The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

New emittance measurement using two PPACs
Akira YONEDA, Hidekazu KUMAGAI, Misaki KOBAYASHI, Masanori KID ERA,Masayuki KASE, Akira GOTO and Yasushige YANO
The institute of Physical and Chern. Research (RIKEN)

Abstract
3. Details of PPA C
A new emittance monitor has been developed withtwo sets of position-sensitive gas counters. It was designed
for the precise measurement of the beam emittance about ahigh-energy beam from the RIKEN Ring Cyclotron. A
preliminary test was done.

1. Introduction
At RIKEN Ring Cyclotron[!], the beam emittance has
been measured with a combination of movable slits andmulti-wire profile monitors[2]. By this monitor, precise data
are obtained about the emittance for the low energy beam,for example, a beam from an ion source and from injectors.
In the case of high energy, for example 135 MeV/u C beam,
this method is not applied, because a thick slit is necessary
for this, therefore a signal from wires becomes small. A
rough estimation of the beam emittance has been only doneusing a normal profile monitor for this beam. In the future,
the RRC will be used as an injector to the next stage
accelerator[3]. For the design of new accelerator, we need
the precise emittance measurement for the beam extracted
from the RRC.

A PPAC system consists of electrodes, housing, alinear actuator and electronics.
3.1 Electrode and Hardware

Fig. 1 shows the electrode assembly and its housingwith two windows.
-To use the PPAC as a two-dimensional-positionsensitive detector, its electrode assembly consists of ananode and two cathodes on both sides of the anode. Theseelectrodes are put parallel to each other with separations of4 mm. The electrode is a thin gold layer (60 !-1g/cm2 inthickness) which is deposited on a 2.5 1-1m Mylar film. The
anode is covered with the gold layer all surface on its bothsides. On the other hands, the cathode has 51 gold strips
whose pitches are 1 mm. The sensitive area is 50 x 50mm2 .
The charge division method is employed for the signal
readout. On one side of the cathode strip, every strip
electrodes is connected to the next one through a 200 ohm
resistor. Two signals leaded from both ends are introduced
to the outside, and used to get the position information.

2. Principle of measurement
We employ two sets of position-sensitive-type PPACs(parallel plate avalanche counter), which is widely used as acounter for nuclear physics experiments in RARF (RIKEN
Accelerator Research Facility). Such a PPAC itself canwork as a beam profile monitor for the low intensity beam.
Suppose that two position-sensitive detectors areinstalled in a straight beam line with a distance of L in theinterval, and that the sensitive areas are large enough toensure all particles in the beam through the both detectors.
When one particle passes though the detectors, two sets ofdata; (Xa, Ya) and (Xb,Yb) are obtained as the twodimensional position data, where the suffix 'a' and 'b' are forthe first (upstream) detector and the second (downstream)
one, respectively.
From the obtained data, a trajectory of the particle iscalculated easily as (Xa,X') and (Ya, Y'), where X'=(XbXa)/L and Y'=(Yb-Ya)/L. If the thickness of the detector isthin, the trajectory is not affected by the existence of thedetector. After accumulating these data (Xa,X') and (Ya, Y')
for a number of particles in the beam, we can get the beamemittance.

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gas_out

gas_insig X1,X2, Y1, Y2

HVFig.1 An electrode assembly of PPAC and its
aluminum housing with two windows.

The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

position ·signals from two cathodes and one feedthough forthe bias vbltage to the anode.
As shown in Fig. 2, the counter box is fixed on one endof a support pipe which is driven linearly by a pneumaticactuator. The counter is set exactly on the beam axis
remotely when in use, and removed out of the beam area
when no use. Two PP ACs have been installed inside thebeam diagnostic chambers separately, a distance between
them is 2.9 m along the beam line just after the extraction ofthe RRC. There is no forcussing element between the twochambers.
Actually the probe is installed in the beam line with anangle of 45 degrelative to horizontal axis as shown in Fig.2,
because the space just above the chamber is reserved for thealignment. Therefore the position data has to be converted
into the normal horizontal-vertical coordinate.
Two gas lines and five signal cables are introduced toatmosphere through a counter support pipe. A gas ofisobuthane is being supplied as a stable gas flow, keeping apressure inside the counter around 8 torr. A gas supply
system consists of a mass flow controller, a leak valve and arotary-pump.

a)

PPAC

Beam Line

Fig.2 PPAC set on the beam line. (a) cross sectional
front view from the upstream of the beam line;(b) cross sectional side view.

