‘rolling mode’) for 136Xe beam on 100Mo target.
Estimate ~ 25hbar in TLFfor ~25% above Coul. barrier. For Eb(136Xe)~750 MeV, in labblf~30o and tlf~50o.
100Mo +136Xe (beam) DIC calcs.
Identification of new isomeric state in 136Ba, N=80 isotone.
N=80 isotonic chain, 10+ isomers(h11/2)-2
I=10+
Structure of 8+ final state changes from 134Xe -> 136Ba ?
Isomer decayalso depends on structureof final state
N=80, (h11/2)-210+ isomers
136Sb
135Te
Use FRS to select projectile fission products (forward boosted ones). Note transmission a few %.
T1/2=565(50) ns state in 136Sb (Z=51, N=85)
M. Mineva et al. Eur. Phys. J. A11 (2001) p9-13
Online-Mass Separation Technique
Select by massSelect by decay times
Lifetimes from grow-in curve
Surrey/GSI/Liverpool,136Xe+Tanat
A=184
A=185
A=186
A=183
A=182
136Xe @11.4 MeV/u on to 186W target in thermal ion source (TIS), tape speed 160 s.
Mass selection achieved using dipole magnet in GSI Onlinemass separator (ASEP).
keVEQQeEAu
QevAuB
60 ,1
2.
Z selection by tape speed to remove activity.
See Bruske et al. NIM 186 (1981) p61
S. Al Garni, PhD thesis, Surrey (2002) Surrey/GSI/Liv./Goettingen/Milano
Gate on electron ( or ec) at implantation point of tape drive, gives ‘clean’ trigger. Use add-back
Use grow-in curve techniqueR=Ao(1-exp(t/
Select cycle length for specific , add together multiple tape cycles.
Basic Technical Requirements for Studies with Isomers
• Beam pulsing, good t=0 reference for short (ns) lifetimes.
• In-flight separator (eg. FMA, LISE, FRS...) for ~microsecond-ms decays.
• Tape drive/helium jet system for 10ms->hours lifetimes
• Traps, cyclotrons etc. for longer lived species
Full-sky Comptel map of 1.8 MeV gammas in 26Mg following 26Al GS -decay.
(a) Spin traps, eg. 26Al, (N=Z=13) 0+ state -decaying spin-trap.
5+, T=0 0 keV, T1/2=7.4x105 yrs
0+, T=1 228.3 keV, T1/2=6.3 secs(decays direct to 26Mg GS via superallowed Fermi+…forking in rp-process
(decays to 2+ states in 26Mgvia forbidden, l=3 decays).
e.g., Diehl et al., Atron. Astrophys 97, 181 (1993); Publications of the Astr. Society of the Pacific 110:637 (1999)
26Al isomer and the rp-process
5+
0+
3+
1+
0228
417
1058
2070 2+
t1/2=6st1/2=0.7My
Thermal photonsequilibrate isomer and ground state populations for T>5x109K. see Runkle et al., Astr.J. 556, 970 (2001)
T<4x108K not in equilibrium, must be modelled as separate isotopes in rp-process path.Jose et al., Astr. J. 520, 347(1999)
rp-process can bypass 26Al ground state, decays via isomer
Mg)(Al)(Si),(Al),(Mg 26026262524
pp
gs26226gs262525 Mg)MeV8.1(Mg)(Al),(Mg)(Al
xEp
Yield of 1.8 MeV s from 26Algs decay (e.g., relative to 22Na decay)gives insight into T and where rp-process forming 26Al occurs.
How do you produce and measure (high-energy) isomers ?
• Produce via nuclear reaction e.g., fusion-evaporation, deep-inelastic, projectile fragmentation…..
• Isomeric targets ? (see A.Tonchev NIM paper).
• Isomeric ‘beams’– Measure, depending on lifetime using
• ns : Use in-beam electronic techniques (eg. start-stop) • ns -> ms: In-flight technique, projectile fragmentation.• 100 ms -> hours: On-line mass-separator (eg. GSI set-up).• > hours: Measure mass differences from ground state using e.g. ion traps, coupled
cyclotrons etc.
In-beam, electronic technique (t)eg, PHR, G.D. Dracoulis et al. Nucl. Phys. A586 (1995) p351
Fusion-evaporation reaction with pulsed beam (~1ns), separated by fixed period (~500ns). Using coincidence gamma-rays to see across isomer
94Zr+16O-> 110Cd*->103Pd+3n
Proton drip line isomer physicsfrom 208Pb fragmentation.N=74 chain of K=8- isomers.Next in chain would be 140Dy, proton decay daughter of (deformed) 141Tb.(See Filip Kondev’s talk)
136Sm, 138Gd Isomers orginally seen in fusion-evap (ANU data)A.M.Bruce et al. Phys.Rev. C50 (1994) p480and C55 (1997) p620
Heaviest odd-odd,N=Z gammas, isobaric analog states ? N=Z=43; 86Tc, C. Chandler et al. Phys. Rev. C61 (2000) 044309
~500 86Tc in ~ 1 week
8+ isomer in 78Zn, real evidence of 78Ni shell closure.J.M.Daugas et al. Phys. Lett.B476 (2000) p213
iitot
i
fii
effimp
tot
b
ttG
TOFTOF
FGbNN
R
1
expexp1
1
expF
, )1(
0
2
2
22
1
11
Isomeric Ratio Calculations
M. Pfutzner et al. Phys Rev. C65, 064604 (2002)
2
3/22
22
2)1(exp
ratio,isomer predicts model off-cut sharp
13
3210178.0
, 2
)1(exp2
12
fJ jth
p
ppf
ffjj
JJdJPR
AAAA
A
JJJP
m
M. de Jong et al. Nucl. Phys. A613 (1997) p435
M. Pfutzner et al. Phys Rev. C65, 064604 (2002)
max31blfmax
31tlf
max
3/13/1
0
221
max
1
172
1
172
fragments. twoebetween th mom. ang. relative the
and , intosplit is limit, mode rolling In the
25.12
cosec1.4
where, approach,closest of distance by thegiven ismax. issection -cross DIC the whereangle The
. and 219.0
is mom. ang. peripheral max. y theclassicall-Semi
l
AA
ll
AA
l
lll
fmAAEeZZd
dgrazing
AAAAVERl
B
T
T
B
blftlf
TBgraz
k
TB
TBCMCM
Bock et al., Nukleonika22 (1977) 529
top related