The generation of radioactive ion beams

Pergamon PII: SOO42-207X(96)00163-2

Vacuum/volume 47lnumber 1 l/pages 1341 to 134411996 Copyright 0 1996 Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0042-207X/96 $15.00+.00

The generation of radioactive ion beams* ALatuszyliski,b D Maczkaarb and Yu Yushkevich, b “Institute of Physics, M. Curie-Sklodowska pl. M.C. Sklodowskiij received

27 March

University, I, 20-031 Lublin, Poland; bJoint Institute for Nuclear Research, Dubna, Russia

1996

A new design is presented ofan ion source of the hollow-cathode type adapted to the on-line isotope separator JASNAPP-2 at JINR, Dubna. To determine its optimal work conditions the source characteristics have been investigated. With the source were generated radioactive fluxes of various isotopes of PO, At, Bi, Pb, Fr and Rn using a 660 MeVproton beam and the Th (p, xpnl reaction. The relative values of the cross-sections for production of the above isotopes are also given. Copyright 0 1996 Published by Elsevier Science Ltd

Introduction

Ion sources adapted for use in various types of particle acceleration systems have received attention for many years. There are important and varied applications of radioactive ion beams in basic and applied scientific research, as well as their traditional applications in electromagnetic isotope separation and implantation processes. An example of such applications is research into off-line and on-line nuclear spectroscopy, which aims at identification and determination of nuclear parameters of shortlived isotopes placed far from the stability line fl.‘,’ In solid-state physics radioactive beams can provide markers in processes based on ion implantation.3 Further examples are: the examination of diffusion of impurities implanted into solid targets, the investigation of their depth distributions, the phenomenon of mixing, recoil implantation, ion sputtering. Also ion beams are important in industrial applications. An ion source is described, which can effectively generate beams of short-lived isotopes of heavy elements. The source has been adapted to the research complex JASNAPP-2 which uses a 660MeV proton beam from the JINR synchrocyclotron in Dubna to produce radioactive isotopes. The operation of the source is based on an arc discharge excited in a hollow-cathode system. In addition to the description of the source’s structure and a limited survey of its characteristics, early results are given on the production of ion beams of short-lived isotopes of PO, Rn, Bi, At, Hg. It is shown that the source operates not only effectively, but sufficiently fast enough to provide ion beams of isotopes with a half-period of a second. The structure of the source

The elements of the ion source are shown in Figures 1 (a) and (b). The device is composed of two essential parts. In one bottom *Revised version of paper presented at the 9th International School on Vacuum, Electron and Ion Technologies, 14-17 September 1995, Sozopol, Bulgaria.

part of the source (elements 4-6, lo), radioactive isotopes are generated as a result of nuclear reactions triggered by bombarding a target material with high-energy protons. The upper region (elements 1, 2, 3) is the ion source proper and the structure of this part is a development of earlier designs reported in Refs 5 and 6. The characteristic feature of the source is that it offers the operator a possibility to select a more effective method for atom ionization: that of electron impact in the discharge plasma, or that of surface ionization. Two modes of source operation correspond to these two ionization methods. Analysis of ionization processes in the source operating in the surface ionization mode, i.e. when ionization of atoms results from their interaction with a hot internal surface of the ampoule (I), is the subject of a separate article.6 Here we are concerned with research into the production of radioactive beams in a source operating under the plasma regime. A plasma of an arc discharge at low gas pressure is formed in the ampoule (l), which acts as a cathode. The top of the ampoule is surrounded by a tungsten wire filament (C#J = 7 + 8 mm), heated by a dc current. The electrons from the filament are then accelerated by the potential 300-600 V towards the ampoule. This implies that the walls of the ampoule are always at high temperature, so their internal surface functions as an emitter of electrons that develops a discharge between the anode and the cathode. The anode is an insulated tantalum tube (3). Radioactive atoms appear in the target container (5) as a result of nuclear reactions. The container is a tantalum cylinder 18 mm in diameter and 100mm long. It can hold about 50g of target material, whose type can be changed depending on isotopes under examination. The container is resistance heated to a temperature ranging from 800 to 2800 K by a dc current. With an input power

of about 2 kW (200 A), the top limit of this temperature can be maintained. The target container is mounted with heat-conducting copper bars connected to the feedthrough cooled by a water-system. To reduce heat losses, the container (as well as the ionization ampoule) is surrounded by radiation shields. The 1341

