On-line isotope separation at the hilac

N U C L E A R I N S T R U M E N T S AND METHODS 78 (2970) 45-69;

© N O R T H - H O L L A N D P U B L I S H I N G CO.

ON-LINE ISOTOPE SEPARATION AT THE HILAC* J. M. NITSCHKEt Lawrence Radiation Laboratory, University of California, Berkeley, California, U.S.A.

Received 8 August 1969 An one-line isotope separator was developed at the HILAC. The heavy ion beams impinges on a thin target, and the recoiling product nuclei are stopped in a solid or liquid catcher. They diffuse out of this catcher at high temperatures and are ionized by a rotating electron cloud. The ions are focused with a rf

quadrupole lens into a Paul mass filter. Thereafter they are subjected to analysis by conventional nuclear radiation spectroscopy. The principle of the instrument and the results of the first on-line runs are reported.

1. Introduction The n u m b e r o f presently u n k n o w n b e t a unstable nuclei including neutron-, p r o t o n - , a n d a l p h a - e m i t t e r s with half lives between 10 - z a n d 102 sec is estimated to be a b o u t 20001). S o m e o f these nuclei will still have long e n o u g h half lives so t h a t they can be studied by c o n v e n t i o n a l techniques including wet chemistry. Identification o f others will be possible by on-line techniques which d o n o t include mass s e p a r a t i o n a n d which are c a p a b l e o f h a n d l i n g h a l f lives as short as 1 msec. M o s t o f the unk n o w n nuclides, however, will require on-line i s o t o p e s e p a r a t i o n techniques for u n a m b i g u o u s a n d direct identification. U s i n g F e r m i ' s t h e o r y o f b e t a decay, one can estimate that, excluding super allowed transitions, the

shortest h a l f lives o f the u n k n o w n b e t a emitters will be a b o u t 10- 2 sec 2). This gives the first a n d very i m p o r t a n t p a r a m e t e r for the c o n s t r u c t i o n o f an on-line separator. Points o f interest besides b e t a emission are a l p h a emission, delayed n e u t r o n a n d p r o t o n emission, new d o u b l e m a g i c a n d d e f o r m e d regions, Q-values and their i m p l e m e n t a t i o n for the semi-empirical mass formula, nuclei o f interest in astrophysics, a n d new t r a n s - L a w r e n c i u m elements, in particular, in conjunct i o n with the next generation o f heavy ion accelerators. A f t e r being convinced o f the usefulness o f on-line s e p a r a t i o n one has to decide on the m e t h o d to be applied, whereby the type o f accelerator available is of p r i m a r y i m p o r t a n c e . On-line systems at p r o t o n synchrotrons, reactors, c y c l o t r o n s (also used as n e u t r o n generators), s p o n t a n e o u s fission sources, a n d heavy i o n accelerators will differ m o s t l y in target arrangements, fast chemistry, t r a n s p o r t a t i o n a n d i o n source while mass analysis, collection, alpha-, beta-, g a m m a - , a n d p-detection a n d d a t a h a n d l i n g will be m o r e uniform. Fig. 1 shows some possible ways o f setting up a n online separation system. The middle b r a n c h is the one m o s t frequently used and also p e r h a p s the m o s t ad-

* This work was done under the auspices of the U. S. Atomic Energy Commission. t Present address: Centre de Spectrom6trie Nucl6aire et de Spectrom6trie de Masse du C.N.R.S., B&t. 108, Campus, 91 - Orsay, France.

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Fig. 1. Possible ways of on-line isotope separation. 45

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J. M. NITSCHKE

vanced (CERN, Orsay). Not included in fig. 1 are some unique systems like the multidimensional fission study by Watson et al., laser time of flight systems, or heavy ion recoil spectrometers. Without discussing in detail the principles used in different on-line systems, a few problems and limitations should be pointed out, particularly in regard to fulfilling the most important requirement that the total transport time should be as short as possible. 1. Reactor fission studies are hampered by the alternative to either use the internal high neutron flux and cope with long (10 m) transport lines, or use the external neutron beam which has orders of magnitude lower intensities. The transport time in the first case would be in the order of several seconds unless the ionization takes place inside the reactor. 2. On-line systems using high energy protons face the basic problem of obtaining short diffusion times in targets of high effective thickness. In order to avoid the high radiation background produced by proton accelerators, on-line projects have to include pump lines which prohibit the study of very short lived isotopes, or the ion source has to be located near the target. 3. The problem common to all projects is the lack of a universal ion source. Reaction products which are in gaseous form at or near r o o m temperature can be used in conventional discharge-type ion sources (for example: Scandinavian type). Alkali, and with lower efficiency, group I1 elements have been used in surface ionization sources. Most other elements are easily lost by condensation on cold surfaces before they get into the ion source and/or are inefficiently ionized. It is not surprising, therefore, that the first elements studied on-line were noble gasses, alkalies and other elements with high vapor pressure at low temperatures. 4. A further problem which will require intensive

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study is the fast chemistry necessary to eliminate the ambiguity in Z. Little is known here. Most projects rely on physical means of separation like differences in surface ionization, catcher diffusion, evaporation rates and characteristic X-rays in coincidence experiments. A heavy ion accelerator eliminates to a large extent the necessity for chemistry, since one can in most cases perform a differential experiment where the bombarding ions differ by one proton. 5. At the H I L A C a very promising approach had been taken a few years ago with the R A M A (Recoil Atoms Mass Analysis) projectS). It corresponds to the top branch in the schematic diagram fig. 1. Recoil atoms of a thin target are stopped in one atmosphere helium gas and can be collected with high efficiency (up to 80%) as demonstrated in many experiments. If one finds a way to get rid of the large amounts of helium without losing too much of the product nuclei, which then could be introduced in a discharge-type ion source, this seems to be a very promising method of on-line separation. It was also thought of directing the recoil carrying helium jet on a surface and removing the deposited activity with a sputtering ion source. None of the very brief considerations given in this introduction points to a "one-and-only method" of on-line separation. Most of them are rather tailored to the specific experimental facilities. This is also the case with the fairly unique H I L A C project. 2. General description of the on-line isotope separator The lower branch of fig. 1 shows schematically the approach taken at the H I L A C on-line separator project. Recoil target, recoil catcher and ionization have been combined into one unit to avoid losses and to gain speed. For more detailed discussion we refer to fig. 2. The heavy ion beam impinges on a target, which is thin enough (about 1 mg/cm 2) to let product

