The -decay of N
12
ECT Trento
3rd October 2001
Hans O. U. Fynbo
Outline
• Why study -decay of light nuclei
• Detection System
12
• N - Preliminary
• Outlook
Why study -decay of Light Nuclei ?
– “Exact” A-body calculations
possible for A12.
• Shell-model states
• Molecular-cluster states
– Break-up mechanism not fixed
by kinematics
• Test case for 2He emission
models
– Crucial for bridging the A=5
and A=8 gaps in Big Bang and
Stellar nuclear synthesis.
Some recent theoretical interest
Experiments
• States near 3-threshold
surprisingly unknown
• 10.3MeV mainly known from
12
12
•-decays of N and B
•The spin-parity of the 10.3MeV
state is unknown
•Important to determine cluster
structure
Latest evaluation
Nucl. Phys. A506 (1990)
Experiments
Schwalm and Povh Nucl. Phys. 89 (1966) 401
Eyrich et al. Phys. Rev. C36 (1987) 416
Lewis et al. Nucl. Phys. A474 (1987) 499
The -decay of 12N
•Unpublished data from
Heidelberg from 1978 (Schwalm)
•Main experimental problems are
• Energy loss in target
•Summing
•Not possible to determine spin
of the 10.3MeV state.
•Indications for new state at
13MeV
12
N Motivation II :Breakup of the 12.71MeV state
Balamuth, Zurmuhle and Tabor Phys. Rev. C10 (1974) 975
R-matrix based sequential break-up with
order-of-emission interference
A.A. Korshenninikov Sov. J. Nucl. Phys. 52 (1990) 827
Hyperspherical Harmonics expansion.
Simultaneous emission.
SAME DATA !!
12
N Motivation II :Breakup of the 12.71MeV state
Experiment
– ISOL type beam
• Mass-separated
• Low energy
• Reduce energy loss
– IGISOL
• Fast
• Chemically blind
Detection System
– Essentials
• LargeSolid Angle
– detect all particles
• High Segmentation
– no summing
– New Complications
• Detection is kinematics
dependent
• Deadlayer in detector
Monte-Carlo simulations
12
N Results
– Required corrections
• Energy Loss in C-foil
• Energy Loss in detector
deadlayer.
• Include in calibrations
– Corrections different for
each strip
• Takes time - but can be
done !
• Allows for much better
resolution.
– Many Cuts possible
• Total momentum
• Veto -particles with E-E
• Efront = Eback ...
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N Results
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N Results : The 10.3MeV state
– Break-up mechanism
known
• Can we determine the
spin-parity?
– Method
• Select 8Be 0+
branches by gating on
diagonal
• Use Monte-Carlo to
correct for detection
efficiency
• Fit
– 0+ favoured
– Preliminary
• Need more X-checks.
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N Results : The 10.3MeV state
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N Results : The 12.71MeV state
– Break-up mechanism
unknown
• Sequential R-matrix?
• Democratic?
– Compared to Data
• Democratic best, but
both models fail for
the highest two peaks
• Surprising similarity
between models
– Still work to do !
– Any ideas ?
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N Results : Total Data
Pending
– Fully understand the detailed response and
acceptance of the detector system
• Detection thresholds
• Energy cut-off due to C-foil
– Is there a state near 13MeV ?
• Rotational band structure built on 0+
• Likely to be 2+
– Understand the break-up of the 12.71MeV state
• Theoretical input most welcome!
• Interesting to compare to the 15.11MeV state
Outlook : Detection thresholds
– Record spectrum in air
from standard triple
source
• Test Monte-Carlo
• Determine threshold
• Easy “calibration”
5
y = 0.0327x - 0.1455
R2 = 0.9961
4.5
Low Energy Cut-off (MeV)
4
3.5
3
300
64
2.5
Linear (300)
Linear (64)
2
y = 0.0191x + 0.091
R2 = 0.9976
1.5
1
0.5
0
0
20
40
60
80
100
120
140
160
CFD Voltage (mV)
Work of Damian King CERNISOLDE Summer Student 2001
Outlook II
–
9
Li available from most
ISOLDE targets
• Determine neutron energy
from E, P conservation
– 9C should be re-measured
with lower thresholds
• Breakup mechanism of low
lying states
–
12
B can be produced
indirectly via 12Be
• Very high yield
– Try to improve yield of 12N
Outlook III
– Several Detector Projects
• G.Nyman + B.Jonson (Guthenburg)
ball of hybrid E-E telescopes
• O.Tengblad (Madrid) cube of
DSSSDs
• J.Aysto (ISOLDE) ball of single
detectors using TOF and pulseshaping
5 cm
•Beam-coolers and Paul-traps
•Reduce effect of C-foil
•Plans underway at GANIL-SPIRAL
and ISOLDE-CERN
10 cm