Design of the EXCYT radioactive ion beam separator

__ __ l!!i8

Nuclear Instruments and Methods in Physics Research B 126 ( 1997) 17-2 I

&

NOMB

Beam Interactions with Materials 6 Atoms

ELSEVIER

Design of the EXCYT radioactive ion beam separator G. Ciavola a.*, D. Rifuggiato a INFN Luhorutcwio b II. Phy.sikolisches

a, H. Weick b, M. Winkler b, H. Wollnik

Nu:ionulr Institut.

de1 Sud, Viu S. Sojiu 44. 95123 Coruniu, Uniurrsity

Girswn.

35392 Giexwn.

’

Italy

Germuny

Abstract The design of a mass separator is described for the radioactive ion beam facility at the INFN-LNS in Catania. The two stages of this separator achieve an energy achromatic mass separation, that should allow one to separate the different elements within one isobar as long as their Qa-values are larger than 5 MeV in the case of nuclei of mass 100. The system also incorporates a preseparator that should retain most of the activity produced in the target and ion source.

1. Introduction

At the LNS research center in Catania, the EXCYT facility (Exotics with CYclotron and Tandem) is being constructed. For this facility the K = 800 superconducting cyclotron will provide a beam of energetic light ions that can produce short-lived spallation products. These exotic nuclei then shall be mass analyzed and accelerated by the Tandem accelerator [I]. Radioactive ion beam (RIB) facilities require that the produced short-lived nuclei are ionized and then brought to the accelerator efficiently but that also all other nuclei are held back as ions or as neutrals. This holds for the isotopes of neighboring isobars as well as of neighboring elements within the same isobar. 1.1. Mass separators

cross

in electromagnetic

contamination

isotope

The target ion source usually can only achieve a modof undesired elements except when surface ionization [2] or tuned laser ionization [3] are used. The main attenuation of ions of undesired isobars or elements must be obtained by deflecting ions of different masses differently [4- 131. Though such separation methods for the purification of ions of mass m, are quite effective, there are limits [9], however, especially if ions of undesired mass m, are abundant. (1) Since the ions move in a vacuum of = 10m7 mbar for a few meters in the EXCYT separator, several percent of them are scattered at residual gas atoms. Some of the

ions of an undesired mass m, are thus scattered such that they pass through a separator exit slit properly positioned for the transmission of ions of mass ma. Such ions of mass m, could be effectively eliminated, however, by a two-stage mass separator. (2) Some of the undesired ions of mass m, can be accelerated to an energy that is a little smaller than the nominal ion energy. Thus their final momentum can be equal to that of fully accelerated ions of mass ma. Such ions of mass m, can be eliminated, however, if the used mass separator consists of two sector fields placed at different electrostatic potentials [3-61. The masses of ions of different elements that belong to the same isobaric chain differ only by their Q, value. Thus the mass separators must achieve large mass resolving powers which requires a careful correction of all image aberrations [3,14.15] as well as very precise adjustments and alignments.

est attenuation

* Corresponding [email protected].

author.

Fax:

+ 39-95-542-302;

0168-583X/97/$17.00 0 1997 Published PI1 SOl68-583X(97)01064-6

e-mail:

1.2.

Layout

for

aberration-reduced

mass

separators

The performance of an isotope separator depends on the ion-optical properties of the magnetic sector fields and the overall system stability. In order to relieve the demands on the ion source stability, one can [I51 place a focusing device between the ion-source and the entrance slit of the actual isotope separator [3-6,101. If in such an arrangement the ion-source properties change a little - the reason for which may only become obvious later - one can restore the deteriorated beam properties to what they had been earlier only by modifying slightly the optical properties of the focusing device in question. In doing so, one has effectively decoupled the beam forming properties of the ion source from its property to generate ions.

by Elsevier Science B.V. All rights reserved

SECTION 11.ON-LINE MASS SEPARATION

18

G. Ciuuofu et al. /Nucl.

ion-source and preseparator

Instr. and Meth. in Phys. Res. B 126 (1997)

