HUA Master’s Thesis Award abstract
Construction of a BGO Calorimeter Systemfor the Σp scattering experiment
Department of Physics, Tohoku University
Michihiko Ikeda
Introduction
We plan to perform a Σp scattering experiment (JPARC E40) for the study of the ΣN interaction. In
this experiment, we are going to determine the diﬀerential cross sections of the Σ+ p, Σ− p elastic scatterings and the Σ− p → Λn inelastic scattering with highstatistics.
In the E40 experiment, π beams are irradiated toa liquid hydrogen target. Σ± are generated by the(π ± , K + ) reactions, and some Σ particles are scattered with a proton in the target. A momentum
vector of the incident Σ± is measured by two spectrometers, that is, the K1.8 beamline spectrometerand the KURAMA spectrometer. In order to identify the Σp scattering events, the relation between thescattering angle and the kinetic energy of the scattered proton is requested to be consistent with theΣp scattering kinematics. For this identification, we
are newly developing detector system, which is composed by a cylindrical fiber tracker (CFT) and BGO
calorimeters. By surrounding the target with this detector system, the scattering angle and the kinetic
energy of the scattered proton are measured with
CFT and the BGO calorimeters, respectively. TheBGO calorimeter is a large BGO scintillation counter(30 × 25 × 400 mm3 ), read by PMT. Here, the resolution of the BGO calorimeters is required to be better
than 3% (σ) for 80 MeV proton.
In the E40 experiment, BGO calorimeters areplaced under a high counting rate (40k - 400 kHz).
The purpose of this research is to develop a readoutsystem of the BGO calorimeters which satisfies required resolution in the E40 condition. Then we willconstruct an actual BGO calorimeter system.

Readout system of BGO calorimeter
Under the high counting rate, the signal from BGO
must be piled up with a high probability due to its
long decay time of 300 ns. In order to decompose
the pile-up signal, we decided to employ a readoutsystem with a Flash ADC. In this system, waveforms
are recorded with Flash ADC and are analyzed to separate pile-up signals oﬄine. A signal from the PMT
is shaped by a integral circuit and are recorded by
CAEN V1724 Flash ADC to keep a reasonable resolution with rather long sampling time of 30 ns. In thisreadout system, the waveform can be reconstructed
suﬃcient accuracy from 70 sampling points.

Evaluation of the readout system
In order to check the performance under the highrate condition, we performed a test experiment using
a 77.8 MeV proton beam at a cyclotron facility in Tohoku University (CYIRC). The proton beam was injected to the BGO calorimeter by changing the beam
rate from 1 kHz to 700 kHz. The stability of PMT
was also studied by changing the applied voltage. Asa result, the resolution was obtained to be 1.3% (σ)
at 450 kHz beam rate by operating PMT with 600 V(rated voltage: 900 V). The eﬃciency of the oﬄine
waveform analysis was also obtained to be 99% at
40 kHz. From these results, BGO calorimeter system
has achieved the required energy resolution under thehigh counting rate.
Furthermore, it is necessary to establish an energy
calibration method as a calorimeter. In the experiment at CYRIC, we obtained the information about
the four diﬀerent beam energies by degrading thebeam energy with the Cu degraders. From the relation between the BGO signals and proton beam energy, an energy calibration formula was obtained. Using this formula, we could reproduce the pp scattering
data with an accuracy of 0.4 MeV in the energy rangeof more than 10 MeV.

Actual calorimeter system
We assembled a frame structure for the BGO
calorimeters and installed 22 BGO calorimeters tothe frame. We have constructed the actual BGO
calorimeter system. In order to operate in the E40
experiment, the actual system was checked its various performances, such as energy resolution and soon. The resolution was obtained to be 15% (σ) at662 keV γ ray from a 137 Cs source. Assuming√ thatthe energy resolution is proportional to 1/ E, theresolution at 80 MeV is expected to be 1.4%. This
resolution satisfies the requirement.
On the other hand, we also construct and tested thedata acquisition (DAQ) system of the BGO calorimeters. From the E40 experiment, DAQ is required tooperate suﬃciently at 5 kHz trigger rate.
By devising the data transfer of V1724, actual
calorimeter system has eﬃciency of 98% at 5 kHz trigger rate. Therefore, it is expected that DAQ can be
performed with suﬃcient eﬃciency.
From these works, the BGO calorimeter system becomes ready for the E40 experiment.