3.2 Electronicsand Software

The two cathodes are set so that the orientations of strip will
be in different way. One is used for the measurement of theX-position and the other is used for theY-position.
These electrodes are set inside the aluminum box
which has two windows (for the beam entrance and exit) of
50 x 50 mm2 in size and the distance of the two windows is
32 mm. The window film is 2f..Lm Mylar on the surface of
which aluminum (8 f..lg/cm2 ) is deposited. This counter box
has a gas inlet and outlet and four feedthroughs for thede!.

In Fig. 3, a block diagram of electronics is shown. Apreamplifier is located nearby the counter to keep theprecise imformation of the positions.
First, a PC takes eight signals, two signals for theX-position and two signals for the Y-position from eachPPACs. If only the eight signals are all above the threshold,
10% of the maximum data range, the PC accumulates thesedata and calculates the X-position and theY-position, which
are Xa, Xb, Ya and Yb. The reason to set the threshold is toNIMBF~n-fin-foutsumshaTFA

CFD

r------~----

HVC.C.
Sch-ADC
CAMAC
Fig.3 Block diagram of electronics-277-computer

The JlthSymposium on Accelerator Science and Technology, Harima Science Garden City, 1997

get a position information accurately. The signals smaller
than the threshold have a possibility to make the accuracy
worth of the position ~ata. The beam profile is taken bysuch way, the beam emittance is also calculated by using
these data.
4. Error estimation

A scattering of the beam by a material of the upstream
counter has the effect on the emittance measurements. A
total thickness of one PPAC corresponds to 1.9 mg/cm2Mylar. A scattering angle was estimated using by theZiegler formula. As a result, the scattering angle of the 135
MeV/u Ne beam after passing PPAC is less than 0.1 mrad.
This value means the effect of the angler scattering isnegligible.
During the measurements with PPAC, the beam ha.S to
be attenuated down to order of 104 pps. Much attention
should be paid in order to attenuate beam without changing
the full-beam emittance. A set of the thin copper plate withmany small holes on it is used for beam attenuation. We
need always check that the beam profile is not changed after
the beam attenuation is done.
The position error of PPAC is estimated to be 0.3 mm.
The error due to it on beam angle is 0.1 mrad, being
negligible.

5. Preliminary Test

A test for the emittance measurement was done with abeam of 110 MeV/u 22Ne. The obtained data of the profileand the emittance of the beam are shown in Fig.4 and Fig.5.
The obtained ernmitances are 20 7tmm·mrad for horizontal
plane and 7.5 7tmm·mrad for vertical. The horizontal data islarger than the expected value. The calibration of position
and angle are under way.
6. Conclusion

From the result of beam test, the PPAC can work as abeam emittance monitor instead of the orthodox
measurement way with using slits and multi-wire profile
monitors. By this method, an accurate measurement may
become available.
References[1] M.Kase et al., "Status Report on RIKEN Ring
Cyclotron", in this proceedings.
[2] M.Kase et al., Proc. 11th Int. Conf. on Cyclotrons andtheir Applications., (1986) 443-446[3] Y.Yano et al., "RIKEN RI Beam Factory Project",in this proceedings.

( Xa,Ya) Srnrnldiv( Xb,Yb) Srnrnldiv......•••: .....
+Fig. 4 A beam profile measured by the PPAC

( x,ex) Smrnldiv, ImR/div.

( y,9y) Smrnldiv, lmR/div.

'Fig.5 A beam emittance measured by the PPAC

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