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O N - L I N E I S O T O P E S E P A R A T I O N AT T H E H I L A C

nuclei recoil into an ion source where they are caught in a solid or liquid catcher of high diffusivity. Electron bombardment keeps the ion source and the recoil catcher at a high temperature so that short diffusion times can be achieved. After diffusing out of the catcher the recoil nuclei are ionized by a rotating electron cloud and extracted into an electrostatic quadrupole which transfers and focuses the ions into the mass analyzer. As mass analyzer, we use a Paul mass filter 4) which will later be described in detail. It is evident from fig. 2 that neutral particles of undesired mass number have a chance to get through the whole separator as is the case in all straight-through machines. It is therefore necessary to separate neutral and ionized particles with a deflection system. In fig. 2 the particles are deflected 90 ° onto an alpha counter which is meant to be an example for the radioactive counting equipment. In addition to the neutral particles traveling down the instrument, an intense light is emitted from the ion source which in order not to be "seen" by the alpha detector has to be trapped in a light trap. The light trap also serves as a Faraday cup to read the ion current. To allow for offline tuning of the whole instrument, artificial recoils of thermal energy can be produced in a mass marker oven near the target (fig. 2). This oven designed as a Knudsen cell produces a molecular beam consisting of ions of the same mass number as the radioactive isobar that is to be separated on-line. If the mass marker oven is charged with a natural element, it will in general produce several isotopic masses. This is a very welcomed feature because it allows adjustment of the resolution and transmission of the mass separator which are the two most important parameters. Except for the temperature of the ion source, all parameters of the separator can be adjusted with the mass marker before going on-line. 3. The ion source The task of the ion source is threefold. It has to stop the recoils, diffuse them out of the stopping material and ionize them. The recoils produced in heavy ion reactions with H I L A C ions of 10.38 MeV per nucleon can vary in energy from several hundred MeV to fractions of 1 MeV, assuming no stopping in the target itself. The majority of all reactions for production of neutron deficient isotopes, however, involves recoil energies between about 5 and 20 MeV. Measurements of electronic stopping show that recoils of this energy can be stopped in 0.5 to 2 mg/cm 2 of material [independent * The figures quotes in this paragraph on diffusion are only for illustration. They vary m a n y orders of magnitude.

47

of Zwithin a factor 2]5). 1 mg/cm 2 is, for instance, equal to 5 x 10 - s cm of tungsten or 5 x 10 -4 cm of carbon. The time ~ it takes for a particle to diffuse through a layer of non crystalline material of thickness d depends on its diffusion coefficient D in this material and is given in first approximation by = dZ/(6D).

Therefore looking for a typical time constant z of l0 -2 sec the diffusion coefficient has to be D = d2/(6~) = 25 x 10-1°/6 x 10 - 2 ~ 4 x 10 -s cm2/sec (for tungsten), and about 4 x 10 -6 cm2/sec in the case carbon is used as recoil catcher. Diffusion coefficients of this magnitude can be achieved only for very few elements at moderate temperatures (i.e., alkalies); most other elements require temperatures which in some cases approach the melting point of the base material. (Example: the diffusion coefficient of Li in ll00°C Si is 1 × 10 -5 cm2/sec and for Bi at the same temperature 1 x 10-14 cm2/sec). Since the diffusion coefficient is highly temperaturedependent D = D O e -E/RT,

a 10% increase in temperature typically results in one order of magnitude higher diffusion. Liquid metals have up to l0 s times* higher diffusion coefficients and can be used as recoils catchers, but containment and high vapor pressures at the melting point create problems which have to be solved. The following opposing conditions have to be met by a liquid catcher material: 1. Melting has to take place at a low vapor pressure ( < 10 -5 torr) so that the ion source does not get saturated with vaporized and ionized catcher atoms, and the back of the water-cooled target and the insulators are not coated. 2. The metal has to have a high melting point so that the recoil atoms have a high vapor pressure in the liquid. 3. The material has to have a high stopping power. A look at the vapor pressure curves of all elements 6) shows that only very few elements yield a reasonable compromise between these conditions. The following materials have been tried as recoil catchers: tantalum, tungsten, porous tungsten, graphite, graphite felt, liquid cerium and liquid zirconium. The third task of the ion source, the ionization of the radioactive atoms, can be achieved in various ways, the most common being the gas discharge which has high

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Fig. 3. Anode current of a simple magnetron as a function of the magnetic field, for different anode voltages. ionization efficiency. The disadvantages of the discharge source for our application are: 1. A fairly long time delay results from the fact that the holdup time is equal to the discharge chamber volume divided by the "pumping speed" off the exit slit. 2. It requires high extraction voltages and yields therefore high energy ions which are incompatible with the quadrupole mass analyzer in its present form. 3. It produces a high background ion beam, which has to be kept at high energy to avoid blow-up. For these reasons and in order not to duplicate work that is done, i.e. at CERN, it was decided to look into the possibility of electron bombardment ionization.

Electron bombardment is in general about 104 times less efficient for ionization than a gas discharge. The ion current I i depends on the electron density Je, the cross section Qe of the electron beam, the path lengths le of the electrons, the differential ionization cross section a for electrons of a fixed energy, and the particle density p in the following form: I i =jeQelc~p. There are three free parameters to increase the ion current: Je, Q~ and I~. The cross section Q~ of the electron beam is limited by the size of the ionization chamber. We have therefore tried to increase the path length and the electron density. For an understanding of the ion source it is useful to first consider a simple magnetron as shown schematically in fig. 3. It consists of an electron-emitting filament wire, a coaxial anode cylinder and two end plates in a magnetic field parallel to the common axis. At zero or weak magnetic fields, the electrons leave the wire to go radially to the anode which is at a positive potential •,. With increasing magnetic field the electrons are bent into circular orbits, until at a certain critical field Bc the bending radius becomes smaller than the inner radius r of the anode cylinder, and the electron flow is cut off. This is illustrated in fig. 3, where the electron current I, is plotted versus the magnetic field H for different anode voltages. Anode voltage Ua at cut off and magnetic field B c are related by Hull's v) equation:

