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Nuclear Instruments and Methods in Physics Research A 362 (1995) 224-228

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iii! EISEVIER

NUCLEAR INSTRUMENTS

&METnoDS IN PHYSICS RESEARCH

SectlonA

An electron-beam transport system for parasitic experiments

T. Saito a, *, T. Terasawa a, 0. Kormo a, S. Ito a, H. Itoh a, M. Oikawa a, Y. Suga a, M. Mutoh a, T. Tamae a, M. Sugawara a, D. Sims b

a Laboratory of Nuclear Science, Tohoku Unioersity, M&mine, Taihakuku, Sendai 982. Japan b School of Physics, Uni~~ersity of Melbourne. Parkville 30.52, Australia

Received 14 December 1994; revised form received 21 March 1995

Abstract A new beam line to allow parasitic experiments in two separate experimental halls has been installed. The total length of

the beam line is about 80 m. Two achromatic 90” deflecting systems have been incorporated. For the beam delivered to the

second experiment, the beam diameter was 2 mm and the emittance was 1 mm mrad. The duty factor was reduced to an

average of 50% from 80%. This decrease was satisfactorily recovered using a deflector system synchronized with the beam

pulse. The tagged photon beam, and experimental conditions were significantly improved when the transported beam was

used. Successful parasitic experiments involving electron scattering and tagged photon experiments have now been demonstrated.

1. Introduction

At this laboratory, both coincidence-electron experi-

ments such as (e, e’p) and (e, e’n), and high energy tagged-photon experiments are used to study nuclear exci-

tation modes, and the decay of excited nuclear states. Both these experiments use the 150-MeV pulse stretcher ring

[l]. Both techniques require long beam time assignments,

and it was considered that they might run parasitically, and hence allow an expanded experimental program.

The average current used for electron scattering experi-

ments is a few p,A, while for tagged-photon experiments

only a few nA are required. Thus parasitic tagged-photon experiments would be possible in conjunction with elec- tron-scattering experiments if the beam exiting an electron scattering target, is supplied to the tagging system.

Until now we have carried out both types of experi- ments in the same experimental hall (Hall 2), in which the

stretcher ring is also located (see Fig. 1). To allow para- sitic experiments, a new beam line was installed to trans- port the beam from Hall 2 to another experimental hall (Hall 1). Conveniently there was a disused tunnel joining the halls, and the beam line utilized this. Using this beam line successful parasitic experiments have been carried out. In this paper we describe the design and construction of an electron beam transport system for parasitic experiments,

* Corresponding author.

and report results of the experiments which demonstrate its successful operation.

2. Beam transport system

A schematic diagram of the beam transport system is

shown in Fig. 1. The system is about 80 m long, and consists of three straight sections and two 90” deflecting

systems. Each 90” deflecting is achieved with an achro-

matic system consisting of two dipoles and a quadrupole magnet [2] as shown in Fig. 1. There is an energy defining slit in the first 90” deflecting system. A quadrupole doublet

is used to transport the beam through each long straight section, along the length of which are coils to compensate for the earth’s magnetic field. A l-cm diameter brass collimator was placed at the middle of the first straight section.

The design of the beam transport system was carried out using the code TRANSPORT [3]. In the design, the

emittance of the beam passed through the first target (for target thickness of l/100 radiation length) is assumed to be N 100 mm mrad estimated from the emittance of the beam from the stretcher ring ( * 20 mm mrad) and increase of the emittance due to its passage through the target. The emittance will be reduced by a factor of about 30 in both directions due to the collimator, which also reduces the primary intensity by a factor of about 1000. Then the initial emittance for the transport system is assumed to be 3 mm mrad. The beam trajectory calculated for the beam with an emittance of 3 mm mrad is shown in Fig. 2.

0168-9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved

SSDI 0168-9002(95)00385-l

T. Saito et al./Nucl. Instr. and Meth. in Phys. Res. A 362 (1995) 224-228

36.098m

18.224 I_ I I.614

I ? 1””

1 1

IP ZnS Q,, IP

L f

IP

E Q 1.2

ti Collimator

ad

vl.8 :

ZnS IP

7.537

Hall 2 Hall 1

Cherenkov detector

Linac

Fig. 1. Schematic diagram of the beam transport system. SSTR pulse stretcher ring, M dipole magnet, Q quadrupole magnet, IP ion pump

and ZnS beam monitor.

Beam ducts are made of .50-mm-diameter aluminum

tube. They are evacuated by four ion pumps and three turbo-molecular pumps to a vacuum of _ lo-’ Pa. The

beam size is monitored at four places shown in Fig. 1 with

z2 . _.- .

