414 Nuclear Instruments and Methods in Physics Research B50 (1990) 474-477

North-Holland

A 3 MV TANDETRON FACILITY AT KFUPM

H.A. AL-JUWAIR, M.M. AL-KOFAHI, A.B. HALLAK and M. RAJEH

Energy Research Laboratory, King Fhad Universi@ of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

A 3 MV General Ionex Tandetron accelerator has recently been tested at the nearly established Energy Resarch Laboratory at the King Fahd University of Petroleum and Minerals. The accelerator features a very stable solid-state power supply which delivers

about 3 MV of terminal high voltage. A beam resolution of about 400 eV was measured. Ions of a wide range of masses, ranging from hydrogen to gold, were accelerated. The configuration of this Tandetron will be described along with a discussion of the facility and research programs.

1. Introduction

The interest in materials research in Saudi Arabia has grown dramatically in the last decade. This is mainly due to the industrialization process that is taking place in the country and the need to understand the behavior of different materials in the Saudi climate. The Re- search Institute at the King Fahd University of Petro- leum and Minerals (KFUPM) is one of the leading institutions in Saudi Arabia in science and technology. In the field of material research, there are facilities for complete characterization of the fine structure of materials using the scanning-electron microscope, the electron-probe microanalyzers and the scanning trans- mission electron microscope. For surface studies there exist several analytical techniques including scanning Auger microscopy, secondary-ion mass spectroscopy, X-ray photoelectron spectroscopy and others. There are also elemental analysis techniques such as atomic absorption, X-ray fluorescence and polarographic analyzers. The capabilities also include spectrophotome- try, chromatography and structural analysis of materials using X-ray diffraction. With the growing interest in ion beam techniques, the university decided to acquire an accelerator to complement the existing conventional materials research techniques, and provide a training and educational support to the academic departments at KFUPM. The choice of the General Ionex 2.25 MV Tandetron system was guided by several factors, among which are the ease of operation (since students are expected to operate the machine) and also the versatility and capabilities of this Tandetron. Besides complement- ing the materials research, several academic depart- ments are interested in nuclear-structure studies through high-resolution (p, y) spectroscopy, light-ion nuclear re- actions, environmental studies (air pollution, water qu- ality, etc.), 14C dating, and mineralogy.

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The machine is now in operation, and is installed in a special laboratory called the Energy Research Labora- tory (ERL). The aim of this paper is to introduce this ion-beam facility and describe the basic components of the Tandetron system and highlight some of the results and tests performed on it. Finally, some research direc- tions will be discussed.

2. The Tandetron facility

2.1. The Tandetron

The Tandetron accelerator was designed to be a 2.25 MV high-current machine and was later modified to run at up to 3 MV terminal voltage. Basically it consists of the following systems (see fig. 1).

2.1.1. The injector assembly The dual-leg injector includes separate ion sources

for heavy and light ions. A cesium sputtering source (model 860B) depending on surface ionization [l] is used for the production of heavy ions. It is followed by an einzel lens and preacceleration tube. For hydrogen and helium, a duoplasmatron-type [2] source (model 358) is used. To provide negative helium ion injection with high efficiency, the ion source is interfaced to a recirculating lithium charge-exchange canal. This canal is followed by a 7 o electrostatic deflector for removal of neutral and unwanted positive ion beams. Each of the two injectors is designed to produce and accelerate ions of different species up to about 80 keV. The beams are steered and focused to the low-energy end of the accel- erating tube by lenses and a 30° injector magnet. The injector magnet has a maximum field of 1 T.

H.A. Al-Juwair et al. /A 3 MV Tandetron facility 475

c INJECTOR ASSEMBLY - ACCELERATOR B HIGH ENERGY EXTENSION -

Fig. 1. Layout of the 3 MV Tandetron accelerator, showing all different sections.

2.1.2. The accelerator system The accelerator system is housed in a pressure tank

containing insulating sulphur hexafluoride gas normally at 135 psi. The accelerator tube, terminal gas stripper assembly, and the 3 MV highly regulated solid-state power supply are contained within the tank. For the utmost in reliability and low noise levels, the system has no moving parts except the pumps.