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ON-LINE

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the magnetron is far from trivial if space charge effects are taken into account. A theory developed by Brillouin 8) describes the electrons as moving as a "solid cloud" around the cathode. The extremely high path length for the electrons makes this device attractive as an ion source. The disadvantage in using the simple magnetron as an ion source is that an ion can be formed near the cathode as well as near the anode, so that the energy spread is nearly equivalent to the anode voltage times the charge of the ion. To avoid this, we have surrounded the cathode wire with a coaxial grid which is on anode potential and accelerates the electrons before they go on their circular orbits inside the anode cylinder. Thus, the anode region if field free and the extracted ions have only minimal energy spread. Fig. 4 shows schematically hairpin-cathode, grid, and anode of the magnetron source. The ions, which have been formed, are extracted by a high transmission grid into the first, the so called transfer quadrupole. The second grid in fig. 4 serves as electron mirror and is biased slightly above cathode potential to prevent electrons from ionizing the residual gas outside the ion chamber. The second improvement achieved by the introduction of the grid is that the electron density is

AT THE

HILAC

49

much higher than in a simple magnetron because the space-charge limited electron flow in a coaxial diode is inversely proportional to the square of the anodecathode distance. At high electron currents though, a space-charge well is formed between grid and anode cylinder, which results in some spread of the ion energy. Fig. 5 is a photograph of four anodes of different materials after several on-line runs. Fig. 6 shows an anode chamber with accelerating grid and several catchers of different materials. The recoils enter through the grid covered hole. To demonstrate the effectiveness of the electron cutoff, the mass-analyzed ion current of 92Mo (the mass marker oven was filled with natural Mo) is plotted as a function of the magnetic field (fig. 7). The peak ion current coincides with the cut-off field at 97 G indicating that it corresponds to the maximum electron path lengths. 4. Technical details o f the ion source

As pointed out in the previous chapter the temperature of the recoil catcher has to be as high as possible to achieve a short diffusion time. Since the recoil catcher is an integral part of the magnetron anode chamber, it was decided to keep the whole at a high

Fig. 5. Magnetron anodes after several on-line runs; (left to right, top row) 0.25 mm tungsten, porous tungsten, (bottom row) tantalum, 0.13 mm tungsten.

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higher t e m p e r a t u r e s would be to m a i n t a i n the structural stability of the c h a m b e r wall, and it w o u l d bec o m e a p r o b l e m to c o n t r o l the t e m p e r a t u r e o f the m a g n e t r o n cathode. ( A t 2727 ° C the electron emission density o f tungsten is 14 A/cruZ.) It was therefore decided to set 2700°C as the upper t e m p e r a t u r e limit. A t this t e m p e r a t u r e the r a d i a t i o n heat load on the target from the m a g n e t r o n a n o d e is a b o u t 80 W, which is a n o t h e r factor preventing higher temperatures. Fig. 8 shows the ion source with the target a n d the extractor q u a d r u p o l e . The outer housing is milled out of one solid piece of O F H C c o p p e r and serves as a spool for two H e l m h o l t z

temperature. This has the a d v a n t a g e that the residence time of the gaseous recoil a t o m s on the c h a m b e r wall becomes extremely short. A n d for some elements we get a large c o n t r i b u t i o n from surface i o n i z a t i o n on the tungsten wall. The m a x i m u m t e m p e r a t u r e at which this ion source works is a b o u t 2700°C, The v a p o r pressure o f tungsten at this t e m p e r a t u r e is 4 x 10 - s tort, which at an i o n i z a t i o n efficiency o f I A / t o r r yields 40 /~A tungsten ion current. A t this current the space-charge b l o w u p reduces the transmission considerably. A further difficulty at even

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Fig. 7. Ion current after mass separation as a function of the magnetic field in the magnetron source.

Fig. 6. Magnetron anode with accelerating grid and catcher. Different catcher materials in front; (left to right) graphite felt, graphite, porous tungsten.

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liquid nitrogen-trapped 6" diffusion p u m p charged with D C 705. The vacuum chamber for the source is fabricated from stainless steel and has metal gaskets. Bake-out is possible but turned out to be unnecessary. The vacuum is in general 10 -8 torr and during operation about 10 -7 tort.

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The transfer quadrupole is the next part of the separator which the ions encounter as they leave the ion source. Since the transfer quadrupole is a special case of the more general mass-analyzing quadrupole, we will delay its discussion until a later chapter. The following serves as a brief explanation of how an electrostatic quadrupole field can be used for mass separation. The basic equations were first derived by Paul et al.4). A three dimensional hyperbolic field is generally described as (p = ~ 0 ( ~ X 2 +/3y2

coils, and supports all other parts of the source. Cooling lines traverse the whole copper block and are able to handle 5 kW heat load. The drawing fig. 8 is in most parts self explanatory. The magnetron anode is surrounded by a filament for electron-bombardment heating. The total power available is about 4.5 kW giving an anode temperature of 2700°C. The heating filaments are surrounded by only one heat shield, so that the target can be m o u n t e d close to the ionizing region. Fig. 9 shows a plot of the heating power and the c~counting rate of a separated isotope versus anode temperature. The ion source will accept recoils that leave the target under angles < + 10 °. The holes in the heat shield and in the magnetron anode, as well as the target anode distance, are chosen accordingly. The magnetron cathode is a hairpin filament made from 0.5 m m non-sag tungsten wire; the accelerating grid consists of 0.075 m m tungsten wire - 20 wires per centimeter. The anode itself is made out of tungsten; we also have tried porous tungsten and (for lower temperatures) tantalum. It should be noticed that all insulators are " c o l d " and well hidden from metal vapors which could coat them. They do not have to be replaced. All insulators are made of b o r o n nitride. The Helmholtz coils, which provide a uniform magnetic field in the ionizing region, are v a c u u m impregnated with epoxy and encapsulated v a c u u m tight in an all-copper housing. There is no outgassing f r o m the coils. The v a c u u m system for the ion source consists of a

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have chosen 7 = 0 and e = -/3 = 1/r 2, so that the potential for x = 0 , y = + r 0 and y = 0 , x = + r o is q~ =q0 o. The electrodes are connected to a dc and a rf potential, so that the potential inside the quadrupole is given by q0 = (U + V cos cot) (x 2 - y2) 1/rg. F r o m this potential we get the electric field in x- and y-direction: E x = (2/r 2) ( U + V cos cot)x, E, ----- (2/r~) (U + V cos cot)y.

