Updated on March 8, 2003.

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.
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. 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).
  • 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.

    Experiments

    KEK-PS E419 (7-Lambda-Li)
    Status
  • Run at K6 kiine 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 at D6 line in December, 1998. Analysis finished and published.
    Results
  • 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.
  • BNL-AGS E930 (16-Lambda-O, 15-Lambda-N and 10-Lambda-B)
    Status
    Run at D6 line in September-November, 2001.
    Results
  • 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 spin-spin, spin-orbit, and tensor interaction strenghts determined from 7-Lambda-Li, 9-Lambda-Be, 16-Lambda-O data.
  • KEK-PS E509 (hyperfragments from stopped K-)
    Status
    Run at K5 line in April, 2002.
    Results
  • 7-Lambda-Li E2(5/2->1/2) gamma rays was observed from stopped K- on 10B and 12C targets.
  • KEK-PS E518 (11-Lambda-B and B(M1))
    Status
    Run at KEK K6 line in September-October, 2002. Results
  • Six 11-Lambda-B gamma transitions were observed and one of them (E2(1/2+ -> 5/2+)) was assigned.
  • KEK-PS E566 (11-Lambda-B and 12-Lambda-C)
    Status
    Run at KEK K6 line in September-October, 2005. Results
  • 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).

  • 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)
  • KEK-PS Proposal (E518)
    "Gamma-ray spectroscopy of 11-Lambda-B" --- submitted on June 18, 2002, approved on July 18.
  • *Gzipped postscript File (0.8 MB)
  • J-PARC Letter of Intent
    "New generation spectroscopy of hadronic many-body systems with S=-2 and -1: Hypernuclear gamma-ray spectroscopy"
  • *Gzipped postscript File (4.4 MB)
  • BNL-AGS Proposal (E964)
    "Systematic Study of Double Strangeness System by an Emulsion-Counter Hybrid Method" --- apporved in Nov. 2001, revised version approved in Sept. 2004. Hyperball will be used for Xi-atomic X ray measurement.
  • E964R home page by Prof. Nakazawa
  • KEK-PS Proposal (E566)
    "Hypernuclear gamma-ray spectroscopy on 12C target" --- submitted in Aug. 2004.
  • *Gzipped postscript File (1.1 MB)
  • J-PARC Proposal (E13)
    "Gamma-ray spectroscopy of light hypernuclei" --- submitted in May 2006.
  • *pdf file (0.9 MB)
  • J-PARC Letter of Intent
    "Gamma-ray spectroscopy of hypernuclei at K1.1" --- submitted in Dec. 2007.
  • *pdf file (0.1 MB)

  • Future Plans

    Upgrade of Hyperball

  • Hyperball-J: For experiments at the J-PARC 50 GeV PS, Hyperball2 will be further upgraded. 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.