Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 41 (2008)

7(LNS Experiment : #2582)

Current Status of the Electro-magnetic Calorimeter
Complex FOREST
T. Ishikawa1 , H. Fujimura1 , R. Hashimoto1 , J. Kasagi1 , K. Maeda2 , M. Sato1 ,H. Shimizu1 , K. Suzuki1 , and H. Yamazaki1
1

Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826
2

Department of Physics, Tohoku University, Sendai 980-8578

A new electro-magnetic (EM) calorimeter complex FOREST with a solid angle of about 4π in total
has been planned, and has been partly constructed [1, 2]. FOREST consists of three EM calorimters
SCISSORS III, the LEPS Backward Gamma detector (BG), and Rafﬂesia II. A plastic scintillator hodoscope is placed in front of each EM calorimeter. In this year, the hodoscopes in front of BG havebeen replaced, and Rafﬂesia II has been constructed. A high speed data taking [3] and a solid hydrogen/deuterium target [4] systems have been also prepared for elementary meson photo-production experiments with FOREST. A beam test for FOREST has been performed, and the π 0 and η peaks are clearlyobserved in the γγ invariant mass distributions. We are now ready to carry out long term experiments.

§ 1. Replacement of the BG Plastic Scintillator Hodoscopes
Since the LEPS Backward Gamma detector (BG) was originally used at the Laser Electron Photon beam facility at SPring-8 (LEPS), the plastic scintillator hodoscope prepared for BG does not ﬁt toFOREST. The space for Rafﬂesia II has been already occupied by the light guides and photo-multipliertubes (PMT) of these scintillators. Figure 1 shows photos and a schematic view of BG. As shown inFig. 1, a space exists between the polar angle of 100 degrees and an upstream plate for supporting leadscintillating ﬁber (Lead/SciFi) modules of BG. We planned to put all the light guides and PMTs of theplastic scintillators in front of BG into this space. The number of plastic scintillators has been enlarged
from 12 to 18. Each plastic scintillator is connected to a small and light metal-packaged single-anode
PMT Hamamatsu R8900U through a ﬁsh-tail light guide. This type of PMTs are also used for plastic
scintillators in front of SCISSORS III [2]. The room for Rafﬂesia II has been obtained upstream of thesupporting plate of BG after the hodoscopes were replaced.

§ 2. Construction of Rafﬂesia II
Rafﬂesia II consisted of 36 SF-5 lead glass Čerenkov counters [5] in the original design of FOREST [1]. The size of the SF-5 counter is 150 mm (W) × 150 mm (H) × 300 mm (T), and its density is
4.07 g/cm3 . Its performance has been investigated [6] by using the positron beams at LNS [7]. Since wefailed many times in making a solid hydrogen target ﬁtted to FOREST, we decided to make a distanceshorter between a target holder and a vacuum chamber for a refrigerator of the target system. The8Space for
Rafflesia IITargetγ Beam

Dead SpaceFig.1. Photos and schematic view of the LEPS Backward Gamma detector (BG). The left andmiddle panels show a photo and a schematic view of BG before the plastic scintillator
hodoscopes were replaced, respectively. The space of Rafﬂesia II was occupied with thelight guides and photomultiplier tubes of the hodoscope. The right panel shows a photoof BG after the hodoscopes were replaced.

chamber occupied some space reserved for the SF-5 counters.
The SF-6 lead glass Čerenkov counters were adopted for a large part of Rafﬂesia II instead of SF-5
ones to keep the large solid angle of FOREST. The size of SF-6 counter is 75 mm (W) × 75 mm (H) ×250 mm, and its density is 5.18 g/cm3 . Since the density of SF-6 is higher than that of SF-5, both havethe same lengths in a radiation length unit (11.8X0 ). The higher position resolution will be obtained byadopting the smaller module. We constructed Rafﬂesia II with the 52 SF-6 and 10 SF-5 counters on the11th June in 2008. Figure 2 shows the schematic view and photo of Rafﬂesia II.

Fig.2. Schematic view and photo of Rafﬂesia II. The left panel shows the schematic view ofRafﬂesia II. Two types of lead glass Čerenkov counters are used. The right panel shows
the photo of Rafﬂesia II.

9§ 3. Beam Test
A beam test of the FOREST was performed from 16th June to 7th July in 2008. A high speed datataking system [3] had been newly developed and ones for SCISSORS III and LEPS Backward Gammadetector had been uniﬁed. Energies and timings of detectors were taken by ADC and TDC modules in 5subsystems, one Tristan KEK Online (TKO), three VME, and one Fast Encoding Readout ADC (FERA)
on CAMAC systems. The digitized data were collected in each subsystem. All the digitized data were
ﬁnally collected with a personal computer (PC), and the data associating with the same trigger werebuilt to one event. Event tags generated with an FPGA module developed at LNS [8] were used toconﬁrm the trigger identiﬁcation.
The trigger condition for the data taking system was described as
([SigmaTagger i] ⊗ [NEMC ≥ 2]) .

(1)

iChannels of STB-Tagger II were divided into 16 groups so that the counting rate of each group shouldbe the same, and SigmaTagger i (i = 1 . . . 16) denotes an OR signal of each group. Crystal signals ofSCISSORS III were divided into 10 groups, and Lead/SciFi signals of BG were divided into 18 groups. Anoutput signal was generated in a SCISSORS III group when a linear sum of signals belong to it exceeded
the threshold. An output signal of a BG group was an OR signal of ones belong to it. The NEMC ≥ 2stands for the signal which was generated when more than two output signals of SCISSORS III and BG
groups were generated.
A neutral cluster was selected as a γ particle. The energy of the cluster was reconstructed as a sumof the modules which joins it, and the unit vector of the momentum was reconstructed from an energy
weighted average of the position vectors of the modules from the target center. The energy calibration
was made so that the π 0 peaks in the γγ invariant mass distribution should be π 0 mass. Figure 3 showsγγ invariant mass distributions measured with FOREST. The π 0 peaks are clearly observed. About
2,000,000 π 0 and 40,000 η produced events are measured a day.

Counts/ 5 MeV

S-S

π0S-B

π0

B-B
ηπ0γγ Invarint Mass (MeV)
Fig.3. The γγ invariant mass distributions measured with FOREST. The left panel shows thatfor the events two photons are detected by SCISSORS III. The middle one shows that for
the events one photons is detected by SCISSORS III and another is detected by BG. The
right one shows that for the events two photons are detected by BG.

10References[1]

T. Ishikawa et al.: Research Report of LNS, Tohoku University 39 (2006) 35.

[2]

T. Ishikawa et al.: Research Report of LNS, Tohoku University 40 (2007) 19.

[3]

H. Fujimura et al.: in this Research Report of LNS, 41 (2008) 26.

[4]

R. Hashimoto et al.: in this Research Report of LNS, 41 (2008) 31.

[5]

A. Ando et al.: KEK Report KEK-79-21 (1979).

[6]

M. Sato: Internal GeV-γ analysis note No. 86 (2008);
R. Hashimoto: Internal GeV-γ analysis note HD No. 105D (2008).

[7]

T. Ishikawa et al.: Research Report of LNS, Tohoku University 40 (2007) 6.

[8]

K. Nanbu, T. Ishikawa, and H. Shimizu: Research Report of LNS, Tohoku University 40 (2007) 27.