Chargeexchange reactions and exploding stars

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Charge-exchange reactions and exploding stars

• Nuclear Astrophysics Pizza Lunch
• February 9, 2004
• Remco Zegers

CE-club_at_NSCL Sam Austin, Daniel Bazin, Arthur
Cole, Wes Hitt, Hendrik Schatz, Brad Sherrill,
Meredith Howard (OSU)
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outline

• Electron-capture and ?-decay in supernovae
• Type Ia Supernovae
• Type II (core-collapse) supernovae
• Experimental techniques for measuring
  Gamow-Teller strength in
• stable nuclei
• unstable nuclei
• Some other applications for charge-exchange in
  inverse kinematics
• ?-process
• neutron-skin thickness

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Type Ia supernovae
white dwarf in binary stellar system
high H accretion rates
growing C/O white dwarf
Chandrasekhar mass (1.4 Msolar) ? contraction
nova_at_pbs
high central density?C fusion
thermonuclear explosion
nova_at_pbs
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Type Ia Supernovae

• A degenerate electron gas (pressure is
  independent of T) with high Fermi energy is
  present? efficient electron capture depending on
• electron-capture rates (40 pf-shell nuclei 60
  protons)
• flame propagation speed (how long at high T?)
• central density (Fermi energy)

YeltZ/Agt is reduced
isotopic composition of ejected material is
altered
if Ye becomes very low (neutron-rich nuclei) the
reverse process (?-decay) becomes important
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Type II (core-collapse) supernovae
Massive star Fe is fusion endpoint
electron capture?pressure drop
collapse
density increases
core bounces back
shockwave ?heating ?expansion
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SnII pre-collapse stage
electron capture reduces the number of
electrons ?-decay increases the number of
electrons
changes YeltZ/Agt
both produce neutrinos carry energy/entropy
from the core
pressure support
Langanke Martinez-Pinedo
Up to pf shell nuclei (A55-65) are
important both stable and unstable
Ye
time till bounce
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SnII Collapse stage
Since neutrinos carry away entropy, composition
is dominated by nuclei and not nucleons.
temperatures and densities are large enough to
maintain nuclear statistical equilibrium (for
given Ye nuclei with highest binding are favored)
electron capture?Ye decreases ?neutron-rich and
heavy nuclei (?-decay)
nuclei with A65-112, including Ngt40 and Zlt40
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EC and ?-decay

• Allowed transitions
• Fermi t? S t?(i) gt DL
  DS 0, DT 0, 1
  0 ? 0 (IAS dominates) Sum Rule Sb- -
  Sb N-Z
• ?-decay
• Gamow-Teller st? S s(i)t?(i) gt DL 0, DS
  1, DT 0, 1 0 ? 1 (Giant resonances)
  Sum Rule Sb- - Sb 3(N-Z)
• EC and ?-decay

strength
cross section in charge- exchange
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charge-exchange reactions
90Zr(p,n)
GT-Strength is distributed, with maximum at GT
resonance
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EC and ?-decay in stellar environment

• Electrons in degenerate gas are sufficiently
  energetic to populate the GTR GT
• F and GT- outside the Q window phase space for
  electron is blocked by electron gas
• due to finite temperature excited states are
  thermally populated and connect to low-lying
  states in the daughter with increased phase space
  (URCA)

GT-
F
GT
Langanke Martinez-Pinedo
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Theory

• Weak interaction rates by Fuller,Fowler,Newman
  (FFN) (1980-1985)
• experimental info from ground-state to low-lying
  excited states
• add collective strength via single state
  representation determined via independent-particle
  model (IPM)

• Experimental results (p,n) and (n,p) indicate
• strong quenching for medium-heavy nuclei
• strong fragmentation
• universal quenching factor
• Need shell-model calculations (Brown,Wildenthal,
  1988)
• take into account residual interactions between
  valence nucleons