* ;I,,,Mh;, Q4.5 MQ”~ Q7.8 3 I I I I I I I

0 10 20 30 40 50 60 70 80

Z(m)

Fig. 2. Envelopes calculated for the incident beam sizes x = y = 3

mm and emittances of E, = E.” = 3 mm mrad.

photoluminescence plates (ZnS) which are remotely con-

trolled. The main parameters of the beam transport system are given in Table 1.

Table 1

Parameters of beam transport system magnets

Dipofe magnets

Radius of center trajectory

Pole gap

Maximum energy

Field strength

Field uniformity

Deflection angle

Entrance and exit angles

Maximum current

Maximum power

Weight

Quadrupole magnets

Geometric length

Aperture radius

Field gradient

Maximum current

80 cm 35 mm

150 MeV

5500 G

1 x lo-’

45”

12.43”

200 A

12 kW

660 kg

10 cm

26 mm

5.1 T/m

10 A

226 T. Saito et al. / Nucl. Ins@. and Meth. in Phys. Res. A 362 (1995) 224-228

3. Test of the beam transport system IO

As a test of the beam quality, the electron tagging

spectrometer [4] was installed in Hall 1, at the end of the beam line.

t“,“““““““““““““~ : 0

o”o 0

-O 0 000 0 Target in

oooo 0000 Ooeo oooeo~)

3.1. Emittance

The emittance of the transported beam was obtained by measuring the profile of the electron beam at the radiator

position of the tagging spectrometer, and 2 m further down

stream. The radii were 2 mm and 3 mm at these positions, giving an emittance for the transported beam of 1 mm

mrad.

0.1

0 5 IO 15 20 25 30

Tagging Channel Number

Fig. 3. Spurious rate measured at Hall 1, with and without the

target (Hall 2).

3.2. Beam transmission factor

The transmission measurements were done using a pulsed beam, as a suitable current monitor for a continuous

beam was not available. The current passing through the

scattering chamber in Hall 2 was measured by a core monitor, while the current exiting the tagging spectrometer

in Hall 1 was measured by a Faraday Cup. A maximum

current of 1 (LA was transported for an injection current of

3 p,A, when the slit was opened to its full width; i.e. a

transmission factor of 0.33. We assumed that the reduction

of primary intensity will be a factor of 1000 in the case of passing through the target in the scattering chamber for the

beam from the stretcher ring. But this transmission factor is that without the target for the primary beam with good

emittance from the linear accelerator.

lem, since they give a spectrometer response for no associ-

ated photon. These electrons are called “spurious” elec- trons. The spurious rate was determined by measuring the

low-energy electron spectrum (0.2-0.8E,, E,: incident

electron energy) with and without the radiator. The spurious rate was measured for the situation when

there was a target (13.4 mg/cm2 of 12C) in the electron

scattering chamber (Hall 2), and when there was no target.

The results are shown in Fig. 3. The spurious rate R is O.l-0.3% when there is no target, and increases by one

order of magnitude when there is a target in the electron

scattering chamber. However, this value of 2-3% is still significantly less than the value of 5-10% obtained for tagging experiments when they were done using the pri-

mary beam in Hall 2. Thus it seems that the use of the beam parasitically using the beam transport system, actu- ally improves the spurious rate.

3.3. Beam profile 3.5. Tagging eficiency

The profile of the transported electron beam was ob-

served using the fluorescent screen (ZnS) at the position of the radiator in the tagging system. The diameter was

measured as _ 2 mm, which is an improvement over the

5-mm beam diameter of the beam spot measured under the same conditions when the tagging spectrometer was in Hall 2, and using beam directly from the stretcher.

A bremsstrahlung radiator of l/1000 r.1. was placed in the tagging spectrometer, to produce a bremsstrahlung beam. The diameter of this beam at 2 m down stream was 7 mm; which is a half that measured when the tagging spectrometer was in Hall 2.

In experiments using a tagging spectrometer, the tag- ging efficiency is a ratio of the number of tagged photons,

to the total number of recoil electrons detected by the

scintillators on the focal plane of the tagging spectrometer. Tagged photons were detected by a lead glass Cherenkov detector placed in the bremsstrahlung beam, and the num- ber of photons in coincidence with a tagged electron was determined. The tagging efficiency was 60%. This is quite comparable with the value obtained in the experiments done with the tagging spectrometer in the original position (Hall 2) using the electron beam exclusively.