52

J. M. NITSCHKE

A n d this leads immediately to the equations of motion : m5~ + (2e/r~) (U + V cos ~ot)x = O,

the x- and y m o t i o n have to be overlapped. The shaded areas give a- and q values for stable particle orbits in the quadrupole (fig. 13).

,ny - ( 2 e / r ~ ) ( U + V cos o~/)y = 0, m~=0. With the following transformations ogt = 2¢,

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x" + ( a - 2q cos 24) x = 0. F o r q = 0 we have the ordinary harmonic oscillator " a " being the spring constant. The cos term introduces a time dependence of this spring constant and it is obvious that if q is small compared to a, stable oscillations will result, otherwise the time dependent term will dominate and render an instabIe solution. One gets a more quantitative picture of these stability problems by looking at the stability diagram, which for low a and q values looks like fig. 11. ~X

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Referring back to the definition of a and q, and skipping constant factors (also frequency!), we have a,,~ U/m and q,,~ Vim; that is, a and q can be adjusted by changing the dc and rf voltage (at constant frequency co). In a practical instrument one operates always in the upper corner of the stability diagram and with a constant ratio U/V. This corresponds to a straight line in the stability diagram (fig. 14). The U / V ratio is chosen so that only one mass number gives an a and q pair which is inside the stability region. By keeping the dc/rf ratio constant but varying their magnitude simultaneously, one can " p u s h " different masses in the stability region and in this way use the instrument as a mass analyzer. Fig. 14 shows mass m 0 inside the stability region; all other masses lie outside on the line called "mass scan line". The drawing is layed out so

Fig. 11 All a and q values in the shaded area give stable solutions for the Mathieu equation. For the x direction we have a direct correspondence between the normal Mathieu equation and the equation of motion. F o r the y direction we have to replace a by - a to obtain the stability diagram of fig. 12.

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ON-LINE

ISOTOPE

SEPARATION

that for m0 = 18, ml would be 19, m 2 equal to 20, m_ x equal to 17, m_ 2 equal to 16 etc. From fig. 14 it is obvious that one can change the resolution by varying the dc/rf ratio. Lowering the dc/rf ratio means that the mass scan line intercepts a wider portion of the stability region and allows stable orbits for more than one mass number. After this brief explanation of the operating principle of the mass filter we can compare it with the more commonly used magnetic, electrostatic or time-offlight analyzer. It is evident from the equations of motion that this instrument is a genuine elm analyzer since neither the energy (like in the electrostatic analyzer) nor the m o m e n t u m (like in the magnetic analyzer) enters. Even if an ion on a stable orbit collides with a rest gas molecule and is not too badly knocked out of its way it will continue through the instrument (we have operated at pressures of 10 -4 torr). The input velocity of the ion is important only, in so far as, to allow for enough time inside the rf-dc field for undesired particles to get unstable. The greatest advantage besides the velocity independence is that the resolution can be adjusted from 0 (scan line on q axis) to infinite (scan line intercepting the corner of the stability region). This is not possible with most other mass separators. A further extremely valuable feature of the quadrupole is that its acceptance angle is about __15° and the entrance aperture 6.5 ram, which again compares favorably with other instruments. Since at the start of the whole project we did not

AT T H E H I L A C

53

have too clearly an idea how the on-line ion source would finally look like, the properties of the quadrupole seemed to be very attractive. Disadvantages are that it is a single-channel instrument, that it requires rather low energy ions, and that it is a straightthrough machine. In the actual instrument cylindrical rods were used instead of hyperbolic ones (for cost reasons) (fig. 15). Their diameter was chosen as to minimize higher order multipole components in the quadrupole field.

6. Computer study of the quadrupgle mass filter Before the mass filter was combined with the magnetron ion source it was studied extensively as a separate unit. It was found that the theoretical relationships between a, q, U, V, e/m and the particle stability were perfectly fulfilled within the accuracy of the measurements (V could only be measured to 0.1%). The parameter having the most influence on the resolution of the instrument was the ion velocity; this is theoretically well understood and became quite clear after the behavior of the unstable particles was studied with a computer. The size of the entrance aperture, which was thought to have a strong effect on the resolution, was found to be fairly uncritical. To get a better understanding of the behavior of charged particles, in particular their amplitudes in a quadrupole field, a computer program for the C D C 6600 was developed to solve Mathieu's equation for

Fig. 15. The rod structure for the quadrupole mass filter (total length between insulators 50.80 cm).

54

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o z~

,i ~ - o o o o o o ez ~ ~ w w t ~ w t ~ • ~ o o o o c J

uJ

[ p o o o o o o o o - a ~a ~ u~ u0 uo t n t n e~ D , , i c~r~

" o \

,

~

oo

oo

oo~'Ul

ooU~bJ

-7

.20E

-01

-9 .60E

Ol

. . . . . mmooe~cn

-1.20E*00

Fig. 16. Computed stable orbits for 6 particles in the quadrupole filter; top: projection of the orbits on the x,y-plane; middle x-component of the particle orbit; bottom: y-component. Particles differ in input vector (mass: 28 ainu).

