EXPERIMENTAL TESTS OF SCREEN MATERIALS FOR HIGH-PRECISION TRANSVERSE BEAM-SIZE MEASUREMENTS AT THE SuperKEKB
INJECTOR LINAC
F. Miyahara†, K. Furukawa, M. Satoh, Y. Seimiya, T. Suwada,High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract
The SuperKEKB injector linac is required to deliver
low-emittance electron and positron beams. Wire scanners
are employed to measure Twiss parameters and to adjust
beam optics conditions. Screen monitors also play important roles for single-shot measurements. However, thebeam size became more than 10-times smaller comparedwith that of the KEKB injection. Beam tests have been performed in order to evaluate materials for high-precisiontransverse beam-size measurements at the injector. The
main purpose of the beam tests is to quantitatively investigate the saturation effect of each screen material for generating the scintillation light, which is strongly depending on
the beam fluence. Several scintillating screen materials including YAG:Ce, LYSO:Ce, BGO and aluminum ceramic
doped chromium oxide (Al2O3:Cr) have been tested withhigh energy and high fluence. The results are comparedwith that obtained by the OTR measurement. The saturation of the luminescence was confirmed for all crystals andevaluated in fluence of the ~1 nC/mm2.

INTRODUCTION
A very thin inorganic scintillator, YAG:Ce crystal is usedin many facilities, is used to measure the charged particle
beam profile or the temporal structure with the rf deflector.
Beam profile measurement using a scintillating screen iseasier to align the optical system than the optical transition
radiation (OTR). In case of ultrashort bunch length, coherent OTR gives incorrect beam profile. By employing optimized imaging system, the resolution of the profile is comparable to the OTR screen monitor [1]. The response ofYAG:Ce crystal to the high brightness electron beam wasstudied and the limitation of the resolution is discussed inRef[2]. In case of YAG:Ce screen, the saturation becomesreal at the electron beam fluence of the order~0.04 pC/mm2 for 100 MeV and this limit scales with energy. In the KEK linac, the electron beam with the bunchcharge of 4 nC, the energy of 7 GeV and the normalized
emittance of 40(Horizontal) / 20(Vertical) µm is required
for SuperKEKB HER injection [3]. Because the beam sizein the linac is ~100-200 µm, the fluence exceeds the limitation scaled by the beam energy. Thus, we have performed
beam tests of scintillating screens, YAG:Ce, LYSO:Ce,
BGO and Al2O3:Cr, to confirm the saturation of the light
output to the high fluence electron beam [4]. Properties of
those scintillators are summarized in Table 1. The saturation of the luminescence of those scintillators, except for_________________________________________† fusashi.miyahara@kek.jp

Al2O3:Cr, was observed at fluence of 1 nC/mm2 for
1.5 GeV beam, but there was no reference measurement
using the OTR screen. Thus, we have measured a beamprofile measurement using those scintillating screens andan OTR screen.
Table 1: Properties of Scintillators
tdecay
[ns]

lmax
[nm]

Light yield
Density3[10 ph/MeV] [g/cm3]

YAG:Ce

70

550

174.6

LYSO:Ce

41

420

33

7.1

BGO300480

97.1

Al2O3:Cr> ms690

Large

4.0

EXPERIMENTThe KEK linac consists of 2 straight sections and 180 degrees arc section between them. The beam profile measurements were performed at a test beam line with a beam
dump located downstream of the first straight section. Thelinac has an RF-gun for the generation of the low emittanceelectron beam and a thermionic gun for low current beamfor PF/PF-AR storage rings or high bunch charge beam toproduce a positron beam for SuperKEKB LER. In this experiment, a bunch charge of 1.15 nC beam generated by therf-gun was used, and the beam accelerated up to 1.5 GeV.
The experimental setup is shown in Fig. 1. The beam passthrough a 30 µm thick stainless steel window and a blackout fabric to block the OTR generated at surface of the window. Scintillating screens and OTR screen are set on a motorized stage. The thickness of these scintillating screens is
0.1 mm. Two imaging system are located in the directionof 90 (OTR screen) and 19 (scintillating screens) degrees
to the beam line respectively. The layout of the imaging
system is also shown in the figure. To avoid saturation ofthe CCD and adjust quantity of light on the CCD for eachscintillating screen, the variable neutral density (ND) filters is used. Because Al2O3:Cr has a long decay time, signal
level of pixel outputs were adjusted by exposure time (500µs for the Al2O3:Cr, 50 µs for other screens). The CCD
camera has a sensor resolution of 659x493 pixels (cell size
7.4 µm) and 12bit ADC. The bandpass filter is used forlimiting wavelength region for the OTR and Al2O3:Cr. Theresolution of the imaging system around beam spot is better
than 20 µm that is 4 times smaller than the previous study.

