SILICON DETECTOR NEEDS FOR THE YORK PHYSICS PROGRAMME

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SILICON DETECTOR NEEDS FOR THE YORK PHYSICS
PROGRAMME TIGRESS Silicon Detector
Meeting Colorado, February 2006 Brian Fulton

• Update on grant
• Physics programme
• Detector needs
• Some questions

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1. UPDATE ON GRANT
Grant bid to EPSRC successful Nuclear
Structure and Nuclear Astrophysics at the ISAC-II
facility Brian Fulton, Bob Wadsworth, Charles
Barton and Alison Laird
Value 1.3M (CN) Duration 4 years Start
date June 2006 (but a few steps taken already)
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Physics Programme
Coulex (build a zero degree Bragg
detector) n-rich Ne (island of
inversion) 72Kr and 68Se (shape
co-existence) Xe (multiple Coulex for octupole
states)
Astrophysics (fund a barrel to augment the
CD) Hot-CNO breakout via 15O(a,g)19Ne measur
ed through 15O(6Li,d)19Ne 18F destruction in
Novae by 18F(p,a)15O measure 19Ne states in
18Ne(d,p)19Ne
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Resources
Personnel Postdoc (4 years) PhD student (3.5
years) Workshop, design, computing support
Travel (enables extended stays by postdoc))
Equipment Zero-degree Bragg detector (57k)
(inc. gas handling and HV supplies) Silicon
barrel detectors (262k) (inc. preamps and
HV supplies)
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Progress so far
Negotiated start date of June 2006 (can place
orders earlier)
PDRA advertisement out (hope can spend time at
TRIUMF)
Student will start October
Initiated discussions with Daresbury design group
on Bragg
TIG48 integrated into UK MIDAS system and ready
for tests

Charles at TRIUMF for March/April
Chance for detailed work on chamber
Brian at TRIUMF for April/May
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2. The Physics Programme
Nuclear Astrophysics Interest Explosive
Sites (Novae, X-ray bursters, Type 1
Supernovae) Brian Fulton, Alison Laird Nuclear
Structure Interest evolution of structure of
exotic proton and neutron rich nuclei Bob
Wadsworth, Charles Barton
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The Physics OpportunityNuclear Astrophysics
18F(p,a)15O reaction Novae 15O(a,g)19Ne
reaction Initiates rp process in x-ray bursters
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The Physics Opportunity18F(p,a)15O reaction
Novae

• Classical Novae
• binary systems
• H accretes from RG onto a WD
• Slow accretion rate leads to degenerate layer of
  H on top of C/O or O/Ne core
• Temperatures of up to 3 x 108K
• Time 100-1000s to eject layer
• Light curve increases to max in hours but can
  take decades to decline
• Absolute magnitude can increase by 11 magnitudes
• Can be recurrent (t105 y)
• Elemental composition of ejecta may be observed

Nova Herculis 1934 AAT
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The Physics Opportunity18F(p,a)15O reaction
Importance of Reaction

• Reaction dominates destruction of 18F
• Some of the ejected materials are radioactive
• Gamma ray emission Clayton Hoyle Ap. J. 494
  (1974)
• Direct observation would provide firm constraints
  on modelling.
• Emission of 511 keV and below dominates flux and
  is strongly dependent on final 18F abundance
• Timescale of decay is almost ideal for satellite
  based observation.

Artists impression of INTEGRAL satellite
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The Physics Opportunity18F(p,a)15O reaction
The Reaction

• Final abundance of 18F depends on rates of
  production/destruction processes
• Production decay of 18Ne and 17O(p,g)18F
• Destruction 18F(p,a)15O and 18F(p,g)19Ne

20Na
21Na
18Ne
19Ne
20Ne
17F
18F
19F
15O
14O
16O
17O
18O
13N
14N
15N
18F(p,a)15O dominates the destruction rate of 18F
12C
13C
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The Physics Opportunity18F(p,a)15O reaction
Current Status

• Reaction rate dominated by resonant contributions
• T9 lt 0.25 ? 38 keV resonance dominates
• 0.25 lt T9 lt 0.4 ? 330 keV resonance dominates
• T9 gt 0.4 ? 665 keV resonance
  dominates
• Recent data from Bardayan et al. and Utku et al..