--

I

p7.q c,,:.i

1St-stage separator

I

17-21

I

~~~_,~‘.

.i.~ m-8 .~~~

_

j.. )_

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I

.’

2nd~stage separator

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Fig. 1. The geometry of the EXCYT mass separator including the final beam transport to the tandem accelerator. Principally this prefocusing device can be an electrostatic round lens [l]. However, it is advantageous to use an astigmatic device instead [IS]. In this case namely it is possible to match the ion beam better to the astigmatic optics of the sector magnet that performs the momentum analysis. In detail one may adjust this prefocusing device such that the beam has a small horizontal image size of 2x0= 0.5 mm at an entrance slit, but is 2y, = 5 mm wide in the perpendicular direction. As long as these 5 mm are still small as compared to the magnet air gap, all fringingfield effects stay small. The main features of such a design are [15]: (1) The position of the intermediate x-image of the ion source can always be adjusted to be exactly at the position of the entrance slit. (2) The sector magnets of the separator require only narrow air gaps. (3) A good correction of second order image aberrations can be achieved already by a slight curvature of the sector field boundaries. The largest image aberrations are usually the aperture aberrations (X 1 AA)ai + (X 1BB)b$. In case of such a system only the first term needs a correction because a, s b, as a consequence of x0 e yo.

2. Specification

of the EXCYT

quadruplet and a magnetic dipole (see Fig. 2). The electrostatic quadrupoles are planned to be built in the narrow gap design already used at the Oak Ridge separator, in

acceleration i

mass separator

The EXCYT mass separator overall system consists of three sections, discussed in the following sections. An overall view is shown in Fig. 1. 2.1. The target ion source and the

preseparator

The proposed preseparator consists of a separately pumped field-free region, an electrostatic quadrupole

41.194 m Fig. 2. Ion beam profiles in the EXCYT mass separator of Fig. I.

G. Ciuoola et al. /Nucl. Instr. and Meth. in Phvs. Res. B 126 (1997) 17-21

19

horizontally to an entrance slit and the beam height in both sector magnets is only about 5 mm [IS].

Fig. 3. The preseparator geometry of Fig. 2 is shown again. However, here representative trajectories of ions of masses 80.90, 100, 110, 120 are shown in order to illustrate that it seems possible to collect all undesired ions on an easily exchangeable beam collector.

which case the distances between quadrupole electrodes are only a few mm ’ [18]. The ion-source is mounted on the high-voltage platform I that requires high power. In addition, the ion-source is highly activated and must be exchanged quite often. Thus, easy access must be guaranteed. Therefore the source is placed in an area where the floor is lowered by 1 m, so that finally the platform mounted on 0.8 m high insulators will be at the level of the standart concrete floor. The main purpose of the preseparator is to eliminate the bulk of undesired ions from the beam of desired ions, so that no heavy radioactive shielding downstream from the preseparator is required. It should allow the system to collect all ions of neighboring isobars as well as of undesired molecular ions on an easily removable collector, so that only ions of interest will pass (see Fig. 3). 2.2. The first-stage

separator and the charge-exchange

cell

The first-stage separator consists of (see Fig. 2): (1) A beam guidance system that takes the ion beam from the output of the preseparator on the high-voltage platform I through a 1 m thick shielding wall to the high-voltage platform II which may be at a slightly different potential. (2) The charge-exchange cell is located between the wall beam guidance system and the first quadrupole quadruplet. This position before the first-stage separator was chosen so that any molecular ions formed in the charge-exchange cell can be eliminated already by the first-stage separator. Note also that the potential of the charge exchange cell is planned to be variable so that ion accelerations or decelerations take place at the entrance and the exit of this charge-exchange cell. (3) The actual first-stage separator consists of a system of two sector magnets of 0.6 m radius preceded and followed by electrostatic quadrupole quadruplets. This separator is designed in such a way that the beam is focused

’ The advantage of this design is, that for progressively longer quadrupoles the required field strengths are smaller and thus also the corresponding image aberrations multiplet are smaller.97/$17.00 0

of the resulting quadrupole 1997 Published by Elsevier

2.3. The second-stage

mass separator