A gas-stripping region located in the terminal hous- ing, between the low- and high-energy halves of the accelerator tube, removes electrons from high energy particles. The negative ions from the low-energy acceler- ation tube lose electrons in the stripper and become positive so that they are accelerated a second time down the high-energy end of the acceleration tube.

The terminal high-voltage power supply takes its input from a step-up transformer. The primary winding of the transformer is fed from a tank circuit of a 5 kW push-pull oscillator tuned at 40 kHz. The output of the rf transformer is fed in parallel to a voltage multiplier consisting of a series of high-voltage rectifiers. A feed- back loop via a generating voltmeter, placed opposite to the terminal electrode, provides for very good stability in the power supply. The terminal voltage is measured by the same generating voltmeter.

2.1.3. The high-energy extension The purpose of the high-energy extension is to focus

the beams emerging from the accelerator and to select the beam of interest. As shown in fig. 1, the major components of the high-energy extension are the elec- trostatic quadrupole triplet and the switching (analyz- ing) magnet. Multiple beams emerge from the accelera- tor because the gas stripper in the terminal forms a combination of neutral atoms and singly and multiply charged ions from the incident ion beam and from the target gas. Focusing of a single ion species is accom- plished by the elecrostatic quadrupole triplet lens. The

selection of this focused beam is made by the analyzing magnet. The maximum field for the analyzing magnet is 1.15 T.

2.1.4. End stations Five ports are available on the analyzing magnet of

the high-energy end of the accelerator for setting up experiments. These ports are located at + 30 O, + 15 O, O”, - 15 O, and - 30 o respectively, with respect to the beam line of the accelerator. Presently, two of the five ports are used: the - 30 o port is used for simultanuous RBS (Rutherford backscattering spectrometry) and PIXE (particle-induced X-ray emission) analysis, and the + 30 o port which is equipped with a 30 in. multi- purpose scattering chamber convenient for proton- induced and light-ion nuclear reactions.

The RBS/PIXE chamber was originally installed as an automated RBS chamber with a remotely controlled goniometer, but modified recently to include a PIXE setup. The goniometer is controlled by an IBM PC computer and equipped with precise stepper motors for controlling the position of the sample holder in three dimensions. The same computer is interfaced to a Nuclear Data ND62 multichannel analyzer which is used with the computer for data acquisition and analy- sis.

The nuclear reaction and scattering chamber is a 30 in. diameter chamber equipped with a rotating base table and two top covers - a flat one equipped with its own rotating table and a 2a cover with a mechanism for detector mounting. The target holder can rotate in the horizontal plane and its motion in the vertical direction is controlled by a stepper motor.

2.2. The target-making laboratory

The target laboratory is a facility that makes availa- ble different techniques for the preparation of samples

VII. ACCELERATOR DEVELOPMENT

416 H.A. AI-Juwair et al. /A 3 MV Tandetron facility

for the various experiments. The major equipment in this laboratory includes an electron beam coating plant (Leybold model Univax 450) with controlled simulta- neous evaporation of materials from two electron beam guns. One of the guns has two pockets which allow coevaporation of two materials using the jumping-beam technique. A total of three materials can be evaporated simultaneously. Other equipment available includes a vacuum-annealing oven, a pellet-making apparatus with different die sizes, an ultrapure water system, an ultra- sonic cleaner, a roller press for the production of thin foils from thick sheets, a diamond saw for slicing thick targets, and a sample polisher. A good stock of most of the elements is maintained.

2.3. The data acquisition system

In addition to the data acquisition station that was delivered with the RBS end station which was wnfig- ured around the IBM PC, the facility has a very power- ful data acquisition and analysis system based on a VAX-11 cluster [3-51. Basically, it uses a VAX-11/785 superminicomputer for both data acquisition and analy- sis. The system has been optimized for speeding up data acquisition and storage and for the efficient utilization of the CPU time and memory resources. Standard CAMAC hardware is used for data acquisition.

