Updated on March 8, 2003.
-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)22-795-6454
- FAX: 81-(0)22-795-6455
- 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.
In 2005, Hyperball has been upgraded to "Hyperball2"
by adding six Clover-type Ge detectors in collabbration with
the CYRIC (Cychrotron Radio-Isotope Center in Tohoku Unive.) group.
The photo-eapk efficiency has been doubled to 5% at 1 MeV.
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).
Nuclear Medium Effect of Baryons
The magnetic moment of a Lambda in a hypernucleus is a good
probe to study possible modification of baryons in nuclear matter.
Although the direct measurement of hypernuclear magnetic moment is
extremely difficult, the g-factor of a Lambda ca nbe derived
from a B(M1) (transition probability) of a Lambda-spin-flip M1
transition in a hypernucleus.
KEK-PS E419 (7-Lambda-Li)Run at K6 kiine 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)
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 42 keV, which confirmed a
very small spin-orbit interaction of Lambda in a nucleus.
Run at D6 line in December, 1998. Analysis finished and published.
BNL-AGS E930 (16-Lambda-O, 15-Lambda-N and 10-Lambda-B)Gamma rays from 16-Lambda-O and 15-Lambda-N were observed.
Fro the ground state doublet (1-,0-) spacing of 16-Lambda-O,
the strength of Lambda-N tensor interaction was first obtained.
A Lambda-spin-flip transition in 7-Lambda-Li (7/2+ -> 5/2+)
was observed from 10B target data
by the gamma-gamma coincidence method.
The spacing energy is consistent with Lambda-N
and tensor interaction strenghts determined
from 7-Lambda-Li, 9-Lambda-Be, 16-Lambda-O data.
Run at D6 line in September-November, 2001.
KEK-PS E509 (hyperfragments from stopped K-)7-Lambda-Li E2(5/2->1/2) gamma rays was observed
from stopped K- on 10B and 12C targets.
Run at K5 line in April, 2002.
KEK-PS E518 (11-Lambda-B and B(M1))
Six 11-Lambda-B gamma transitions were observed and
one of them (E2(1/2+ -> 5/2+)) was assigned.
Run at KEK K6 line in September-October, 2002.
KEK-PS E566 (11-Lambda-B and 12-Lambda-C)
Three 11-Lambda-B gamma transitions were observed and assigned.
Three 12-Lambda-C gamma transitions were observed and assigned.
Both ground state doublet spacings were measred and the Lambda-N
spin-spin interaction strength was found to be smaller
(Delta=0.33 MeV) than the value from 7-Lambda-Li (Delta=0.43 MeV).
Run at KEK K6 line in September-October, 2005.
Proposals and LOI's of Experiments
KEK-PS Proposal (P392)*Version without figures. (Postscript File) (0.2MB)
"High-Resolution Gamma-Spectroscopy of 7-Lambda-Li Hypernucleus"
--- submitted on June 18, 1996, withdrawn on '97-July 22, 1997.
KEK-PS Proposal (E419)*Version without figures. (Postscript File) (0.2 MB)
*Cover letter in Japanese. (Postscript File)
"Measurement of E2 Transition Rate in 7-Lambda-Li Hypernucleus"
--- submitted on 'June 23, 1997, approved on 'July 22, 1997.
BNL-AGS Proposal (E930)*Full version. (Postscript File) (10 MB)
*Small version without Fig.1,2. (Postscript File) (5 MB)
*Cover letter to Tom Kirk. (Postscript File)
"High-Resolution Gamma-Spectroscopy of Hypernuclei
using Large-Acceptance Germanium Detector"
--- submitted on Sep.3, 1996, approved on Oct.10, 1996.
Proposal to DOE (E930)*Postscript File (7.4 MB)
"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.
KEK-PS Proposal (E509)*Gzipped postscript File (2.3 MB)
"Gamma-ray spectroscopy of hyperfragments using stopped K- method"
--- submitted in Feb.2002, approved on March 18,2002.
Letter of Intent for JHF 50 GeV PS*Gzipped postscript File (1.6 MB)
"Gamma-ray spectroscopy of hypernuclei"
KEK-PS Proposal (E518)*Gzipped postscript File (0.8 MB)
"Gamma-ray spectroscopy of 11-Lambda-B"
--- submitted on June 18, 2002, approved on July 18.
J-PARC Letter of Intent*Gzipped postscript File (4.4 MB)
"New generation spectroscopy of hadronic many-body
systems with S=-2 and -1: Hypernuclear gamma-ray spectroscopy"
BNL-AGS Proposal (E964)E964R home page
by Prof. Nakazawa
"Systematic Study of Double Strangeness System by an Emulsion-Counter
--- apporved in Nov. 2001, revised version approved
in Sept. 2004. Hyperball will be used for Xi-atomic X ray measurement.
KEK-PS Proposal (E566)*Gzipped postscript File (1.1 MB)
"Hypernuclear gamma-ray spectroscopy on 12C target"
--- submitted in Aug. 2004.
J-PARC Proposal (E13)*pdf file (0.9 MB)
"Gamma-ray spectroscopy of light hypernuclei"
--- submitted in May 2006.
J-PARC Letter of Intent*pdf file (0.1 MB)
"Gamma-ray spectroscopy of hypernuclei at K1.1"
--- submitted in Dec. 2007.
Upgrade of Hyperball
Hyperball-J: For experiments at the
J-PARC 50 GeV PS, Hyperball2 will be further
Faster readout electronics based on waveform digitizers
and faster background suppression counters using PWO scintillators
are being developed.
The project is funded and will be completed in 2009.