•(ntue 8Z ssetu) sai~ue aseqd 1.i ltlaJajj.lp l'e alodnJpenb aql olu! palaa[u! salo!laed aA!_-I "LI '~!ar m

Do

a~

ZO

300"~

~ m D .

m

tJ

00~m • c, m ~

ZO

30E

,

" 9

z, •

z

~z

20

300 " L

~ -

Y "0

\ z

Z 0 - : 1 0 0 " 6

XO"

30Z

"~

Do

~r,-SrlS glq

I

.g

:1C. f3 " U -

o , ,

•

L ~

ZO-3OO't,-

Z

~z

~0"

300"

Z-

ZO "300"Z

ZO-300"~

-13ooI lndu! aql 'A a g'0 snu!tu pue snld aJe uo!laaJ!p-a" pue -x u! sluauodtuoa ~ a a u a aql 'A a 08I uo!laaa!p z u~ fi~aaua sl! 'ntue gZ s! ala!laed palaafu! aql jo ssetu aq.L •m d m o aalndtuoa aql jo aldtuexa ue s ~ o q s 9I "~!:I ~

:DVqlH 3 H I

ZC0

300 "

ZO

300"8

~0

300"~

9

•lold aa!loadsJad 13 pue au~ld-Dx oql oluo uo!laa.foad 13 se llam s~ aueld-z'~f pug - z ' x aql u! Sl.lqJo ~[a]lJed ~LII Sh'd3Jp a~l -]old dtuo D l e d zu!I-UO u V "suo.tl.lpuoa le!lIu! luzazJj!p

J.V NOIJ.V~IVd~tS ~ t d O I O S l ~ t N I I - N O

56

J.M. NITSCHKE

tions are - 0 . 3 3 , 0, + 0 . 3 3 c m (which corresponds to an entrance aperture o f k"). W i t h these entrance locations and two velocity vectors for each location, 6

orbits are calculated in the x , z - and in the y,z-plane. a and q were chosen so that double focusing in x and y results. This represents particles in stable orbits.

3 .6OE*Ot

2.iGE+Ol

? .20E+O0

i

z

O.

, ~r4 o~ z~ z

?.20E,W0

-

";?????? ~ o ~ w w w m u ~ m m o o o o o o o o m z u . . . . . .

m

~

T

Z.tGE+Ut

i

u~

-

o

wu, o o me)

w w c~o me~

-2.BBE*0%

,o,, × 4 g d j ~ ; g 7 .20E-Ol

2.40E--01

z O.

m "~ z~

-2.40E-0t

ff , - - I , l i P i •

~

,

o

o

o

o

o

o

~ o o o o o o o m z ~ . . . . . . m

D

i

,

,

o o

o o

~m

mm

Ol

\

\. m o~

m ~ o o m m ,,

2.2OE+OJ

Fig. 18. Example for instable particle orbits. All parameters the same as fig. 16, except mass: 27 amu.

ON-LINE ISOTOPE SEPARATION AT THE HILAC T o s h o w the influence o f the rf phase angle at the time the particle is injected into the quadrupole field, fig. 17 shows the orbits for 6 different phase angles 0, x3 ~ , 3-z~z~ ~ ' 3m ~, ~r~, 2~*. The particles enter at the center

57

x a n d y energy c o m p o n e n t s of 0.01 eV. Beside the phase angle, the influence of the radial velocity on the amplitude is very obvious, if one compares fig. 17 with fig.

* Because of the strong influence, the rf phase angle at the time of the injection of the particle has on its amplitude inside the quadrupole, we have tried to bunch the injected beam. This about doubled the transmission and improved the resolution• i.2OE÷O0

9•60E-01

?.20b-UL

4.80E-0~

A

II (~ c~

2.40E-01

ii

z

0.

__L

E

m

"~ c]c~ z~

-2

.40E

0 l-

g. ~- c l o

V

o c~ cJ o

cq o ~ W ~ U a W b J U a •

~ J o o o c ~ o o

( p o o o o o o o o m ~ j . . . . . . • tumtr) m m m m cq .~ No m o

\ \

rl

i

i

-7

.20E

Ot

l

oc,

o o

wm

mul

cao

oo

?.

:. .H~p.

e n c ~ o o ~

0~

'

r

o~ m

3.60E-0~

z

0.

m "el

-3

.60E-Of

-2

.20E-05

-1

.08E*O0

-~-

.44E*00

o~ zr~ ~0

g-

,4,4,-4o,4,4 ~- I I I I I I e~ C~ ~ t~ tu uJ tu w ta~ m o o o o o o o ~ e~zJ . . . . . . -(xmknmmu~mm m (n

o o

~

,~ [Dc~ ~.o -

oo i I tdW

o ~ I i LUUJ

\

e3 ~ o

,I

o

e3 m

-~,BOE*O0 m

m

~

o

r--

m

Fig. 19. Same parameters as fig. 16, but mass: 29 amu.

ol

o

r--

u,

58

J. M. NITSCHKE

16. All other parameters are the same as in fig. 16. Without changing any of the parameters of fig. 16, the mass of the injected particle was decreased from 28 to 27 amu and the resulting particle orbits are plotted in fig. 18. In fig. 19 the mass was increased to 29 amu, one amu above the stable mass 28. It is obvious that the particles of mass 27 will not be transmitted through the quadrupole filter because the field-producing electrodes limit the maximum excursion to _+ 1.64 cm.

ion beam was hollow (which was expected because of the accelerating grid in the magnetron) and that the field between the electron mirror and the einzel lens acted strongly defocusing. Fig. 20 is a very simplified case where the ions are created in a plane perpendicular to the axis in the middle of the ion source. The computer program solves the particle orbit equation by taking the magnetic field of the Helmholtz coils, the electrostatic field of the electrodes, and the space charge created by the beam itself into account. Several unsuccessful attempts to improve the transmission of the ion optical system lead to the decision to apply the strong focusing principle of a quadrupole lens. Instead of building an ordinary electrostatic quadrupole doublet or triplet in which the ion beam, loosely spoken, is first "squeezed" in x and then y direction (z being the axial direction), we found that this can be done with one quadrupole, if it is connected to a rf source instead of dc. In this case the ion

7. The transfer quadrupole The connection between the ion source and the mass analyzer was originally accomplished by conventional ion optics consisting essentially of an einzel lens and a pair of deflecting electrodes. It turned out, however, that this straightforward approach yielded a very low transmission. A computer study of the extraction ion optic, for which fig. 20 is an example, showed that the

67

SS

97

0

0

[

- 1 • ,

•

.

•

" .

i .

"

~ "

, . " . ' .

•

: --

•

-

-

,

ks

•

-

,.

Fig. 20. Computed particle orbits in the extraction region of the magnetron source; extractor potential : 50 V, electron mirror: 50 V, Helmholtz field: 100 G. Dotted lines are equipotential lines.