3. Performance tests

The maximum voltage delivered by the terminal power supply was originally 2.25 MV, but it was later modified to give more than 3 MV. This has been accomplished by raising the turns ratio of the rf trans- former from 18 to 22. Consequently, the tank pressure was increased from 122 psi to 135 psi to accommodate the increase in the terminal voltage. The stability of the terminal voltage is measured over a 4 h period and found to be better than 0.03% at 2 MV.

Several resonance and threshold nuclear reactions were used to calibrate the terminal voltage of the Tandetron. Among the reactions are: 19F(p, y)160 at E, = 340.45, 484 and 872.1 keV; 27Al(p, y)28Si at E, = 992 keV; 7Li(p, n)7Be at Ethrrshold = 1880.5 keV. A typical excitation function of the 27Al(p, y)“Si reso- nance at E, = 992 keV is shown in fig. 2 and was used to obtain the energy spread of the beam. The data corresponds to a thick-target yield curve with a FWHM of 400 eV. This resolution is considered to be a remarkable feature of the Tandetron.

A variety of ion species of different charge states were accelerated in the machine. Table 1 shows the beam currents of some of the ion species obtained at the injector and the high-energy end of the Tandetron. Beam currents range up to 200 pA for gold ions mea-

r

/ 25%

J . l

.

/ 400eV 7

I I

991

4,ot.n Ike'4

992 993

Fig. 2. Thick-target yield curve for the “Al(p, y )%i reaction showing a beam energy spread of 400 eV; E, = 991.87 +0.06

keV.

L I

sured at the injector. The transmission of protons and helium ions was measured at energies as low as 200 keV.

4. Research directions

Recently we have finished several tests and calibra- tion runs on different kinds of samples aiming toward perfecting the machine for RBS and PIXE analysis (see figs. 3 and 4). A program on MeV ion implantation is under progress and data have been collected on the ranges of several important heavy ions in silicon at MeV energies. Ion beam mixing is also being examined especially for the preparation of high-T, superconduct-

Table 1 Typical beams obtained from the KFUPM Tandetron

Ion trpe

H+

Energy Current Ion VW [particle- type

PAI

4500 30 04+ He’+ 3000 0.5 Si+

B- 60 35 Si’+

B2+ 4500 11 Si3+

02+ 4500 18 CU- 03+ 6000 1 AU-

Energy Current lkeV1 [particle-

IN

7500 2.5 1000 1.2 4500 20 6000 5

60 60 60 200

H.A. AI-Juwair et al. / A 3 MV Tandetron facility 417

30

25

4

GO

a

s 115

z

0.5 I

Energy (MeV) 1.0 1.5

I I

CU

100 200 300 400 500

Channel

Fig. 3. RBS analysis of Y,-Ba,-&r-O, superconducting thin film; 2 MeV He++, 20 PC, 3.38 keV/channel.

ing films. Furthermore, we have identified several re- search programs which are geared towards solving the industrial problems in the kingdom such as metal corro- sion, polymer research, and pollution studies.

The authors acknowledge the support provided by the Research Institute, KFUPM, and wish to thank the General Ionex team for their continued interest in this project.

Energy (MeV)

61 0.005 0.010 0.015

I I , 1

lb0 140

Channel

Fig. 4. PIXE analysis of ambient Nnclepore membrane filter; 2 MeV

channel.

References

300

aerosols collected on a He++, 80 PC, 55.6 eV/

[l] H.H. Andersen, IEEE Trans. Nucl. Sci. NS-23 (1976) 959. [2] R.A. Demirkbanov, H. Froblich, V.V. Kursanov and T.T.

Qurkin, BNL 767 (1967) p. 218. [3] H.A. Al-Juwair and R. Abdel-Aal, IEEE Trans. Nucl. Sci.

NS-36 (1989) 611. [4] R. Abdel-Aal and H.A. Al-Juwair, IEEE Trans. Nucl. Sci.

NS-36 (1989) 687. [5] R. Abdel-Aal and H.A. Al-Juwair, IEEE Trans. Nucl. Sci.

NS-36 (1989) 692.

VII. ACCELERATOR DEVELOPMENT