After accelerating the ions to the ground potential, their energy is relatively high, i.e. 250 keV to 350 keV. Because of this acceleration the initial phase space for the second stage is about one half of what it was for the first-stage separator. For the second-stage separator an arrangement as described in Ref. [15] is even more important than for the first-stage separator since the overall mass separation is mainly achieved in the second-stage mass separator. Thus, here a correction of image aberrations as well as the ability of easy adjustment is of utmost importance. Since the ion beam must enter and leave the acceleration column as a round beam. a quadrupole quadruplet must focus it to the entrance slit of the second-stage separator such that it forms a narrow focus of 2.x” = 0.5 mm horizontally whereas its height is 2y,, < 15 mm vertically. Similarly as the first-stage separator also the secondstage separator is assumed to consist of two larger sector magnets of po, = po2 = 2.6 m bending radius and 90” of bending angle each (see Figs. 1 and 2). Though in these large sector magnets the horizontal width of the beam is about 200 mm or 300 mm, its vertical extension is only about 15 mm. Within limits the horizontal beam width can be adjusted and thus the overall mass resolving power M/A h4 can be adjusted [ 141 according to

where A, and A, are the areas occupied by the ion beam in the two sector magnets and 2x,, Ta,, is the horizontal phase-space area of the ion beam. Though the entrance and exit field boundaries of the two sector magnets are curved to eliminate the aperture aberration (X 1 AA)a& two adjustable multipoles are foreseen: (1) Two electrostatic multipole elements, one placed after the entrance slit and one in front of the exit slit. (2) Surface coils in both sector magnets, which can form ion-optically advantageous multipole components in perfectly homogeneous sector magnets or which can alternatively make a slightly inhomogeneous magnet homogeneous [14,16]. This is especially important for the low field magnets used here which have problems with locally varying coercitive forces in the magnet steel [17] and that often show a dip of the field in the middle of the field region. The mass resolving power of this second-stage mass separator should reach up to M/AM = 20000, so that one should be able to fully separate ions of a mass of = 100 u from all other elements of the same isobar if their Qp-values are > 5 MeV.

SECTION II. ON-LINE

MASS SEPARATION

G. Ciuuolu et ul. / Nucl. Instr. and Meth. in Phys. Res. B 126 f 1997) 17-21

20

3. Overall system performance Overall the three separation stages should reach the following goals: (1) The preseparator (with M/AM I 200) should separate ions of one isobaric chain from those of the neighbouring chains. (2) The first-stage separator (with M/AM I 2000) should improve the mass separation of the preseparator and additionally remove all molecular ions possibly formed either in the ion source or in the charge exchange cell. (3) The second-stage separator (with M/AM I 20000) should separate the different elements within one isobaric chain from each other, provided the corresponding Qp-values do not become too small. As an example the calculated mass resolving powers are illustrated in Fig. 4 for monoenergetic ions. The overall energy and mass dispersions D, and D, of the two stage EXCYT separator are calculated as D,=&D,=D,-D,M,,

KD,M?,

(1) (2)

Fig. 5. EXCYT (see Fig. to have resolving

Fig. 4. Calculated mass resolving powers for the three separator stages of EXCYT. After the preseparator, monoenergetic ions of mass 101 and mass 99 could be held back, while ions of mass 100 could be allowed to pass. If the ions of mass 100 would consist of elements that differ in mass by QB-values of 20 MeV the first-stage separator could barely separate and the second-stage separator would separate them very well from each other.

For a different setting of the quadrupole strengths in the achievable mass resolving powers are plotted again 4). However here the assumed 60 keV ions were allowed an energy spread of *20 eV. Note that the final mass is comparable to the one of Fig. 4.

with D, and D, being the dispersion and M, and M, the magnification in the two separator stages, and K = K,/K, the ratio of the corresponding ion energies. Since M, depends on the setting of the quadrupole lenses before and after the acceleration column to ground, one can always achieve D, = 0 which would make the attainable mass resolving power independent of the ion energy spread to first order. Thus also ion sources of large energy spread could be used. Such an operation is illustrated in Fig. 5 in which an ion source is assumed that delivers 60 keV ions with an energy spread of 10 eV but that still allows a mass resolving power of better than 10000. At this point it may also be. worthwhile to mention that the vertical extension (y-direction) of the ion source can be extended from f 0.2 mm to + 2 mm with only limited degradation of the system performance, while horizontal widening of the ion source (x-direction) would decrease the mass resolving power linearly. However, any initially inclined ion trajectories are only accepted if their angles of inclination stay smaller than + 20 mrad.

G. Ciawlrr

et al. / Nucl. lnstr. and Meth. in Phys. Res. B 126 (I 997) 17-2 I

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SECTION

II. ON-LINE

MASS

SEPARATION