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schematics
Example 56Ni28?56Co27
realistic shell model
IPM
f5/2
f7/2
Quenched
protons
neutrons
protons
neutrons
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Large-scale shell-model calculations (A55-65)
FFN (IPM model) data (n,p) (TRIUMF)
Caurier et al. (1999) SM
Caurier et al. folded with experimental
resolution
Quite good, some problems and what about
unstable isotopes?
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New EC/?-decay rates
?-decay
EC
Martinez-Pinedo Langanke (shell-model) FFN
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Effect on pre-collapse stage of SNII
lower core entropies smaller iron-core mass
LMP
Counteracts Ye reduction Cooling, reduction of
entropy
WW Woosley, Weaver (FFN) LMPLanganke,
Martinez-Pinedo Heger, Langanke (SM)
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Collapse stage of SnII

• EC in nuclei with A65-112, including Ngt40 and
  Zlt40
• Before EC is treated schematically (IPM for a
  few representative nuclei) For Ngt40 Zlt40 No EC
• residual interaction/finite temperature Pauli
  unblocking

IPM
Realistically
0g7/2
0g9/2
1p1/2
0f5/2
1p3/2
pauli-blocked
pauli-unblocked
example 74Ge?74Ga
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Effect on collapse stage of SnII

• EC capture rates calculated via shell-model
  Monte Carlo (level densities too high for
  state-by-state calculations SMMC?T-dependent
  occupation numbers ?RPA for EC) Koonin,Dean,
  Langanke, Kolbe

SMMC
but Yp small
Ye
IPM Bruenn et al
ve
enclosed mass (Msol)
Langanke et al.
location of shock formation is shifted
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Effect on SnIa
Brachwitz et al.
FFN SMMC SMMCextrapolation
New results less neutron rich at same ignition
densities Together with measured abundances
postulating that 55 of Fe-group elements in the
Galaxy come from SnIa?new constraints on ignition
density
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Determining the GT strength

• Experimental
• ground-state ?-decay minor part of strength
• ?-capture difficult
• charge-exchange! but need too many
• Cannot measure from thermally excited states

• Calculations are necessary to cover wide mass
  range.

Use experimental values where possible Test
theory with experiment
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CE reactions
?Tz-1
?Tz0
?Tz1
(n,p) (d,2He) (t,3He) (7Li,7Be) Heavy ion CE
(p,n) (3He,t) Heavy ion CE
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Measuring GT strength

• proven to work well for (p,n) (but there are a
  few exceptions)
• for other reactions proof is less complete but
  under certain conditions seems to work well.
• one needs calibration points (e.g. ?-decay where
  available)

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B(GT)-? linearity and CE probes for stable nuclei

• Away from q0 Tensor-? component of
  nucleon-nucleon (NN) interaction
• Multi-step contributions to the cross section
• If probe not uniquely spin-flip, choose Ebeam so
  that ??-component of NN-interaction gtgt
  ?-component
• Choose sufficiently high beam energy (gt 100
  MeV/n)

• In general light-ion probes are preferred over
  heavier ions
• because reaction mechanisms are simpler.
• (n,p) simplest, but bad experimental resolution
• (d,2He) high-resolution and spin-flip only but
  complex probe 2He is unbound?possible linearity
  breaking
• (t,3He) relatively straightforward, but triton
  beam
• Heavy-ion complex but some possible advantages

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NN-Interaction
voc
vT? if qgt0 roughly follows V?
V(q0) (MeVfm3)
v??
v?
v?c
Love Franey
E/A (MeV)
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(d,2He)

• 3S1 deuteron ? 1S0 di-proton if relative p-p
  energy is below 1 MeV
• Pure spin-flip probe but
• complex final-state distribution is mass and q
  dependent and not well-known? linearity breaking
• d-breakup background Agt90 difficult

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GT probes for stable nuclei

• (n,p) data from TRIUMF (120-200 MeV) - early
  90s
• (d,2He) data from KVI (80 MeV/n) 2000gt
• (t,3He) data from NSCL (125 MeV/n)
• lt2003 primary ?-beam
• 2003gt primary 16O beam

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(d,2He) and (n,p)
58Ni target
(n,p)
Hageman et al. KVI
(n,p) and (d,2He) do not always match
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(t,3He) at NSCL
Sherrill et al. (1999) Daito et al. (1998)
58Ni(t,3He) (2000) in analysis
Dec. 2003 PAC approved new experiments 24Mg,53Cr,5
0V,74Ge,94Mo
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(No Transcript)
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Measuring GT in unstable isotopes

