RESPONSE OF SCINTILLATING SCREENS TO HIGH CHARGE DENSITYELECTRON BEAM
F. Miyahara†, S. Kishimoto, M. Satoh, Y. Seimiya, T. Suwada, High Energy Accelerator Research
Organization (KEK), Tsukuba, Japan
M. Koshimizu, Tohoku University, Sendai, Japan
Abstract
Scintillating screen such as YAG is widely used forbeam size/profile diagnostics. However, the inorganic
scintillator shows light output saturation. That causes adegradation of the diagnostics. However, response ofscintillating screens to high current density electron beam
is not clear yet. We have evaluated the saturation effect on
the beamsize measurement by using a simple model.
Beam tests of screens, YAG:Ce, LYSO:Ce, BGO andAl2O3:Cr2O3, were performed at KEK e+/e- injector linac.
Scintillating crystals, YAG, LYSO and BGO, showedsimilar resolution, but the saturation of the light output
was confirmed for those crystals in the charge density of0.5-9 nC/mm2.

INTRODUCTION
Inorganic scintillating screens are very useful tool tomeasure transverse profile of charged particle beams. Thecerium-doped yttrium:aluminum:garnet (YAG:Ce) crystal
scintillator is used in many accelerating facilities. Thescintillating screen shows good resolution comparable tothat of OTR screen [1]. However, response to high charge
density electron beam has not been clarified. Saturation of
the fluorescence of scintillating screens causes broadening of measured beam size. In case of YAG screen,
Murokh et al. reported the saturation on YAG:Ce screen
for the high brightness ultra-relativistic electron beam [2].
They found that the saturation becomes real at the beam
intensities of the order ~0.04 pC/mm2 for 100 MeV beam.
In another report, the saturation was observed when theelectron beam charge density exceeds 1.5 nC/mm2 [3]. In
KEK e+/e- linac [4], the charge density (sigma) of theelectron beam for SuperKEKB high energy Ring will
exceed 25 nC/mm2. Thus, beam tests were performed on
cerium doped YAG (Y3Al5O12), LYSO (Lu1.8Y0.2SiO5),
and BGO (Bi4Ge3O12) crystals for 1.5 GeV, 1 nC/bunchelectron beam at the linac.

OVERESTIMATION OF THE BEAM SIZEDUE TO SATURATION
The degradation of the beamsize measurement causedby the saturation has been estimated by using a simple
model. The saturated distribution g(x) is given by( )= ( ) ( )____________________________________________† fusashi.miyahara@kek.jp

(1)

1 2

− 1
r (s) = − 2ss 1+ e
(2)

f ( x)s0
f ( x = 0)

(3)

s=

where f(x) is an original distribution, r(s) is reduction
factor due to the saturation which is known as a logistic
function, s0 stands for the strength of the saturation. TheEq. (3) represents decrement of the saturation in the outside of the distribution. The plots in the Fig. 1 (a) show
saturated distributions of the Gaussian distribution with
σ=1 for some s0. Beam sizes for those distributions are
estimated by fitting the Gaussian to the data. In the caseof s0=1.0, 25% decrease of the signal at the center isshown. The beamsize is over estimated by 10% althougha discrepancy between the distribution and fitted function(green line) is small. Figure 1 (b) shows the degree of theoverestimation as a function of the decrement factor at thecenter for the Gaussian distribution. In actual beam size
measurement, it is difficult to estimate the amount of thesaturation which leads to more than 10% overestimationof the beamsize.

Figure 1: Saturation of the Gaussian distribution (a) anddegree of over estimation of the beamsize as a function ofthe strength of the saturation at the center of the distribution. Solid lines in (a) show the Gaussian distributions
fitted to the saturated data.

EXPERIMENT

Scintillating Screens
We have tested three different scintillating crystals andan alumina-ceramic that are listed in Table 1 together with
their properties. Those crystals were chosen because they
have visible light output enough for the beam profile
diagnostics, good radiation hardness and processing char-acteristics to make thin flat surface screen (no cleavability). Thickness of those screens is 100 μm. While
YAG:Ce and LYSO:Ce show extrinsic luminescencebased on Ce, pure BGO has intrinsic one. The saturation
is caused by some quenching processes. Those processes
depend on luminescence mechanism and properties ofmaterial (e.g. Concentration quenching depends on concentration of doped ion). Thus, different response to highcurrent density electron beam is expected.
Table 1: Properties of Scintillatorsτdecayλmax[ns][nm] output

