A hermetic semi-spherical electron-positron pair spectrometer for
high energy nuclear transitions.
A poster presentation by
Athanasios Hatzikoutelis , Indranil Mazumdar2, Istvan Diosezegi3, Peter Paul
Nuclear Structure Laboratory, Stony Brook University, Stony Brook NY, USA.
1
1-currently at Lancaster University, Lancaster UK, (presenting).
2-currently at TIFR, Mumbai, India.
3-currently at BNL, Upton NY, USA.
We report about an array of 65 phoswich detectors arranged in a closely packed
hemispherical configuration (figures 1). The array was designed and constructed at the
Nuclear Structure Laboratory of Stony Brook University (figure 4) to measure dilepton
(e+-e-) decays of the excited nuclear states due to Internal Pair Conversion (IPC)
process. The design of the spectrometer enables it to measure the e+-e- pairs at high
correlation angles with higher statistics than achieved before. The measurement at higher
correlation angle is crucial to distinguish the pairs produced by the IPC of Giant
Monopole (E0) Resonance states from the E1 pairs peaking at much lower correlation
angle. The absolute pair-energy calibration was
carried out by measuring the IPC of the 22.6-MeV
transition of the Giant Dipole (E1) Resonance
state to the ground state in 12C. The angular
correlation distribution of the pairs was found to
be in good agreement with the Born and pointnucleus approximation for E1 angular correlations.
Further evaluation of the capabilities of the array
was carried out by measuring the IPC of M1
transitions from two discrete states in 12C. The
measurement also aimed to look for any excess
yield of pairs above the M1 contributions.
Motivation.
The Giant Resonances (GR) built on excited states are an important tool in the
understanding of nuclear structure at finite nuclear temperatures up to T~5MeV. The
electric dipole (GDR) mode has been extensively studied but the other multipole modes
are much weaker. The most important of these is the isoscalar giant monopole (ISGMR)
resonance as it relates to the nuclear compression modulus of finite nuclei. The
compressibility of hot nuclear matter plays an important role in heavy-ion reactions and
can also be extended into describing the behavior of broad hot nuclear systems such as
Neutron Stars and Supernovae.
The ISGMR strength distribution has been systematically studied by inelastic
scattering and the compressibility of cold nuclear matter has been determined. However,
any GMR, built on excited states, has not yet been observed.
The GDR and quadrupole (GQR) modes are present in γ-decays. The GMR can
only be found in the Internal Pair Conversion (IPC) spectrum.The main objective of the
detecting system is the efficient energy and direction identification of electron-positron
pairs that are converted from high-energy nuclear electromagnetic transitions.
IPC in Nuclei
The Internal Pair Conversion (IPC) is a well-understood
channel of nuclear decay since the early `40s when
Oppenheimer introduced it. The distributions of pair angular
correlations from these channels have been used to differentiate
between the multipolarities of electric and magnetic transitions
in nuclei. Rose [M.E. Rose Phys. Rev. 76(1949) 678; 78(1951)
184 ; 131(1963) 1260] was the first to calculate the angular
distribution of the pairs from dipole transitions in nuclei using
the Born and point nucleus approximations. The most extensive
description to-date of the theory of the IPC is offered in [P. Schluter, et. al. Phys.Rep.
(Rev. sec. of Phys. Lett.) 75 No.6 (1981) 327-392.]. In these references it is shown that
the approximations are accurate for high-energy transitions and light nuclei, which is the
case for the present transitions of interest. The calculations of the theoretical curves for
the method described here are based on these approximations.
The essential characteristic of the IPC is the shape of the angular correlation
distributions of the pair members' trajectories (i.e. their momentum vectors). The
correlations strongly favor small angular separation of the pairs, and relatively
independent of multi-polarity (compare the E1 and M1 for angles < 50o) but vary
drastically at large separations (compare E0 to E1 and M1 at angles > 50o).
E1
E0
M1
Pair Emmision Prbability per Correlation Angle
0.008
0.006
0.004
0.002
0
0
18
36
54
72
90
108 126 144 162 180
Pair Correlation Angle (deg)
Spectrometer Design
The traditional approach of performing -spectroscopy
is using small detectors in a sizable vacuum chamber. This is a
new approach. The target centre is 9.6 cm from the faces
(Figure 1) providing adequate discrimination by time of flight
(TOF) between slow and relativistic particles. A phoswich
design of fast and slow scintillators is adopted (figure 2) to
measure the signature energy loss and total energy in the same
element.
Calibration and Test-runs.
Calibration runs have been conducted on the beam line of the Stony Brook heavy
ion LINAC using the reaction 11B(p,e+e-)12C (Ep=7.2 MeV) to populate the GDR of
12
C. The observation of the IPC from the 22.6-MeV E1 transition to the ground state of
12
C established the pair-energy line-shape and produced an absolute pair-energy
calibration (figure 3 left).
Additionally using the following stripping reaction 11B(d,n)12C* (Ed=7.2 MeV)
populating the (1+,1) 15.11-MeV and (1+,0) 12.7-MeV states, tested the energy
reconstruction and angle correlation resolution of the detector. The angle correlations of
these two tests are in figure 3.
2000
E1 theory

70000
60000
M1 theory
1500
11.1-14.3MeV
40000
1000
cnt/bin
pairs per bin
50000
30000
500
20000
10000
0
9
27
45
63
81
99
117
PCA (bin 18deg)
135
153
171
0
9
27
45
63
81
99
117
PCA (bin 18deg)
135
153
171
Figure 3 (left) E1 angular correlations. (right) M1 angular correlations
Angular correlation distribution of the pairs folded with the detector response and
normalized to the total number of events with angle separations above 36o are in
agreement with the Born and point nucleus approximation in both lines.
Figure 3 (right) Search for Axions with Mass between 5 to 10 MeV
In 1977 Peccei and Quinn introduced into the Standard Model a pseudoscalar (Jπ
= 0 ) Nambu-Goldstone particle, the Axion. Recent papers present tantalizing evidence
that a 9.5- MeV Axion may be observed in competition with M1. It would appear as
deviations from IPC in the form of excess events at large angle correlations. The
produced correlation distribution (figure 3 right) shows a deficiency of events in the
region of high angular separations, when compared to the theory of IPC of M1, in
contrast to the reported evidence thus excluding the possibility of the effect seen in
isovector M1 transitions. Only the highly suppressed isoscalar (12.7MeV) that still
exhibit the anomaly (with low statistics from previous experiments) remain candidates
for having a boson (or many) competing. (arXiv:hep-ph/0511049 v2 6 Jan 2006)
Conclusions and Outlook
 The efficient detection of high energy GMR decay pairs requires a large
number of detector elements with opening angle 60o.
 The USB Pair Spectrometer Detector Array has been tested extensively
and is well understood.
 The capability to recombine the measured energies of e+-e- pairs, from
IPC of the E1 in GDR and M1 in 12C, into a transition energy has been
successfully demonstrated.
 The transition energy resolution is clearly sufficient to separate the
transitions to ground state and the first three excited states.


The array can operate in air as it has a reaction chamber of carbon fiber. It
make it independent of vacuum chambers and therefore free to move to
places that have appropriate beam lines.
Until RIA experiments take off and emphasis can be given to these
studies, there is the Stony Brook Pair Spectrometer. Experiments on stable
nuclei can be performed with it, at energies around 60 MeV/u that have
given positive results in the past. It can improve on them with higher
statistics reduced background, and better determine the structure of the
GMR modes.