Figure 1: Experimental Setup.

exceeding half maximum estimated by OTR measurement
and relative light output of those scintillating screens. The
YAG:Ce and the BGO show the same rate (~7%/mm2/nC)
of light reduction with increasing fluence, but reduction
rate of the LYSO is about twice as large as them. It is assumed that the main cause of saturation is ionizationquenching. An index related to ionization quenching is thea/b ratio which is given by the ratio of the light yield toalpha and beta rays with a kinetic energy of ~5.5 MeV (thea/b ratio of many crystal is 0.2-0.3), and a large ionizationquenching means a small a/b ratio. The a/b ratio ofLSO:Ce, which is very similar to LYSO:Ce, is 0.14, that issmaller than the ratio of 0.2 (BGO) and 0.3 (YAG:Ce) [5].
This is considered to be one reason why LYSO:Ce is highly
saturated. In the LYSO:Ce, the saturation clearly worsens
the resolution. At the most convergent point (OTR: 0.18mm x 0.26 mm), the beamsize is about 1.2 times that ofother scintillating screens and OTR screen. The reason why
the light output of the Al2O3:Cr increases with fluence isnot clear. In this paper, we could not determine the threshold at which saturation began for those scintillators. In
YAG:Ce case, by scaling the intensities reported in Ref. 2
to 1.5 GeV, the saturation may appear from ~0.3 nC/mm2.

The electron beam was focused on the screen by using aquadrupole triplet which was set 9.7 m long upstream. Inorder to investigate the change in the amount of light withrespect to the beamsize, the last quadrupole magnet in thetriplet used to change the beamsize. To suppress an effectof the afterglow to the measurement, the repetition rate was
set to 2 Hz. The data acquisition of the profile measurements were synchronized with measurements of BPMs
which gives accurate bunch charge of the beam. Variationsof the bunch charge and beam position at the screen wereless than 1.5% and 50 µm respectively.

RESULT
Sample of beam 2D beam profiles and hor./vert. beam size
measured by 5 different screens are shown in Fig. 2 andFig. 3 respectively. Since the beam is not Gaussian, thebeam sizes were evaluated by the RMS of the distribution
with background subtraction. In previouse work, theAl2O3 :Cr shows a poor resolution compared with otherscintillating screens. In this study, beamsizes measured
with the Al2O3 :Cr are consistent with the beam size measured with the others. The degradation of the resolution inpreviouse work is considered to be caused by chromaticaberration due to wavelengths longer than 700 nm. However, since the Al2O3:Cr is formed by sintering, 2D profile
with the Al2O3:Cr appears to be blurred.
Variations in the quantity of light output which normalized by measurement value with maximum beam size withrespect to the strength of the quadrupole magnet are shownin Fig. 3. Saturation of the luminescence can be confirmed
for those scintillating screens except for the Al2O3:Cr. Figure 4 shows correlation between the fluence of a region

Figure 2: Beam profiles measured by scintillating screensand OTR screen.

LYSO:Ce and Al2O3:Cr, were performed, and quantity ofluminescence and beam size were compared with measurement using OTR screen. Those scintillating screens, except
for Al2O3:Cr, show the saturation of luminescence for thefluence above ~1 nC/mm2. YAG:Ce and BGO have thesame rate of decrease of luminescence with respect to thefluence, and LYSO:Ce has the rate more than twice that ofthose scintillators. In the case of A2O3:Cr, it was found thatthe quantity of the light output increased with the fluence.
In case of beam size is larger than 0.18×0.26 mm2 (RMS)
which has a fluence of 4.2 nC/mm2, YAG, BGO andAl2O3:Cr did not show no increase in beam size comparedwith the OTR, but the measured beamsize with LYSO:Ce
was increased by about 20%. We could not clarify the fluence at which saturation began for those scintillators. Thus,
more detailed analysis and experiments are required.

REFERENCES

Figure 3: Relative light output and beamsize as a functionof the quadrupole gradient. Plotted data is an average of 10
measurements with the bunch charge of 1.150±0.005 nC.
The light output is normalized by the data at 11.64 T/m.

[1] R. Ischebeck et al., “Transverse profile imager for ultrabrightelectron beams”, Phys. Rev. Accel. Beams, vol. 18, p. 082802,
Aug. 2015, doi:10.1103/PhysRevSTAB.18.082802[2] A. Murokh et al., “Limitation of the resolution of YAG:CE
beam profile monitor for high brightness electron beam”, inProc. 2nd ICFA Advanced Accelerator Workshop, Los Angeles, USA, Nov. 1999, pp. 564-580.
[3] K. Furukawa et al., “Rejuvenation of 7-GeV SuperKEKB injector linac”, in Proc. IPAC’18, Vancouver, Canada, Apr.May
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doi:10.18429/JACoWIPAC2018-MOPMF034[4] F. Miyahara et al., “Response of scintillating screens to highcharge density electron beam”, in Proc. IPAC’17, Copenhagen, Denmark, May. 2017, pp.268-270, doi:10.18429/JACoW-IPAC2017-MOPAB067[5] W. Wolszczak and P. Dorenbos, “Nonproportional responseof scintillators to alpha particle exitation”, IEEE trans. Nucl.
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TNS.2017.2699327

Figure 4: Relative right output, normalized by the mostconvergent point, and fluence of the beam.

CONCLUSIONIn order to investigate the response of the scintillating
screen to a high fluence and high energy electron beam,
beam tests for scintillating screens, YAG:Ce, BGO,