ON-LINE

ISOTOPE

SEPARATION

b e a m does n o t have to m o v e f r o m one q u a d r u p o l e to the next in o r d e r to get squeezed in o p p o s i t e directions, b u t the rf causes the q u a d r u p o l e electrodes to change p o l a r i t y fast enough to achieve the same effect in a single step. A n analytical t r e a t m e n t o f the m o t i o n o f the ions in this q u a d r u p o l e is very simple since it is a special case o f the Paul mass filter with no dc voltage, c o r r e s p o n d i n g to a = 0 (section 5). The equations of m o t i o n therefore are:

AT

(0)

THE

59

HILAC

Uio n = 1 2 0 V

H

(b)

Uion = IOOV

H

URF

URF

mS~ + (2ex/r2)V cos cot = 0,

(2ev/4)v c o s

{c~

Uion

: 80V

(d)

U~on = 6 0 V

cot = o,

m,~ = 0. The orbits in the x,z- and y , z - p l a n e are the same except for a phase factor. The c o m p u t e r p r o g r a m developed for the mass filter was used w i t h o u t any modifications. The transfer q u a d r u p o l e acts as a high pass filter for ions. Referring to the stability d i a g r a m fig. 14, the case a = 0 c o r r e s p o n d s to the q axis being the mass scan line, a n d only particles with q values between 0 and 0.92 will have stable orbits. This c o r r e s p o n d s for a given rf a m p l i t u d e V, frequency ~o a n d radius r0 to a mass range

4eV /(O.92rZco 2) <_ in <- oo. The higher the rf a m p l i t u d e the higher the cutoff for low mass ions*. This is a very useful feature, because it allows to cut m o s t o f the b a c k g r o u n d ions off, which are caused by the residual gas, a n d the catcher m a t e r i a l (in the case o f low Z catchers). The space charge burden on the b e a m is thereby greatly reduced. To d e m o n strate the effectiveness o f the high pass filter, a spec-

Fig. 21. Mass spectrum of natural Kr [VSKr (0.35%), 8°Kr (2.27%), S2Kr (11.56%), S3Kr (11.55~), 84Kr (56.90~), 86Kr (17.37~)] cut off at mass no. 83 by the quadrupole high pass filter.

0

H

m l .

U~F

URF

Fig. 22. a, b, c, d. Ion current through the transfer quadrupole as a function of the rf voltage on the quadrupole electrodes, for different ion energies. t r u m o f n a t u r a l K r was cut off at 83Kr (11.6%). N o S2Kr (11.6%), 8°Kr (2.3%) or VaKr (0.35%) can be seen in the mass s p e c t r u m (fig. 21). W h e n the r f was varied to a t t a i n o p t i m u m transmission of a certain mass, distinct m a x i m a were encountered. Fig. 22 shows the ion currents t h r o u g h the mass filter for different ion energies as a function o f the rf voltage. The transmission m a x i m a occur when a waist o f the b e a m is located at the entrance a p e r t u r e of the mass filter. The ratio of waist to average b e a m d i a m e t e r becomes smaller with lower ion energies causing the small transmission variations shown in fig. 22d. To s u p p o r t this e x p l a n a t i o n , the rf a m p l i t u d e s for the four m a x i m a in fig. 22b, together with the necessary i n f o r m a t i o n s a b o u t mass number, energy, and g e o m e t r y o f the transfer quadrupole, were " f e d " into the c o m p u t e r which calculated the particle orbits. The entrance a p e r t u r e for the mass filter was located 17.5 cm f r o m the starting p o i n t of the ion. The fig. 23 shows that indeed a waist in x and y occurs at this location. Fig. 24 shows the particle orbits c o r r e s p o n d i n g to the rf a m p l i t u d e at the second m a x i m u m in fig. 22b. A g a i n a waist in the x,z- and y,z-plane occurs at a b o u t 17.5 cm. F o r the t h i r d and f o u r t h m a x i m u m fig. 25 and fig. 26 are ob* Since quadrupole mass filters are used more and more "online" with gas chromatographs this might be a good way of filtering out large amounts of He, as well as suppressing primary ions in sputtering experiments.

60

J. M. NITSCIrlKE

1

44E~00

lOs£~O0

t.20£-Ot

£z

~~'

n~6°£-°I. [

'~ Z~

360£ Oi !

• w

\

,,>ocooo,~

/

.~LLA'D

r~oooooooo -~ c,,~mmmw~, c~Z

_1

f

. . . . .

10t3£~9f?

aCOE-01

7,

0.

2

2

~ z ~

. . . . . .

oo oo oo

oo g

a0£-00 oo

c:

o

~

=

£

'2

%

Z

2

£

Fig. 23. Particle orbits t h r o u g h the transfer quadrupole at a rf voltage corresponding to the first m a x i m u m in fig. 22b.

ON-LINE

ISOTOPE SEPARATION

rained. The waists are far less pronounced, and in fig. 26 the particle is already unstable. This is also indicated by the sharp drop in the ion current in the middle of

t

eoi,c9

".

ce£-

AT THE H I L A C

the fourth maximum of fig. 22b. In praxis only the first or second maximum are used for operating the quadrupole.

13 0

g

~ c : r J O O O o o o c.z~ •

S

-,o oo

oo oo

o ¢;

o E,

t

eo~*o.

~

oof

~)t

6

oof

ot

-1

~c,

?0¢-0J

c l o a o o o o

te~i

.

.

.

.

o~

oog

oo

oo

or, ?

61

)t)~*)3

Fig. 24. Same as fig. 23 except rf voltage is higher (2nd maximum in fig. 22b).

62

J. M. N I T S C H K E

With a slight modification in the rf power supply the transfer quadrupole can also be used for steering the ion beam; dc voltages of opposite polarity (contrary

to the mass filter!) superimposed on the rf are fed to each pair of opposing electrodes, so that the beam can be deflected slightly. This is very helpful in directing

a.,,t-O0

10et*O0

7.~0£

O:

60E

ot

60t

~l

O.

~

z.7Ot-O:

~Ot-O0

7.70t-00

a .eot-O0

~.OOt

-1

0l

eot~oO

Fig. 25. Same as fig. 23 except rf voltage is higher (3rd maximum in fig. 22b).

ON-LINE

ISOTOPE SEPARATION

the beam into the entrance aperture o f the mass filter. Fig. 27 shows the magnetron anode, extractor - and electron-mirror grid, and the transfer quadrupole.

