High Precision
γ-ray Spectroscopy of
4Λ
He and
19 Λ
F at
J-PARC
S.B. Yang1, M. Agnello2,3, Y. Akazawa4, N. Amano5, K. Aoki6, E. Botta7,3, N. Chiga4, H. Ekawa5, P. Evtoukhovitch8, A. Feliciello3, T. Haruyama6, S. Hasegawa9,
S. Hayakawa10, R. Honda4, K. Hosomi9, S.H. Hwang9, Y. Ichikawa5, Y. Igarashi6, K. Imai9, S. Ishimoto6, R. Iwasaki6, S. Kanatsuki5, K. Kasami6, T. Koike4, J.K. Lee11, J.Y. Lee1, S. Marcello7,3, K. Miwa4, S. Nagao4, M. Nakagawa10, M. Naruki6, H. Sako9, V. Samoilov8, Y. Sasaki4, S. Sato9, T. Shiozaki4, K. Shirotori12, H. Sugimura5,
S. Suzuki6, H. Takahashi6, T. Takahashi6, H. Tamura4, K. Tanabe4, K. Tanida1, Z. Tsamalaidze8, M. Ukai4, T.F. Wang13, T.O. Yamamoto4, and Y. Yamamoto4 1Department of Physics and Astronomy, Seoul National University, Seoul 151-747, Korea 2Dipartimento di Fisica, Politecnico di Torino, via Duca degli Abruzzi, 10129 Torino, Italy 3INFN, Sezione di Torino, via P. Giuria 1, 10125 Torino, Italy
4Department of Physics, Tohoku University, Sendai 980-8578, Japan 5Department of Physics, Kyoto University, Kyoto 606-8502, Japan
6Institute of Particle and Nuclear Studies (IPNS), High Energy Accelerator Research Organization
(KEK), Tsukuba, 305-0801, Japan
7Dipartimento di Fisica, Universit di Torino, Via P. Giuria 1, 10125 Torino, Italy 8Joint Institute for Nuclear Research, Dubna ,Moscow Region 141980, Russia
9Advanced Science Research Center (ASRC), Japan Atomic Agency (JAEA), Tokai, Ibaraki
319-1195, Japan
10Department of Physics, Osaka University, Toyonaka 560-0043, Japan 11Department of Physics, Pusan National University, Busan 609-735, Korea 12Research Center of Nuclear Physics, Osaka University, Ibaraki 567-0047, Japan
13Research Center of Nuclear Science and Technology (RCNST) and School of Physics and Nuclear
Energy Engineering, Beihang University, Beijing 100191, China E-mail: maruchi2@snu.ac.kr
(Received July 16, 2013)
A new γ-ray spectroscopy experiment on4ΛHe and19ΛF via the (K−, π−) reaction (J-PARC E13 1st Part) is being prepared at the J-PARC hadron facility. The measurement will provide information on the degree of charge symmetry breaking effect in ΛN interaction and information on the effective ΛN spin-spin interaction in sd-shell hypernuclei. In May 2013, a commissioning beam time was used to check beam conditions and performance of individual detectors as well as the whole system. Production of Σ+ and12
ΛC through the (K−, π−) reaction was performed with a CH2 target. A CF2
target was also tried to test the system in theγ-(K−, π−) reaction coincidence mode.
KEYWORDS: Λ hypernuclei, γ-ray spectroscopy, Hyperball-J, J-PARC
1. Introduction
The study of ΛN interaction has been a key to extend our understanding of nuclear force to the general baryon-baryon interaction. It is also believed that such a study will provide important information regarding neutron star core, which is largely unknown at present. High precisionγ-ray spectroscopy using an array of germanium detectors is a very powerful tool to studyΛN interaction.
BH1 Mass slit Beam line spectrometer 0 5 m BFT D4 Q12 Q13 BC3,4 Q10 Q11 BH2 spectrometer SKS magnet BAC1,2 SAC1 Target Hyperball-J SDC1,2 SP0 TOF SDC3,4 SAC3 Iron block SMF
Fig. 1: The E13 experimental setup at the J-PARC K1.8 beam line.
Since 1998, we have succeeded in measuringγ rays from several Λ-hypernuclei (7ΛLi,9ΛBe,11ΛB,12ΛC,
15
ΛN and16ΛO) at KEK and BNL [1–7].