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ions b

7

6

al b) Figure 1. (a) Ion source and (b) target assembly: (1) ionization cavity; (2) tungsten filament; (3) anode transfer tube; (4) Cu bars; (5) target container; radiation shields; (6) target-source connection; (7) feedthroughs; (8) gas inlet; (9) source holder; (10) target material. The scale oni) / concerns Figure

target container is connected to the ion source by a Ta anode tube 60mm long and 15mm in diameter. The nuclear reaction products formed in the target are directed through this tube to the source where they are ionized. The structure of the target-ion source assembly also provides for the introduction of a suitable support gas passing through the target container. This gas not only stabilises the discharge in the source, but also facilitates the transfer of radioactive atoms produced in the target to the ion source. Several source holder units will be built which will make it possible to easily exchange a burnt-out element by removing the whole unit and storing it sufficiently long to allow the accumulated reaction product activity to decay. The source holders are attached to the mass separator by a flexible bellows allowing an alignment of the exit hole with the extraction electrode. Operation characteristics The source in question underwent theoretical examinations and the conclusion was that its basic observable ionization efficiency /?, can be described by the following approximate relation:

exp(-xl-d,

(1)

where r and b are the radius and the length of the ionization chamber, n, is the electron density in the plasma, n, is the atom density, n: is the excited atom density, T, is the gas temperature, k is the Boltzmann constant, M is the mass of the atom, a is the area of the plasma meniscus, A is the area of extraction opening, and Q,., are ionization coefficients. The ions diffuse along the x axis. The rate constants k I) k, are defined in terms of the electron velocity distribution fTv,) and the velocity-dependent cross sections: 1342

Measurements have proved the relations between individual operation parameters of the source defined by Formula (1). For example, Figure 2 presents typical dependence of ionization efficiency on the gas pressure in the source and on the length of the ion extraction opening. The curves obtained refer to the source working in the off-line systems, i.e. when a beam of highenergy protons from the accelerator do not reach the source. It was only gas carrier (Xe or Ar) that was dosed into the ionization ampoule via the target container. Aside from ionization efficiency measurements, overall testing of the whole system was performed with both the source and the target container in operation. In those measurements the target material was a mixture of Th and graphite, while the container was kept at high temperature. Au and Ag atoms were added as markers to the target material. As an example, Figure 3 shows the dependence of ion current of elements emitted from the target on the target temperature obtained in one of the tests. The falling of the parts of the curves

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0.8

Figure 2. (a) Source efficiency as a function of the Xenon flow into the ion source. (b) Dependence of the source efficiency on the length of the ion extraction

opening.

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Table 1. List of radioactive isotope beams

Element

Isotopes and r,!,

Hi?

“‘Hg (3, 6 s); “‘Hg (48 s) 19’Bi (90 s); “‘Bi (8 min); ‘95Po (4, 5 s; 2 s-m); 196Po (5 s); 19’Po (58 S; 86 s); “*PO(1, 8 min); *“PO (11, 4 min); 2’3Po (48 min) 19*At (5 s); ‘*At (7, 2 s); 2WAt (48 s, 4, 3 s-m) *“Fr (45 s).

Bi PO At Fr

Figure 3. The temperature-ion current curves.

for higher temperatures is caused by the shortage of a given element in the container.’