63

AT THE H I L A C

M a x i m u m ion energy for unit resolution

(Am = 1 ainu): E~ ax = 105 eV; Mass range: 0-300 amu; Resolution: m/Am = 250 (design value). To test the resolution, 50 eV Xe ions were injected

8. Experimental evaluation of the quadrupole mass filter The quadrupole mass filter which was built for the on-line isotope separator has the following parameters : rf frequency (constant): ~o/2:z = 1.000 M H z ; G e o m e t r i c field parameter: r o = 1.6418 cm; Radius of the quadrupole electrodes: R = 1.8915 cm; Ratio R/r o for m i n i m u m higher order multipole c o m p o n e n t s : R/r o = 1.152; Length: L -- 57.5 cm;

f z

d~

,q~

-4

eO._+.)3

ze z

~ n a o o o o o o

- i ~ - . . ~

z

/

\/

\Z

z

/

~naaaaaoa

g~, ~a £,. aa

22

ao na ~ o o o o

7,

~

~

~

?,

3~

o°

?,

Fig. 26. S a m e as fig. 23 except rf voltage is higher as in all previous cases (4th m a x i m u m in fig. 22b). Particle is m an instable orbit.

64

J. M. N I T S C H K E

by pulse counting. It should be noted t h a t even at low resolution the interference o f adjacent peaks can be m a d e small. F u r t h e r m o r e the l o g a r i t h m i c display in fig. 29 is a g o o d d e m o n s t r a t i o n o f the p e a k a s y m m e t r y which is a basic artifact o f all q u a d r u p o l e - and m o n o pole-mass filters. To u n d e r s t a n d this we have to refer back to the stability d i a g r a m fig. 14. It should be r e m e m b e r e d t h a t on the mass scan line m l > m0 > m_ ~. If one wants to bring a larger mass, say m l , into the "stability c o r n e r " , one has to increase the rf a n d dc voltages U, V. M a s s ml then enters the stable region under a very flat angle so t h a t noise on the rf a n d dc produces a large " n o i s e " in mass n u m b e r ; which results in the flat slope of the mass peaks on the side o f the lower mass (in this case mo). If one increases the rf and dc voltages further mass m~ leaves the stability region nearly p e r p e n d i c u l a r to the stability b o u n d a r y where rf and dc instabilities have little influence on the mass n u m b e r ; which thus results in a steep slope o f the mass p e a k on the side o f the higher mass (mz). The stability of U and V is 3 x 10-4; the stability o f the frequency is 1 x 10 -6. The angle of entrance into the stability d i a g r a m would n o t have any influence on the slope o f the mass peaks if the rf and dc voltage as well as the frequency were a b s o l u t e l y stable. 9. Performance of the complete separator and on-line runs

Fig. 27. Magnetron anode, extraction grid, electron mirror and transfer-quadrupole (from top to bottom).

Several tests of the c o m p l e t e s e p a r a t o r were m a d e before it was installed at the accelerator. In p a r t i c u l a r it was tried to determine the over-all efficiency. In o t h e r words, the chance o f a recoil a t o m leaving the target, to get into the ion source, to defuse out o f the catcher before it decays, a n d to become ionized, to be transferred into the mass filter, to get t h r o u g h the

129 (26%)

into the q u a d r u p o l e . Fig. 28 is a p l o t o f the c u r r e n t at the F a r a d a y cup versus mass number. The resolution is m/Am = 850. The isotopic a b u n d a n c e s are n o t rep r o d u c e d correctly because of a slight inaccuracy in the t r a c k i n g of the rf and dc voltages. This effect disa p p e a r s at lower r e s o l u t i o n and is o f little i m p o r t a n c e in an i s o t o p e separator, M o s t tests concerning the p e r f o r m a n c e of the quad r u p o l e as a mass s p e c t r o m e t e r were m a d e with a 20 stage p h o t o multiplier, counting the t r a n s m i t t e d ions and storing the n u m b e r o f counts in a multi-channel analyzer which was o p e r a t e d in the multi-scaler mode. The stepping off t h e M C A was synchronized with the mass scan. Fig. 29 shows a typical spectrum o b t a i n e d

Xe spectrum Ion energy: 50volts

5×~o-~2A 13r(21%)

4 ~ s g

132(27%)

5

2 128(2%)

o

126(.09%} J ~

YJ

/ - A ~ - = 850

134(10%) /36 (9%)

A ,

Fig. 28. High resolution mass spectrum of Xe. Ion current measured with a Faraday cup.

ON-LINE

ISOTOPE

SEPARATION

AT THE

65

HILAC

hOOE+ 05

t.OOE+04

I.OOE+03 c

hOOE+02

I.OOE+ OI

I.OOE+O0 0

50

I00

150

200

250

500

550

400

Channel number

Fig. 29. Low resolution Xe-spectrum. Ion pulse counting:with 20 stage multiplier and MCA storage. mass filter and to be deflected onto the detector. Only very few of these variables can be theoretically deduced with any degree of confidence, whereby the linkage of the following parameters will cause large variations: 1. The type of nuclear reaction will influence the recoil energy and consequently the penetration depth into the catcher and the diffusion time. 2. The type of nuclear reaction and the target thickness will influence the absorbtion in the target and the angular spread of the recoils. 3. The kind of element that is produced in the nuclear reaction will influence the diffusion out of the catcher and the cross-section for electron bombardment and surface ionization. 4. The transmission that can be achieved through the quadrupole mass analyzer depends on the mass number of the isotope to be studied. Preliminary results concerning these four points are: 1. Sm bombarded with 166 MeV 160 produces among other nuclei 154yb with a half life of 0.39 sec9). The recoil energy is 15 MeV. By comparing the yield for lsgYb with the yield for longer lived isotopes made during the same bombardment and taking into account the reduced cross section, it was found that no holdup due to slow diffnsion in the catcher can be observed at this half life. 2. From measurements of the angular spread of recoils by Donovan et al.a°), it can be estimated that with the present geometry about 20% of the recoils enter the ionization chamber. 3. The sensitivity of the magnetron ion source is