3.4. Spurious rate 3.6. Duty factor

When the tagged photon system is operated, electrons There was on observable decrease in the duty factor of

which have produced bremsstrahlung in the radiator are the beam after transporting it to Hall 1. At the output of

momentum analyzed, and their detection identifies the the stretcher the duty factor is 80%; this was reduced to an

energy of the bremsstrahlung photon that may subse- average of 50% for the beam delivered to Hall 1. This

quently interact with the sample. Electrons in the incident decrease may result from a change in the angle of the

beam that have lost energy due to multiple scattering extracted beam from the ring due to the change of the

during the beam transport system produce a serious prob- phase space size in the stored beam [l]. This change of the

T. Saito et al. / Nucl. Instr. and Meth. in Phys. Rex A 362 (1995) 224-228 227

I

ZO

z -1

iz = -2

0 -3

1

30

5 -1 t 1 -2 u

-3

1

ZO

z -1 2 2 -2

u -3

Smsec Fig. 4. Beam shape after the beam transport system against current

for the deflecting coil. (a) Without operation of rhe deflector. (b)

Corrected for the second and third bursts. (c) Corrected for all

bursts.

angle causes the change of the beam position at the collimator at the entrance of the beam transport line, and the beam is partly lost there.

To improve the duty factor, it is necessary to keep the incident angle constant. This was achieved by installing a deflector which was synchronized with the beam burst. This deflector was installed after the scattering chamber as shown in Fig. 1. Fig. 4 shows beam shape against current for the deflector coil. The beam shape was observed with a plastic scintillation detector of tagging system at Hall 1.

Observed beam shapes without operation of the deflector

show different widths for four successive bursts as shown

in Fig. 4a. The result corrected for the second and third

bursts using the deflector is shown in Fig. 4b. The widths were improved, but that for the third burst is not enough. This may be due to another effect, a change of energy

spectrum of the beam caused by the instability of the

commercial power source with 50 Hz. In operation with 200 pps the successive four bursts show different widths as

shown in Fig. 4a. This pattern is repeated in 200 pps

operation. The beam shapes are shown in Fig. 4c with

deflector current used for correction for the instability of

the power source in addition to the angular change at beam

extraction. We can see that the duty factor is satisfactorily improved using the deflector.

x lo4

. .

0 2 4 6

Excitation Energy (MeV)

@I

i T ‘*cc Y* p)

0 loo0 2ooo 3ooo 4ooo

ADC Channel (E)

Fig. 5. Parasitic experiments. (a) Scattered electron spectrum from ‘*C at E, = 130 MeV and 0 = 30”. (b) AE-E scatter plot for the “C(r, p) reaction.

228 T. Saito et al. /Nucl. Instr. and Meth. in Phys. Rex A 362 (1995) 224-228

4. Preliminary parasitic experiment

We carried out a preliminary parasitic experiment using

a new beam transport system: electron scattering from “C

in Hall 2, and in Hall 1, a tagged photon measurement of the ‘*C(-y, p) reaction. The beam delivered to the tagged

photon measurement in Hall 1, did indeed pass through the 104.2 mg/cm * ‘*C target (N 0.0023 radiation length) that

was in the scattering chamber in Hall 2.

In Fig. 5 are shown the scattered electron spectrum

from “C, and a A&E scatter plot from the 12C(y, p) reaction. In the ‘*C(y, p) scatter plot, proton events are

clearly separated from the background, and it seems that the data is better than data obtained when a similar experi-

ment was performed in Hall 2, using the primary beam.

Beam intensities during the experiment were 100 n4 and 1 n.4 at Halls 2 and 1 respectively. The reduction of the

primary intensity is a factor of 100, which is better than the design value of 1000. It is due to the thinner target than

the thickness used in the design. As the maximum intensity from the pulse stretcher ring is about 3 kA, the maximum

intensity at Hall 1 is anticipated to be about 30 nA for this

kind of experiment. From this experiment, the feasibility of performing

parasitic experiment utilizing the installed beam line was confirmed.

Acknowledgements

The authors would like to acknowledge Professor M.N.

Thompson for a critical reading of the manuscript. We

would like to thank the linac crew of the Laboratory of Nuclear Science for providing the high quality beam. The

work is supported financially in part by the Grant-in-Aid for Scientific Research No. 03402003 and the Grant-in-Aid

for International Scientific Research Program No.

05044030 of Ministry of Education, Science and Culture

of Japan.

References

[l] T. Tamae et al., Nucl. Instr. and Meth. A 264 (19881 173. [Z] S. Penner, Rev. Sci. Instr. 32 (1961) 150. [3] K.L. Brown and SK. Howry, SLAC-report no. 91, Stanford

Linear Accelerator Center, Stanford, Calif., USA (1970). [4] T. Terasawa et al., Nucl. Instr. and Meth. A 248 (1986) 429.