Research Report of Laboratory of Nuclear Seience, Tohoku University Vol. 40 (2007)

19(LNS Experiment : #2536, #2582)

LEPS Backward Gamma Detector Reassembled
T. Ishikawa, K. Mochizuki, K. Suzuki, Y. Okada, R. Hashimoto, J. Kasagi,
S. Kuwasaki, K. Nawa, Y. Onodera, M. Sato, H. Shimizu, and H. Yamazaki
Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826

A new electro-magnetic (EM) calorimeter complex FOREST covering a solid angle of about 4π sr
is under construction. It consists of three calorimeters: the EPS forward one made up of pure CsI
crystals, the middle one of lead scintillating ﬁber (Lead/SciFi) modules, and the backward one of lead
glass Čerenkov counters. We have reassembled the middle calorimeter, comprised of 252 Lead/SciFi
modules, which used to be the LEPS Backward Gamma detector system utilized in a LEPS experimentat SPring-8. A performance test for the gamma detector system has been made with a photon beam in
the GeV γ experimental hall. The π 0 peak is clearly observed in the γγ invariant mass distribution.

§ 1. Electro-Magnetic Calorimeter Complex FOREST
Nucleon resonances were experimentally studied via π 0 and η photo-production by using an electromagnetic (EM) calorimeter SCISSORS II in the GeV-γ experimental hall at Laboratory of Nuclear Science (LNS), Tohoku University. The π 0 and η mesons are identiﬁed as a peak at speciﬁc places in theγγ invariant mass distribution. It was difﬁcult, however, to detect all the γ’s coming from π 0 or η decay
since the solid angle of SCISSORS II was only 12.6% in total. In addition to that the process of multi-πproduction is dominant in the GeV energy region. There is a huge background made up with wrong
combinations of γ’s, which do not form a peak of π 0 nor η in the γγ invariant mass distribution. Tosuppress the background due to the wrong combinations, a large solid angle calorimeter is required sothat a fraction of undetected γ’s decreases.
A new EM calorimeter complex called Four-pi Omnidirectional Response Extended Spectrometer
Trio (FOREST) with a solid angle of about 4π sr has been planed [1]. It consists of three calorimeters.
The most forward region is covered with pure CsI crystals ‘SCISSORS III’ [2]. The detector systemcalled ‘Backward Gamma’ is placed at the central region. And a set of lead glass Čerenkov counters
covers the backward region. Figure 1 shows a schematic view of FOREST and a side view of Backward
Gamma. The Backward Gamma detector system covers 30◦ –100◦ in the polar angle and 0◦ –360◦ in theazimuthal angle, consisting of 252 Lead/SciFi modules. Each module has a 10◦ coverage both in thepolar and azimuthal angles. Backward Gamma was originally used at the Laser Electron Photon beamfacility at SPring-8 (LEPS).

20a)b)
Plastic
Scintillator
TargetLead/SciFi

Fig.1. a) Slant view of the new EM calorimeter complex FOREST. b) Side view of the LEPSBackward Gamma detector. Backward Gamma consists of 252 Lead/SciFi modules. Each
module covers 10◦ both in the polar and azimuthal angles.

§ 2. Rebuilding of LEPS Backward Gamma Detector
The LEPS Backward Gamma system was reassembled from 9th to 23rd Nov. in 2006. The frameof Backward Gamma was rotated ﬁrst of all so that the plane corresponding to the polar angle of 90◦

Fig.2. LEPS Backward Gamma. The left panel shows a photo of the LEPS Backward Gamma
when the 90◦ –100◦ , 80◦ –90◦ , and 70◦ –80◦ Lead/SciFi modules were stacked. The rightpanel shows a photo of the constructed Backward Gamma together with SCISSORS III.

became horizontal. The largest Lead/SciFi modules covering a polar angle of 90◦ –100◦ were installed
into the frame at ﬁrst. Then 80◦ –90◦ modules were stacked on the 90◦ –100◦ ones. The 70◦ –80◦ and 60◦ –70◦ modules were installed in the same way. The left panel of Fig. 2 shows the middle stage of stacking
procedure. The 50◦ –60◦ , 40◦ –50◦ , and 30◦ –40◦ modules of the same azimuthal angle were stacked atonce so that the modules could not drop inside the frame during installation. The frame was rotated21back by 90◦ , after the installation of all Lead/SciFi modules was completed. Then the symmetry axis ofBackward Gamma was aligned to the axis of the GeV-γ beamline.

§ 3. Beam Test for Backward Gamma Detector
A beam test of Backward Gamma was performed from 5th to 7th June in 2007. A data acquisition
system for Backward Gamma was developed independently of that for the SCISSORS III. The energy
deposited in each module was digitized with a LeCroy 4300B Fast Encoding and Readout ADC (FERA)
in a CAMAC system. The data were collected and stored with a universal I/O (UIO) module [3] in a VME
system through a LeCroy 4301 FERA driver in the CAMAC system. The timing signals were measured
with CAEN V1190A TDC modules in the VME system. All the digitized data were ﬁnally accumulated
in a personal computer (PC) through the VME-bus. The trigger signals are made under the condition[TotalSigmaTagger] ⊗ [NBG ≥ 2],(1)

where TotalSigmaTagger stands for an OR signal of STB-Tagger II, and NBG ≥ 2 denotes the signal
which is generated when more than two modules are responded.
The energy calibration was made in such a way that the position of the peak corresponding to π 0events in the γγ invariant mass distribution should be at the π 0 mass. Figure 3b) shows the γγ invariant
mass distribution measured with Backward Gamma. The π 0 peak is clearly observed.

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Fig.3. a) Dataﬂow of the data acquisition system for Backward Gamma. The digitized data ofenergies in FERA modules in a CAMAC system are collected in a UIO module through
FERA driver. Those of timing signals are obtained with TDC modules in a VME system.
b) The γγ invariant mass distribution measured with Backward Gamma. The π 0 peak isclearly observed.

Acknowledgment
We appreciate Dr. T. Yorita and Dr. T. Matsumura for many suggestions and advices in the reassembling process of Backward Gamma.

22References[1]

T. Ishikawa: LNS Experiment #2536 (2005).

[2]

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

[3]

Y. Sugaya and M. Nomachi: Nucl. Instrum. Methods Phys. Res., Sect. A437 (1999) 68.