Astrophysical S-factor, taken from Coc et al.,
2000
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The Physics Opportunity18F(p,a)15O reaction
Yields too low to do direct measurement, so try
and get p-capture widths for 19Ne states by
indirect means. Measure n-capture to analogue
states with 18F(d,p)19F (need coincident gamma
to achieve resolution)
Also, analogue assignments not clear for key
6.437/6.999 states, so may want to measure
18Ne(d,p) 19Ne and distinguish order from gamma
decay branching
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The Physics Opportunity 15O(a,g)19Ne reaction
15Oa capture is one of ways to break out of
Hot-CNO cycle in explosive hydrogen burning
scenarios (this occurs in sites such as
novae, x-ray bursters, supernovae etc.)
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The Physics Opportunity 15O(a,g)19Ne reaction
This breakout provides the seed nuclei for the
subsequent rp-process which can produce
elements up to A100
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The Physics Opportunity 15O(a,g)19Ne reaction
Rate never been measured. Have to calculate
based on estimates of the alpha width for
capture states (no measurements) Theory Widths
from analogue states Upper limit of Ga/G from
experiment
UNCERTAINTIES
Need new experimental measurements of alpha
widths of states just above alpha decay threshold
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The Physics Opportunity 15O(a,g)19Ne reaction
Also a key in determining whether SMS (Super
Massive Stars) end their life by exploding
(higher 15O capture rate) or collapsing to a
Black Hole (lower 15O capture rate). The present
rate limit cant distinguish between these.
Recent paper (J L Fisker et al.
arXivastro-ph/0410561) claims this reaction
rate may be the key to understanding the recent
observations of Superbursts
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The Physics Opportunity 15O(a,g)19Ne reaction
Direct measurement unlikely for some time
(possible on DRAGON) so need to use indirect
approach to determine alpha widths. Measure
alpha transfer with 15O(6Li,d) 19Ne Extract
alpha spectroscopic factor Convert
spectroscopic factor to alpha width
Problems Yields low so need thick target but
then particle resolution too low
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The Physics OpportunityNuclear Structure
24,26,28,30Ne Island of Inversion NZ, A70
Studies Shape Coexistence Kr and Se
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The Physics OpportunityNeutron rich Ne and the
Island of Inversion
sd pf shell model space The energy gap is
smaller than anticipated problems include shell
model space truncation 1p3/2 0f7/2 residual
interaction with continuum neutrons
Scheit et al.
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The Physics OpportunityNeutron rich Ne and the
Island of Inversion
Theory
Ne
Mg
Si
OXBASH
QMC Shell Model Calculation
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The Physics Opportunity NZ, A70 Studies
Shape Coexistence
NZ
Region many different nuclear shapes lie within
a small range of excitation energy Determining
the location of states belonging to different
shapes test state-of-the-art nuclear models
Kr
72
78
Br
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Se
70
76
74
68
Stable
Oblate?
Oblate?
709 keV
2
0
671 keV
Prolate band-head
0
72Kr
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The Physics Opportunity NZ, A70 Studies
Shape Coexistence
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Nuclear Structure Coulomb Excitation Reactions
Typical Excitation in Even-Even Nuclei
Experimental Technique
An important technique for nuclear
structure studies with Radioactive or
Stable Nuclear Beams
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4
2
2
0
0
Start with 1 matrix element Build up to multiple
excitations Large cross sections (10s
mb) Well-understood process Simple spectra
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3. Experimental Challenges

• Beam Challenges
• Isobaric Contamination
• Stable Beam Contamination
• Charged Particle Detection
• Z-identification
• proton detection (angular resolution)

Today just concentrate on silicon array issues
and use one experiment to outline requirements
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Determining the a15O radiative capture rate from
a measurement of the 15O(6Li,d) reaction
Rate never been measured. Have to calculate
based on estimates of the alpha width for
capture states (no measurements)
Need new experimental measurements of alpha
widths of states just above alpha decay threshold
Measure the alpha transfer onto 15O e.g.
15O(6Li,d)19Ne
Problems
Low yield Use efficient device (Si array)
States close together so cant resolve Take
coincident gamma (EXOGAM)
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Important levels for astrophysics
4.379 7/2
4.38 7/2
4.197 7/2-
L4
4.140 9/2-
Gamow Window T0.3GK
L4
4.033 3/2
4.03 9/2-
3.529
L1
4.00 7/2-
15N a
3.91 3/2
3.529
15O a
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DWBA 3.5 2.5 3.5 0.8
4.379 7/2
Have to resolve from gs defines DEd required
4.197 7/2-
L4
4.140 9/2-
Separate with g
L4
Gamow Window T0.3GK
4.033 3/2
L1
Eg 4.14 (85), 1.58 (15)
Eg 3.96 (20), 2.69 (80)
3.529
15O a
Eg 2.63 (100)
Eg 4.03 (50) 3.80 (5), 2.50 (15)
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Take current experimental limit of Ga/G
4x10-4 Combine with current estimate of G 9
fs Gives estimate of Sa 0.16
Angular range of rear detector is qlab120o-160o
(qcm 10o-30o) Integrate DWBA yield in this
range Scale by Sa 0.16 gives cross section of
37mb
To separate 4.197 MeV state beam energy loss must
be lt 0.5 MeV Use SRIM to find target thickness lt
120 mg/cm2 Gives effective thickness of Li of 50
mg/cm2
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Gamma efficiency Estimated as 17 at 1 MeV.
This should extrapolate to 6 at 4 MeV, where
we need to work.
So have Beam 108 pps Target 50mg/cm2 Cross
section 37mb Efficiency 6
This gives a count rate of 4/hr so 500 in 5 day
run
Should be able to push limit lower at least
factor of 10
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4. Some Questions
We came up with an initial design for the array
(CDBarrel) at the time when the grant request
was prepared
Here today to (1) See if this is still
sensible (2) See what we can learn from
others (3) Explore some changes which
have been suggested
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Barrel as envisaged at the time of grant
submission
Segments go from 4 to 8 (azimuthal
resolution) 5cm long so can use 4-inch wafer and
so thicker detectors so can avoid
punch-through Particle ID from DE-E arrangement
(140mm and 1000mm) Variable layout capability
COULEX TRANSFER
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Some questions for the meeting
Is 5 cm optimum could go a bit bigger on 4-inch
What is optimum thickness for DE detector?
Why not double sided say 8 x 25 (2mm
pitch)? better energy resolution better time
resolution (coincident particles?) possibility
of rise time identification?
Any advantage in making strip widths variable?
How to mount and get signals out?
Preamps specs. and supplier?