At the J-PARC hadron hall, a newγ-ray spectroscopy experiment on4ΛHe and19ΛF is being pre-pared at the K1.8 beam line (J-PARC E13 1st Part) [8, 9]. In the experiment, we intend to measure γ-ray transition of4
ΛHe(1+→ 0+). In comparison with its mirror nucleus4ΛH [10], it provides clues to
charge symmetry breaking effect in ΛN interaction. It will also be the first measurement of the cross section of hypernuclear spin-flip state via the (K−, π−) reaction. On the other hand, the19ΛF run will be the firstγ-ray spectroscopy of sd-shell hypernuclei. In particular, through the measurement of the energy spacing of the ground state doublet of 19ΛF(3/2+ → 1/2+), the strength of the effective ΛN spin-spin interaction in sd-shell hypernuclei can be extracted. By comparing it with that in p-shell hypernuclei, we can investigate the radial dependence of the strength ofΛN spin-spin interaction. In addition, the γ ray from E2(5/2+ → 3/2+, 1/2+) transition may show a shrinkage of hypernuclear size known as an impurity effect of Λ to nuclear structure [2].
2. Experimental Setup
In the experiment,4ΛHe and19ΛF are produced through the (K−, π−) reaction with the beam mo-mentum of 1.5 GeV/c and 1.8 GeV/c, respectively. The produced hypernuclei are identified by SKS (Superconducting Kaon Spectrometer) and the K1.8 beam line spectrometer [11]. In coincidence,γ rays from the hypernuclei are detected by a dedicated germanium detector array, Hyperball-J. 2.1 K1.8 Beamline Spectrometer and SKS
The momentum and path of beam particles are analyzed by a dipole magnet (D4), a fiber tracker (BFT), and drift chambers (BC3 and BC4). By using the aerogel Cherenkov counters (BAC1, BAC2
] 2 Missing Mass [GeV/c 1.15 1.16 1.17 1.18 1.19 1.2 1.21 1.22 1.23 1.24 1.25 2 Counts / 0.5MeV/c 200 400 600 800 1000 1200 (a) Preliminary + Σ [GeV] Λ -B -0.04 -0.03 -0.02 -0.01 0 0.01 0.02 0.03 0.04 Counts / 2MeV 0 20 40 60 80 100 (b) Preliminary states Λ s states Λ p
Fig. 2: (a)Σ+missing mass spectrum from the K−p→ π−Σ+reaction. (b)Λ-binding energy spectrum from the12C(K−, π−)12ΛC reaction. These two spectra are obtained at the 1.5 GeV/c beam momentum.
and SAC1) which are installed in up- and down-stream of a target, we can choose beam K− and scattered π− at the trigger level. The time of flight of a beam particle between two plastic counters (BH1 and BH2) is used to identify K− in the offline analysis. By using the SKS magnet and drift chambers (SDC1, SDC2, SDC3 and SDC4), we can analyze momenta and paths of the scattered particles. The scatteredπ−is selected in the offline analysis by combining the additional information coming from the time of flight between BH2 and TOF. Sinceπ−s andµ−s from decays of K−, namely K− → µ−ν¯µ and K− → π−π0, cannot be distinguished at the trigger level only by the Chrenkov counters, a muon filter (SMF) and a pion-zero veto counter (SP0) are used to reject these decay events from the trigger. SP0 consists of alternating layers of plastic scintillators and lead sheets to detect high energyγ rays from π0 → γγ decay. SMF which consists of an iron block and a charged particle detector (LC) is placed at the most downstream of the SKS to detect the muon. These decay counters are essential in order to reduce the trigger rate low enough for our DAQ system and the background in the analysis.
2.2 Hyperball-J
Hyperball-J is a new array of germanium detectors for the J-PARC experiments [12]. The array was designed in view of high beam intensity provided by J-PARC. In the E13 1stpart, 28 germanium detectors are installed at Hyperball-J. In this case, a total photo peak efficiency of ∼5.3 % is obtained by simulation for a 1 MeVγ ray. Each detector is cooled by a compact pulse tube refrigerator. PWO counters surround each germanium detector for background suppression. The PWO crystals have a short decay time (∼10 ns) and are operational under an order of magnitude higher counting rate than conventional BGO counters. Furthermore, a LSO counter is housed closed to each Ge detector for monitoring of in-beam live time of individual detectors. Its working principle is a β-γ coincidence from176Lu in the LSO crystal between the LSO counter and the Ge detector.
3. Commissioning Beam Time in May 2013
In May 2013, about 50 hours of beam time was allocated to the commissioning of the experiment at J-PARC K1.8 beam line. Several test measurements were performed to check performance of each detector and the experimental system as a whole.
[keV] γ E 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Counts / 4keV 10 2 10 3 10 (a)(b) (c) (d) (e) (f) (g) Preliminary
Fig. 3: γ-ray energy spectrum after selecting (K−, π−) events. The black line is after background suppression with PWO counters, and the red dot line is without the suppression. Severalγ rays from CF2target are marked. Theγ transitions are as follows: (a)19F(1/2−→ 1/2+). (b)19F(5/2+→ 1/2+).
