OBSERVATIONS OF BEAM LOSS PROTON TRAJECTORIESIN J-PARC LINAC*H. Sako#, T. Maruta, and A. Miura, Japan Atomic Energy Agency, Tokai, Japan
Abstract
In J-PARC linac, highest beam loss has been observed inthe ACS section at the H- beam energy of 181 MeV. The
primary source of the loss is considered to be H0's
produced by the electron stripping of H-'s by the remnant
gas inside the beam duct. The H0 almost conserves themomentum of the H- at the point where it has been
produced, and almost 100% of the H0's are converted intoH+'s (protons) through the beam duct. The H+'s areexpected to have small angles around 5 degrees withrespect to the beam, with energies 100-150 MeV,according to a simulation. To detect H+'s, we developed
six-plane scintillating fiber detectors. The 2 planes arelocated at an upstream position and the other 4 planes arelocated about 1.1 m downstream of the beam. We
measure each PMT signal by defining a trigger signal bycoincidence of dynode signals of the most upstream andthe most downstream PMT's.
We present preliminary results of time-of-flightmeasurements of charged particle tracks consistent withprotons, and distributions of tracks along the beam axis.

INTRODUCTION
The H0 ion produced by a beam H- interacted with aremnant gas atom is considered to be the main source ofthe beam loss in the J-PARC ACS section. The primary
aim of the present work is to identify H+'s converted from
H0's when passing through the beam duct. For thispurpose, we constructed six scintillating fiber detector
planes. Each plane consists of 16 4 x 4 x 64 mm3 plastic
scintillating fibers. Each pair of planes are connected to a
multi-anode photo-multipliers (PMT's) Hamamatsu
H8500C. This pairing is to reduce inefficiency when acharged particle passes through a boundary between
adjacent fibers in a plane.
Charged particle tracks are reconstructed with the fiber
scintillating detector planes. Energy of each track ismeasured with the time-of-flight method in 1.1 m distance,and energy loss in each plane is also measured by lightsignal amplitude in a PMT to identify protons and rejectother particle species such as electrons, X/gamma rays,and neutrons.
Another aim of this research is to measure beam loss
source positions along the beam duct and identify hotspots, by projecting each track to the beam axis.
We have measured beam loss with fiber scintillator
fiber detectors in ACS section at J-PARC linac since 2010
[1,2]. We already observed charged particle tracks___________________________________________*Work supported by JSPS KAKENHI Grant Numbers 23656063,24510134.
#hiroyuki.sako@j-parc.jp

successfully due to beam loss. Detailed description of thescintillating fiber counters and data acquisition system,see [1,2]. Since 2012, we use additional two fiber planes
to measure vertical positions, and in total we use twoupstream horizontal-measuring planes (H0, H1), two
downstream horizontal-measuring planes (H2, H3) 1130
mm apart from H0/H1, and downstream verticalmeasuring planes (V0, V1) 58 mm apart from H2/H3. In2012 measurements, we changed signal cables from LAN
cables to coaxial cables, which resulted in good signal
shapes without satellite tails. The trigger signal was also
improved as follows. Until 2011, we used a scheduled
timing of the macro pulse as the start signal for TDC(Time-to-Digital Converter) CAEN V785 and the gate
signal for QDC (charge (Q)-to-Digital Converter) CAEN
V792. There were two serious problems. First, since theTDC has no multi-hit recording function, if there isanother hit before relevant signal, a wrong combination oftimings in fibers is recorded, and thus signal-tobackground (S/N) ratio is significantly deteriorated. To
solve this problem, we utilized final-stage dynode signals(DY) which are available for the PMT H8500C. The DYsignals are opposite polarity (positive) signals to thenegative anode signals. Since the PMT is a multi-anode
PMT, the DY signal served as analog sum signal over all
anode channels. We discriminate the DY signals of thethree PMT's (DY1 for H0/H1, DY2 for H2/H3, and DY3for V0/V1), and form coincidence signal of discriminated
DY1 and DY3 for the start timing of TDC's and thetiming gate of QDC's. The DY1 and DY3 coincidence
signal (DY1&DY2) is used as a trigger of a charged
particle track penetrating from H0 to V1. Timing andcharge of individual anode signal can be measured withlow accidental background by narrowing the duration
between the start timing and the signal for TDC's, and thegate width for QDC's. We also formed a time gate (TG)from the scheduled beam start timing to define the timing
within the macro pulse. The coincidence DY1&DY3&TG
is used for the data taking. The timing offset and timeinterval per TDC channel for each fiber was calibrated byinjecting a pulse generator signal split to all the detector
signal cables end and measuring the timing with the dataacquisition system.

RESULTS
The analysis shown here is based on the data takenfrom Apr. 2012 to Jun. 2012. The fiber geometry for H0,H1, H2, H3 is shown in Fig. 1. We have moved H2 andH3 to cover three different track angles through H0-H3
with respect to the beam axis. The angle acceptance is
3.6o-5.0o, 6.0o-7.0o, and 9.0o-11.0o.

