Development of Beam-Profile Monitoring Methods in SecondaryHadron Beamline(Report on T539)
Atsushi Sakaguchi, Osaka University
The high-intensity proton accelerator project at the Japan Proton Accelerator ResearchComplex (J-PARC) is in progress. The nuclear and particle physics facility to use the 50
GeV proton-synchrotron beam is under final-designing and the construction of the facility
will start soon. The facility will provide us high-intensity secondary hadron beams, thanks
to the high-power primary proton beam (0.75MW). Although the quality of the beam is basically determined by the design of the secondary beamline optics, some tunings of beamline
elements are necessary to obtain the designed performance of the beamline. The tunings
have been done by looking at the beam intensity, the purity of the beam particle species andthe beam profile at the secondary target position. The beam status or beam profile between
the primary and the secondary target positions have never been monitored due to technicaldifficulties, and beam focusings and effects of various slits in the secondary beamline havenever been confirmed directly.
We are developing several beamline monitoring detectors optimized for the profile measurement of the secondary beam. The difficulties of the profile measurement in the secondary
beamline mainly come from the intensity of the beam in the beamline. There are several
detectors for the beam profile measurement of the primary beamline, at where typical beam
intensity is 1012 particle/spill. In such high beam intensity, detector measures integrated
signal of beam particles and signal levels are considerably high. Other types of detectors
for the beam profile measurement are used in low beam intensity environment, say 106 particle/s, and the measurement is usually made by counting every beam particle. We will
be faced with beam intensities from 108 to 1010 particle/spill in the secondary beamline
at the new facility. Beam monitoring detectors in the intermediate beam intensity are notestablished yet.
We are testing following prototype detectors: (a) Segmented Parallel plate Ionization
Chamber (SPIC) modified for a small-signal readout with an amplifier, (b) glass fiber profile
detector based on Čerenkov radiation in the fiber with a PMT readout, and (c) plastic
scintillation fiber detector with a PMT and amplifier readout. In the T539 experiment, weinstalled the prototype detectors near to the Intermediate Focus (IF) position in the PS/K6
secondary beamline, and typical beam intensities were a few 10M particle/spill and 4M
particle/spill at IF and at the secondary target, respectively. We observed a clear image ofthe vertical acceptance slit of the K6 beam line by the modified SPIC detector as shown inFig.1. Estimated beam intensity was about 2M particle/spill for each readout strip (2.5mmin width) of SPIC. This was the first time that a profile of a beam in such a low intensity
was measured clearly with a position sensitive ionization chamber.
Stable operation of the fiber detectors (glass and scintillation fibers) in the secondary
beam line was confirmed, at least, up to about 5M particle/spill/fiber. We noticed a propershielding of PMT was necessary to avoid background from stray radiations in the secondary
beam line for the glass fiber profile detector. This is a important point to be considered
for designs of beam profile detectors in the secondary beamline. The integrated yield of
the background radiations is almost same level as that from the secondary beam, so someinsensitivity to the background particles is required for the monitoring detectors.

Signal Level

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Signal Level

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Figure 1: Signal level from every SPIC strip when the acceptance slit was open (top) andthe slit was closed down to 12 mm (bottom). The shape of the secondary beam can be seenclearly in the bottom plot.