Updated on April 15, 2002.
-High-Resolution Gamma Spectroscopy of Hypernuclei-
We have constructed "Hyperball" and drastically (1000 times) improved
the energy resolution to investigate structures of Lambda
hypernuclei. By precisely measuring gamma-rays from hypernuclei with this
germanium detector array, we have succeeded in revealing fine structures
- Hirokazu TAMURA
- Department of Physics, Tohoku University
- TEL: 81-(0)22-217-6454
- FAX: 81-(0)22-217-6455
- Email: firstname.lastname@example.org
- Address: Aramaki, Aoba-ku, Sendai 980-8578 Japan
Hyperball is a large-scale germanium (Ge) detector array
dedicated to gamma spectroscopy of hypernuclei.
It consists of fourteen N-type coaxial Ge detectors of 60%
relative efficiency (crystal size of about 70 mm phi x 70 mm).
The distance between the detector end cap and the beam axis is about 15 cm,
and it has a photo-peak efficiency of about 2.5% at 1 MeV.
Hyperball is equipped with special fast electronics which works under the
condition of extremely high counting rate and energy rate
(close to 1 TeV/sec).
The signal from each Ge detector is processed with a transistor-reset
preamplifier and a gated-integrator amplifier.
Each Ge detector is surrounded by six BGO counters
which provide veto signals to suppress backgrounds from Compton
scattering, high-energy gamma rays from pi0 decays, and high-energy
Hyperball was constructed by the Grant-In-Aid for Scientific Research
Priority Area "Strangeness Nuclear Physics" (Spokesperson: K.Imai, Kyoto Univ.)
from The Ministry of Education of Japan, No.08239102.
Hyperball (Hypernuclear Germanium Ball)
High-resolution gamma spectroscopy with Hyperball enables us to
obtain detailed information on hypernuclear structures and
approach the following physics subjects:
The spin-dependent Lambda-Nucleon interactions
(spin-spin, spin-orbit, and tensor interactions)
have not been well known.
Since they are difficult to investigate
by Lambda-p scattering experiments,
they are expected to be extracted from detailed studies of hypernuclear
when a Lambda is added to a normal nucleus,
a level with spin J splits into two levels with J-1/2 and J+1/2 due to the
spin-dependent interactions between Lambda and nucleons.
Such splittings are expected to be small (of the order of 100 keV
or less) and hypernuclei have a characteristic level structure
which is to be called as hypernuclear fine structure.
A few keV resolution of Ge detectors is essential to resolve
each level of fine structures and to extract information on
Impurity Nuclear Physics (Nuclear structure change caused
by a Lambda)
Since a Lambda in a nucleus is free from the Pauli effect and subject to
Lambda-N interactions much different from NN interactions,
structure of a nucleus, such as size, shape, clustering feature,
collective motions, may be drastically changed
when a Lambda is added to the nucleus.
when a Lambda is added in the 0s orbit of a loosely-bound
nucleus, the nucleus shrinks into a more compact system, as shown in the
figure for the 7-Lambda-Li case.
This effect called ``gluelike role of Lambda" was predicted by Motoba et al.
in 1983. Such a shrinkage can be observed
from E2 transition probability, B(E2).
KEK-PS E419 (7-Lambda-Li)Run in May-July, 1998. Analysis completed. Being published.
First observation of hypernuclear gamma transitions
with germanium detectors. Four transitions were observed.
The strength of spin-spin interaction between a Lambda and a nucleon was
derived from the observed energy of the spin-flip M1 (3/2+ -> 1/2+)
transition. The value of Delta in the Milleners parameterization was
found to be 0.50 MeV. (See  in the publication list.)
E2 transition probability B(E2) was measured with Doppler-shift attenuation
method for the E2 (5/2+ -> 1/2+) transition. The obtained value was three times
smaller than the corresponding B(E2) of the core 6Li(E2:3+ -> 1+), which
indicates that the size of 6Li is significantly contracted
when a Lambda particle is added, by 19+-4% in the cluster-model
interpretation by Hiyama et al. (See  in the publication list.)
The ground state spin of 7-Lambda-Li was determined to be 1/2
from the yield of the 7B(429keV) gamma-ray emitted after the weak decay,
7-Lambda-Li -> 7Be* + pi-. It gives the sign of the Lambda-N
BNL-AGS E930 (9-Lambda-Be)
Run in December, 1998. Analysis finished and published.
Two E2 transitions (5/2+->1/2+, 3/2+->1/2+) were separately
observed. The level spacing
of the doublet (5/2,3/2) was found to be only 31+-2 keV, which confirmed a
very small spin-orbit interaction of Lambda in a nucleus.
BNL-AGS E930 (16-Lambda-O, 15-Lambda-N and 10-Lambda-B)
Run in September-November, 2001.
Under analysis. Gamma rays from 16-Lambda-O and 15-Lambda-N were
KEK-PS E509 (hyperfragments from stopped K-)
Running in April, 2002.
Proposals and LOI's of Experiments
- KEK-PS Proposal (P392)
"High-Resolution Gamma-Spectroscopy of 7-Lambda-Li Hypernucleus"
--- submitted on June 18, 1996, withdrawn on '97-July 22, 1997.
- *Version without figures. (Postscript File) (0.2MB)
- KEK-PS Proposal (E419)
"Measurement of E2 Transition Rate in 7-Lambda-Li Hypernucleus"
--- submitted on 'June 23, 1997, approved on 'July 22, 1997.
- *Version without figures. (Postscript File) (0.2 MB)
- *Cover letter in Japanese. (Postscript File)
- BNL-AGS Proposal (E930)
"High-Resolution Gamma-Spectroscopy of Hypernuclei
using Large-Acceptance Germanium Detector"
--- submitted on Sep.3, 1996, approved on Oct.10, 1996.
- *Full version. (Postscript File) (10 MB)
- *Small version without Fig.1,2. (Postscript File) (5 MB)
- *Cover letter to Tom Kirk. (Postscript File)
- Proposal to DOE (E930)
"Determination of All the Lambda-N Spin-Dependent Forces using Hypernuclear Gamma Spectroscopy"--- included in the proposal by Hungerford titled
"A Proposal to Fund Two Fixed-Target Experiments at the AGS
" --- submitted in September, 1999.
- *Postscript File (7.4 MB)
- KEK-PS Proposal (E509)
"Gamma-ray spectroscopy of hyperfragments using stopped K- method"
--- submitted in Feb.2002, approved on March 18,2002.
- *Gzipped postscript File (2.3 MB)
- Letter of Intent for JHF 50 GeV PS
"Gamma-ray spectroscopy of hypernuclei"
- *Gzipped postscript File (1.6 MB)
- Letter of Intent for KEK-PS
"Gamma-ray spectroscopy of 11-Lambda-B"
--- submitted on March 18, 2002.
- *Gzipped postscript File (0.8 MB)