(c)10B(1+→ 3+). (d)18F(3+→ 1+). (e)19F(5/2−→ 1/2−). (f)18O(0+→ 2+). (g)18O(2+→ 0+).
Missing mass resolution of SKS and K1.8 beam line spectrometer was measured through theΣ+ and12ΛC production with a 2.9 g/cm2 CH2 target. Figure 2(a) shows theΣ+missing mass spectrum
with a resolution of 6.5 MeV/c2(FWHM) at the 1.5 GeV/c beam momentum. The absolute Σ+missing mass is shifted by∼10 MeV/c2because an energy loss in the target is yet to be accounted for. For the analysis of12ΛC binding energy, ¯νµandπ0from the K−beam decay were removed by applying a cut on the decay kinematics. A scattering angle between 3 and 10 degrees was selected. Peaks corresponding toΛ’s in s and p shells in12ΛC at the 1.5 GeV/c beam momentum are clearly seen in Figure 2(b). The absolute binding energy is also shifted by 10 MeV as in theΣ+missing mass distribution.
The γ-(K−, π−) coincidence was tested using CF2target (20 g/cm2) at the 1.8 GeV/c beam
mo-mentum. Performance of the Hyperball-J was checked under (K−, π−) trigger condition, the overall energy resolution in a summed energy spectrum of all detectors was 4.8 keV (FWHM) for 1.3 MeVγ ray. Aγ-ray energy spectrum in Figure 3 is obtained after selecting the (K−, π−) reaction event in the offline analysis where several γ rays from the target was observed. The measured Eγwas determined with±0.6 keV accuracy. An energy spectrum with background suppressed using the PWO counter is overlayed in Figure 3. An improvement of the signal to noise ratio can be seen, and a suppression fac-tor, which is a ratio defined as un-suppressed over suppressed counts, is 2.2 at 1 MeV. Identification ofγ ray(s) from Λ-hypernuclei is in progress.
In addition, the tuning of K− beam at K1.8 beam line was carried out under 2.5×1013 /spill primary beam rate . The K−beam rate is 290 k/spill (K−/π−=3.4) and 320 k/spill (K−/π−=2.8) at the 1.8 GeV/c and 1.5 GeV/c beam momentum, respectively.
4. Summary and Outlook The γ-ray spectroscopy of 4
ΛHe and 19ΛF will give new information of s-shell and sd-shell
Λ-hypernuclei. Especially, the charge symmetry breaking will be studied fromγ-ray measurement from
4
ΛHe(1+ → 0+) transition, and the 19ΛF(3/2+ → 1/2+) transition measurement will show the radial
dependence of the strength of ΛN spin-spin interaction. As a result of the commissioning beam time in May 2013, the operation of the entire system was successfully confirmed under the same condition as the real experiment. Even at an early stage of the present analysis, theΣ+missing mass and the bound states of12ΛC are clearly observed. A new germanium array, Hyperball-J also showed satisfactory performance. The preparation for the E13 1ststage is fully completed. We look forward
to the resumption of the J-PARC facility and of our physics run at the earliest time possible. Acknowledgment
The authors thank to all the project members who participated in development of detectors for SksMinus and Hyperball-J. This work is supported by Grants-in-Aid Nos. 17070001, 21684011, 23244043, and 24105003 for Scientific Research from the Ministry of Education of Japan. The re-search is also supported by WCU Grant No. R32-10155, NRF Grant No. 2010-0004752, and Center for Korean J-PARC Users Grant No. K2100200173811B130002410 in Korea.
References
[1] H. Tamura et al.: Phys. Rev. Lett. 84 (2000) 5963. [2] K. Tanida et al.: Phys. Rev. Lett. 86 (2001) 1982. [3] H. Akikawa et al.: Phys. Rev. Lett. 88 (2002) 082501. [4] M. Ukai et al.: Phys. Rev. Lett. 93 (2004) 232501. [5] H. Tamura et al.: Nucl. Phys. A 754 (2005) 58c. [6] Y. Miura et al.: Nucl. Phys. A 754 (2005) 75c. [7] M. Ukai et al.: Phys. Rev. C 73 (2006) 012501. [8] H. Tamura et al.: Nucl. Phys. A 881 (2012) 310. [9] H. Tamura et al.: J-PARC E13 Proposal. [10] M. Bedjidian et al.: Phys. Lett. 62B (1976) 467.
[11] T. Takahashi et al.: Prog. Theor. Exp. Phys. (2012) 02B010.
[12] T. Koike et al.: in Proceedings of the IX International Conference on Hypernuclear and Strange Particle Physics, SIF and Springer-Verlag Berlin Heidelberg, 2007, ed. J. Pochodzalla and Th. Walcher, p. 25.