• CE-reactions in inverse kinematics are very
  difficult

probe (light nucleus)
Heavy beam (A,Z)
S800
(A,Z-1)
recoil (few MeV)

• low resolutions
• physical background (mixed beams, charge states)
• channel definition
• decaying residual
• statistics

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(7Li,7Be) in inverse kinematics
First experiment performed _at_ NSCL (2002) in
analysis

• Measure both gammas from
• 56Co (excitation energy) and
• 7Be (spin-flip tag)
• low detection efficiencies
• charge-states
• multi-step channels

½- (0.43 MeV-)
?S1
3/2-
7Be
3/2-
7Li
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(d,2He) in inverse kinematics
d-target
56Ni
56Co
S800
2He
Monte-Carlo Simulations in progress
p
p
charge-particle detector (HIRA?) need good energy
and angle resolution!

• Possibility of measuring recoil
• selective to spin-flip
• selective to charge-exchange channel
• go beyond particle-decay threshold of residual
• Measure final-state distribution!
• resolution 1 MeV
• thin target and thus low rates

(t,3He)?
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Other applications for charge-exchange probes in
inverse kinematics

• ?-process in collapse of massive stars
• Measurement of neutron-thickness

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?-process in collapse of massive stars
Core Collapse SN
Neutron star
large ?-flux
transmutation possible (energetic ? ?
neutrinos) (Z,A)??(Z,A) ? ?(Z,A-1)nv
? ?(Z-1,A-1)pv ? ?(Z-2,A-4)?v
? largely via first forbidden transitions

• produces
• 7Li,11B,19F,138La,180Ta
• 10B,15N,22Na,26Al,31P,35Cl,39,40,41K,45Sc,47,49Ti,
  50,51V,55Mn,59Co,63Cu

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Forbidden L1 weak interactions

• Forbidden Strength
• Giant Dipole Resonance Sr(i)t3(i) gt DL 1, DS
  0 DT 0, 1 0 ? 1- Giant
  resonance Sum rule (TRK) ? NZ/A
• Spin-Dipole Res. Sr(i)?s(i) t3(i) DL 1,
  DS 1, DT 0, 1 0 ? 0-, 1-,
  2- Giant resonance Sum rules model
  dependent

GDR
SDR
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Probing the L1 forbidden weak transitions
Measure the L1 giant resonances via (p,n)-type
reactions

• (p,n) in inverse
• kinematics?
• neutron is not
• easily stopped in
• target.

Is BJ linearly related to ?(p,n)? Not validated
yet Theoretical Dmitriev, Austin, Zelevinsky
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Neutron-skin thickness
S/S- via theory (e.g. RPA)
Alternative Ex(GTR)-Ex(GTR) neutron skin
thickness (Vretenar et al.)
Langanke Martinez-Pinedo
Krasznahorkay et al.
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(p,n) in inverse kinematics
Example
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Charge-exchange in inverse kinematics
?Tz-1
?Tz1

• (p,n) in inverse kinematics
• Isovector GRs in unstable nuclei
• Macroscopic microscopic info
• Neutron skin
• SDR sum-rule
• Ex(GTR)-Ex(IAS)
• astrophysics e.g. ?-process
• Experimental considerations
• High cross sections (10mb/sr)
• Complete kinematics (use S800)
• low-E n no energy loss in target
• Neutron detection
• aim 300 KeV resolution in Ex
• required ???10 ?En60 keV
• flight path 2 m
• Timing resolution 1 ns
• granularity 3.5 cm
• must be flexible

• (d,2He) in inverse kinematics
• Physics
• Astrophysics (GT in unstable nuclei)
• Isovector GR in unstable nuclei
• Reaction study (pp-final state)
• Experimental considerations
• 2He?pp
• Complete kinematics (use S800)
• thin targets (CD2)
• Proton detection
• aim 1 MeV resolution in Ex
• Monte Carlo simulations in progress
• HIRA
• Beam tracking
• Careful tuning of setup
• must be flexible
• (7Li,7Be?) in inverse kinematics
• 56Ni analysis difficult
• PID, low yield, reaction mechanism

(t,3He)?