Updated on April 15, 2002.

Hyperball Project

-High-Resolution Gamma Spectroscopy of Hypernuclei-

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 charged particles.
Hyperball was constructed by the Grant-In-Aid for Scientific Research on the Priority Area "Strangeness Nuclear Physics" (Spokesperson: K.Imai, Kyoto Univ.) from The Ministry of Education of Japan, No.08239102. High-resolution gamma spectroscopy with Hyperball enables us to obtain detailed information on hypernuclear structures and approach the following physics subjects:
  • Lambda-N interactions
    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 structure. As illustrated, 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 those interactions.
  • 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. For example, 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).

    Experiments

    KEK-PS E419 (7-Lambda-Li)
    Status
  • Run in May-July, 1998. Analysis completed. Being published.
    Results illustration
  • 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 [1] 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 [2] 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 spin-spin interaction.
  • BNL-AGS E930 (9-Lambda-Be)
    Status
    Run in December, 1998. Analysis finished and published.
    Results illustration
  • 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)
    Status
    Run in September-November, 2001.
    Under analysis. Gamma rays from 16-Lambda-O and 15-Lambda-N were observed.
    KEK-PS E509 (hyperfragments from stopped K-)
    Status
    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)