First results of on-line experiments

The source was also tested in the on-line system, which provided beams of short-lived isotopes. For this purpose, the complex JASNAPP-2 based on the synchrocyclotron of the Laboratory of Nuclear Problems (JINR, Dubna) was used. The first information on the structure of this system appeared in Ref. 9. The proton beam from the accelerator was transported over the distance of 55 m to a special room where the ion source-target system was located. The target material was a mixture of Th (2.55 g) and powdered graphite (5.45 g) exposed to 660 MeV protons. The beam spot on the target container was 0.5-l.Ocm*. The proton beam currents were limited to about 1 PA, because of target activation, beam scattering and limited handling system. The products of proton beam interaction went from the target container to the source ampoule via tube (3). After ionization, acceleration up to the energy of several dozen of keV and analysis in the magnetic field, they reached the collector chamber where they were implanted into the ‘coin’ type targets. Since the currents of radioactive ions were too weak to be measured with ordinary instruments, their intensity was estimated from the activity collected on those coins. The activity was determined by the a detectors; the whole experiment was computer-controlled. Attention was paid to two main operation parameters of the source-target system: its efficiency and operation speed. The latter factor obviously affects the possibility of production of beams of appropriately short-lived isotopes. It was found that the efficiency of the source generating radioactive elements is in general well described by eqn (1) which, however, must be supplemented with factors taking into account the conditions for a production of nuclides in the target and their radioactive decay. A lot of effort by many groups has gone into the understanding

and describing of the processes taking place in such source-target systems.* We have decided to take a more pragmatic approach and have empirically determined the operating parameters that provide the maximum radioactive yield. Since the yield of activity critically depends on the temperature of the source and of the exposed target, thermal conditions of the system were investigated thoroughly. It was found that those conditions must be determined separately for almost every radioactive element. The knowledge of the optimum temperature of the target container has turned out to be of a special importance. That temperature could not be too low so as to avoid a decrease in the speed of diffusion of nuclides from the target exposed to protons. In contrast, it could not be too high; otherwise the flow of Th, ThO, ThOz vapour into the source increased, which could reduce the efficiency of the source. This effect is clearly seen in Figure 2(a), which presents the dependence of the source efficiency as a function of the Xe gas flow dosed into the source. For example, the optimum temperature of the target container at the production of PO isotopes was about 2000 K. With this source, ion beams of a number of radioactive isotopes of Hg, Bi, PO, At, Rn, Fr were produced (Table 1). The data gained, including the knowledge of activity gathered on the collector for individual radioactive isotopes, made it possible to also determine cross-sections for production of the nuclides by bombardment of the Th target with protons. In all cases the proton energy was 660 MeV. The results are shown in Figure 4. To conclude, the authors wish to draw attention to a very favourable feature of the source, namely the possibility of using it to produce beams of radioactive ions with relatively short lifetimes. Among others, ‘95Poisotope beams (r, ,? z 2 s) with the energy of several dozens of keV were produced. This work was partly supported by the Committee of Scientific Research (Poland) under Contract No. 3P 407 032 OS.“

mass number 0)

massllurnber b)

Figure 4. The measured radioactive

isotopes.

cross-sections of Hg, PO, Bi, Fr, At and Rn The solid curves are only to guide the eye. 1343

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References 1. Proc 11 th Int EMIS Conf, Los Alamos, USA (August 1986) Nucl Inst and Merh, B26 (I 987). 2. Proc 12th Int EMIS Conf, Sendai, Japan (September 1992), Nucl inst and Meth, B70 (1992). 3. J Sielanko and M Sowa, Nucl Znst and Meth, 209/210, 483 (1983). 4. A Latuszynski, D Maczka and Yu V Yushkievich, Proc 4th Int Conf on Ion Implantation, Berchtesgaden, Germany, H Ryssel and H Glawisching (Eds). Springer, Berlin, 106 (1983).

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5. A Latuszynski, D Maczka, Yu V Yushkievich and K Kiszczak, Vacuum, 5,263 (1986). 6. A Latuszynski and D Maczka, Study of ionization process in the surface ion source, to be published in Vacuum. 7. A Latuszynski, D Maczka, Yu V Yushkievich, N Yu Kotovski and V X Tichonov, Int EMISConA Sendai Japan, Abstr, book, C-P5 (1991). 8. H L Ravn, Nucl Inst and Meth, B70, 107 (1992). 9. V G Kalinnikov et al., Nucl Inst and Merh, B70,62 (1992).