- 638 -With TDC hits, we reconstruct a 3-dimensional trackwith the horizontal position of the fiber xhiti, the hit timing
thiti, and the position along beam axis zi (upstream is thepositive direction) at Plane Hi (i=0, 1, 2, 3). We require
TDC hits in all 4 planes in H0-H3, and apply straight line
fits as the following formulae in z-x, and z-t plane
simultaneously;
x track = X 0 + X 1 z track,t track = T0 + T1 z track
where X0, X1, T0, T1 are the fit parameters. In Fig. 2, thetime residual thit1- ttrack1 is plotted, where ttrack1 is ttrack at
z=z1 (H1). A sharp signal peak for charged tracks withvery low background is observed. The width fromGaussian fit is 0.40 nsec in r.m.s.

Figure 1: Fiber geometry is shown as a function of the zposition (mm) along the beam axis and the horizontal xposition (mm). The upstream fiber positions (z around2170 mm) are common, while there are three
configurations for downstream fiber positions (z around1050 mm).

Figure 2: Residuals of hit timing (nsec) at H1 with respect
to the projected timing from the track. The width of 0.40
nsec in r.m.s. is obtained.

Fig. 3 shows QDC amplitude at H0 as a function oftime-of-flight (thit2-thit0). At the time-of-flight around 10.5
nsec and the amplitude around 450, a single clear peak isseen. The peak time-of-flight value corresponds to protonenergy of about 72 MeV. The horizontal bands around
100 and around 450 are due to accidental wrong
combinations between a H0 hit and a H2 hit. No electron
peak with β=1 around 3.8 nsec is observed.
Fig. 4 compares time-of-flight distributions at different
track angle acceptance. The signal peak is the highest at
the lowest angle range. The energies at the peak are 72.49
MeV at 3.6o-5.0o, 114.7 MeV at 6.0o-7.0o, and 79.05 MeV
at 9.0o-11.0o. These energies are consistent with simulated
H+ energies of 40-110 MeV. The observed energies havetendency of lower energy at a higher angle, which may be
due to higher energy loss since the track passes throughlonger effective thickness of the beam duct.

Figure 3: QDC amplitude (channel) at H0 as a function oftime-of-flight (nsec) between H0 and H2 at 3.6.

Figure 4: Time-of-flight (nsec) between H0 and H2 at thethree geometry configurations.

- 639 -Fig. 5 shows the projected z-position of each track tox=0, namely to the beam axis. The raw data (solid line)
shows decreasing numbers of tracks toward upstream
positions. However, as shown in Fig. 4, S/N is lower fordownstream positions, and we estimated H+ numbers
from these distributions. The results are shown as dashedlines in Fig. 5. After the correction, the H+ peak heights
are similar at 6.0o-7.0o and 3.6o-5.0o, and corresponding zranges of 5400-6300 mm and 6900-9000 mm, while
lower by the factor of 6 at 9.0o-11.0o and 4400-5000 mm.

We have examined linearlity of the trigger counts(DY1&DY3&TG) at TG width of 100 µsec with beamloss. We assume beam loss is proportional to the beamcurrent, and measured bem loss rates and beam current
while moving the beam stopper (scraper) position at
upstream MEBT1 as shown in Fig. 6. The beam current ismeasured by a Slow Current Transformer at the ACS
section. The line shows a straight line fit. The linearity isgood, but a slight saturation seems to be observed at twopoints with high beam current. In this measurement, weused a special geometry configuration. The distance
between H0/1 and H2/3 was set to be much shorter (a few
10 mm), and the horizontal distance to the beam duct was785 mm. In the normal geometry for time-of-flightmeasurements, the beam loss rate per pulse (the y-axis inFig. 6) is around 235.

CONCLUSIONS

Figure 5: Projected-z positions of x-z tracks at threegeometry configurations. Solid lines show raw numbersof tracks including accidental tracks, whereas the dashedlines are after the time-of-flight cuts and corrected toobtain proton numbers.

A clear time-of-flight signal consistent with H+ has
been observed with the improved trigger method with
PMT dynode signals. Assuming H+, the energy is 72.5114.7 MeV, which is cosistent with simulation. In themeasured angle ranges, the H+ rate is highest at the lowest
angle ranges of 3.6o-5.0o. The beam loss distributions
along the beam axis are demonstrated.
During summer shutdown period in 2012, we plan
several detector upgrades for the next beam operation
starting from Oct 2012. We are going to increase thenumber of fibers to read out for each plane and are
currently constructing two fiber planes for vertical
position measurements, which enables vertical trackreconstruction. We will install a remotely controlled
detector moving system in x- and y-positions. With these
upgrades, we expect to improve beam loss distribution
measurements significantly. We are also preparing
detailed detector simulation in order to estimate detector
acceptance for absolute beam loss rate estimation.

ACKNOWLEDGEMENT
We thank J-PARC linac monitor group for supports ofdetector installation and tests. This work was supported
by JSPS KAKENHI Grant Numbers 23656063 and24510134.

REFERENCES

Figure 6: Beam loss rate per 100 µsec macro pulse withthe trigger DY1&DY2&TG as a function of the peakbeam current at a Slow Current Transformer (SCT) at theACS section (mA).

[1] H. Sako, et al., “Beam Loss Particle Tracking in J-PARC
Linac,” 8th Annual Meeting of Particle Accelerator Society
in Japan, Tsukuba, Japan, p. 501 (2011).
[2] H. Sako, et al., “Measurement of Beam Loss Tracks by
Scintillating Fibers at J-PARC Linac,” IPAC11, San
Sebastian, Spain, 2011, p. 1251 (2011).

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