Relative(NaI:TlRadiation
hardness[rad]=100)

YAG:Ce7055035>106

LYSO:Ce

41420

75>106

BGO

300

480

21>105-6

Al2O3:Cr2O3> ms690

LargeHighsaturation of the 12-bit CCD camera. The beam repetition
rate was 1 Hz to avoid the effect of the afterglow.
A magnetic quadruple triplet which was set 9.7 m long
upstream was used to focus the beam. After the optimization of the optics to get minimum beamsize at the screen
position, the quadrupole scan using the last quadruple wasperformed for each screen. The electron beam was stable
during the measurement; the beam current variation andposition jitter at the screen were less than 0.3% (sigma)
and 0.1 mm (RMS), respectively.

RESULT
Figure 3 shows beam profiles taken by 4 different
screens. Because the ceramic is formed by sintering thatcauses diffuse reflection in the screen, the beam profile onthe Al2O3:Cr2O3 is larger than that of scintillating crystal
screen. In case of alumina-ceramic, due to a long exposure time (20 ms) for the CCD camera, many white spots
exist on the image. The exposure time for other screen
was set 10 μs.

Experimental Setup

Figure 2: Experimental setup.
The beam profile measurements were performed at thebeam dump line of the first straight section of the linac.
The experimental setup is shown in Fig. 2. The electron
beam was generated by the thermionic gun and accelerated up to 1.5 GeV. Horizontal and vertical normalized
emittances of the incident beam were about 50 and
100 mm mrad, respectively. The beam passed through a
30 μm thick stainless steel window. The distance between
the window and the screen was 11 mm. The screen at theexit of the window could be moved by using the motorized stage. The beam size spread at the screen due tomultiple scattering was estimated to be 3 μm (sigma) byusing EGS5 code [5]. This effect is negligible for theexperiment. The variable ND filter was used to avoid

Figure 3: Background corrected beam images and projected beam profiles at 1 nC/bunch beam (single shot) on
four different screens. The red line represents a Gaussianfit.
The beam size which is 10 shots average as a functionof the quadrupole magnet is shown in Fig. 4. The strength
was changed from -14.03 to -13.07 T/m. The aluminaceramic shows bad resolution as seen in Fig.3 and theshape of the scan curve is different from the others that
can arises from the afterglow. Three crystal scintillating
screens exhibit same beamsize except for defocusing stepafter the bottom of the curve in the left figure. We assume

that the difference is attributed to a difference of the degree of the saturation. The saturation effect seems to remain for a time because beamsize discrepancies wereobserved in large beamsize area in defocusing step.

Figure 4: Horizontal and vertical beamsize (10 shots average) as a function of quadrupole strength.
Figure 5 shows relative total light output as a functionof the charge density. Because of the beamsize is different on the each screen for the same condition, the size
measured by BGO for corresponding quadrupole strengthis used to estimate charge density. The charge density isgiven by Qtotal [nC]/(πσxσy) [mm2]. The relative output
should be 1, because the bunch charge was fixed to 1 nC.
However, decrements of the light output along with thecharge density are shown for all scintillating screens. Itsuggests that the saturation of luminescence is occurred
above 1 nC/mm2 for those scintillating screens. The saturation on the YAG consists with Ref. [3]. The decrementof the light on the LYSO is much larger than the BGO andthe YAG. It could be related to larger beamsize than thatof the BGO at the bottom of curve in Fig.4. In this experiment, we used only scintillating screen. Thus, simultaneous measurement using OTR or wire scanner is required
to estimate more accurate charge density and amount ofoverestimation of the beamsize.

Figure 5: Variation of total light output and charge density at the screen position. The relative light output is normalized by total light output at the quadrupole strength of-14.03 T/m which corresponds to the start point of thequadrupole scan.

SUMMARY AND PROSPECT
We have studied the saturation of the scintillating
screens and its influence on the beamsize measurement
for the high charge density electron beam. In case of thesimple saturation model, more than 10% overestimationof the beamsize can be occurred if above 25% decrease ofthe signal at the center of the distribution. The beam testof scintillating screens, YAG:Ce, LYSO:Ce and BGO,
were performed at KEK e+/e- Linac. Those screensshowed same resolution in charge density within a rangeof 0.5 to 9 nC/mm2. However, the saturation was observed on those screens above charge density of
1 nC/mm2 for 1.5 GeV beam. In order to estimate thequantity of the saturation and its effects accurately, weplan to experiment with OTR.

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