easily established for gasses. Using common gasses like N2, 02, CO2, He, Ne, Ar, Kr, and Xe it was found that the sensitivity varies between 2 A/tort and 20 A/tort. It has to be realized, however, that the pressure inside the ionization region is not known precisely. We therefore used another independent method to establish the sensitivity. A small piece of Mo wire was put in the ionization chamber and the temperature raised to 1981°K. The resolution of the mass filter was adjusted so that all Mo isotopes were transmitted. This resulted in an ion current of 50 nA. The vapor pressure of Mo at 1981°K is 1.4 x 10-7 torr 11). If one assumes a loss of about 80% at the entrance aperture of the quadrupole, the extractor grid, and in the transfer quadrupole, the sensitivity of the ion source is about 1.5 A / t o m This experiment was repeated at different temperatures. A sensitivity of 2 A/torr is equal to an ionization efficiency of about 1%. The weak point of this method is that an error in the temperature measurement results in a large error in the vapor pressure and consequently in the sensitivity. Therefore a third determination of the ionization efficiency was done with 2*lAm. A known amount of activity was placed on a small platinum disc, which then was spot welded onto the inner wall of the ionization chamber; moderate heating released the americium. The ionized portion of the activity which was extracted out of the ion source was collected on a plate and counted. After a correction for the neutral par-

66

J. M. NITSCHKE 15V 3A

I

_

~ "/

OUT

oov: uo O

DISTANCE i~

~

/

E ton TOTAL 100

\

/

(ev) 200

Fig. 30. Electrical potentials at different parts of the on-line separator (arrows with circles indicate variable and regulated power supplies).

I.OOE + 05

52 Sm- spectrum ! Ion energy :. 50, eV Enfrence - eperature-= 9.5 mm Sm 152 current :-20 nA

9.00E+ 0 4 8.00E+ 04

154

7.00E+04

f

.....

6.00E+ 0 4

147

~9

E= 5.00E+ 0 4 0

co 4.00E÷ 0 4

M

3.00E+ 0 4

150

2.00E ÷ 04 144

I.OOE + 0 4

50

,oo

15o 200 2so choonel o~mber

300

]Fig. 3]. Mass spectrum o f natural Sin as used for separating 1~Sm (refer to section 9).

3~o

400

ON-LINE

ISOTOPE

SEPARATION

IOOO

800

E

600 -g 13

400

m

"7~

200

O

3 o~

o

Mass analysis of separated Sm 152

Fig. 32. Mass spectrum of separated la2Sm sample, as analyzed with a magnetic mass spectrometer. ticles that might have reached the collector the ratio of the activities gave an ionization efficiency of 3.5%. All measurements were taken with the following operating parameters of the ion source: Magnetron anode voltage: U, = 250 V, Electron current measured at the cathode: I, = 500 mA; Extractor voltage: Uex = 35 V; Electron mirror voltage: Uml,r = 300 V; Ion energy: Uio. = 80 V; Magnetic field: H = 100 G (approx.). Fig. 30 shows schematically-typical potentials at different parts of the separator. (The current values indicate the upper limits of the power supplies). 4. In the present quadrupole analyzer the transmission has to be reduced to a b o u t 1% to obtain less than 1% interference from adjacent peaks at mass number 150. This is demonstrated in an experiment where 152Sm was separated from natural samarium (off line). The mass marker oven (see fig. 5) was charged with natural samarium and heated so that the ion current at the 152Sm peak was about 20 nA. The adjustment of the proper resolution necessary to separate lSZSm was done by observing the spectrum fig. 31. The ion current onto the F a r a d a y cup was amplified, fed into a current-to-frequency converter, " s t o r e d " in a multi-

AT

THE

67

HILAC

channel analyzer, and transferred onto a tape. The plotting of the spectrum was done by a computer. After stopping the scanning o f the spectrum and adjusting the mass filter for mass 152, a current of 10 n A 15ZSm was deflected onto a 1" platinum collector which was located at the position o f the alpha detector in fig. 3. 53 ng tS2Sm were collected. The sample was submitted to our mass spectrometer group which analyzed it with a magnetic mass spectrometer. The result is shown in fig. 32*. The separation factors t are 224 for 154Sm, 820 for 149Sm, and 860 for 147Sm. The other isotopes cannot be used to calculate separation factors because of possible interference f r o m other rare earths. These separation factors can be considered satisfactory for most on-line work. They might have been even better, considering that there was an accident during the separation which involved natural samarium atoms. In conclusion the following very preliminary estimate of the over-all transmission o f the on-line separator can be made: 1. Transfer of the recoils into the ionizing chamber:

qt=5xlO-a; 2. Ionization efficiency: t/z = 1 x 10-2; * Thanks to M. Michel who performed the analysis. t The separation factor is defined as the ratio of the natural abundance of an isotope versus the abundance after separation; both ratios taken with respect to the isotope to be separated. I

I I I I 1 Mass no 152 separation

I

30

Er 152_ (Ha 153)

Ha 152

(Er 153)/~ (Er 153)

0

2

I

150

Sm (C Iz, x n)

I00

Dy 150 50

o

Dy J51 ~'~ ,~ Ert54 'i

22" ;,"

O

IO

20

,.4." -,.,,I

Ha 151 ~;/!~

I

30 4 0 50 60 Channel number

-Er152

I

70

80

Fig. 33. A l p h a s p e c t r u m o f Sm (,~2C, x n ) b o m b a r d m e n t

90 (bottom).

Same spectrum after turning on mass filter for separating mass 152 (top).

68

J.M.

NITSCHKE

3. Extraction from ion source, transfer, and collimation: qa = 2 x 10-a; 4. Transmission of the mass filter (at mass 150): r/4 = 10 -2. Total transmission: ~/= qlr/2r/3q4 = 1 x 10 -5 This figure has to be compared with the actual figure deduced from on-line runs. For the first on-line runs we chose the reactions Sm(lZC,xn)Er, Sm(~4N,xn)Tm, and Sm('60,xn)Yb. With full energy beams (10.38 MeV/nucleon) and x equal to 6, 7, 8 or 9, alpha-emitting isotopes of the rare earth are produced. They require only a simple surface barrier detector for unambiguous identification by their a-decay energy and half life, and have high enough mass numbers to prove the usefulness of the mass filter. Fig. 33 shows the result of the separation of 15ZEr which has a half life of 10.7 sec. The lower part of fig. 33 is the alpha spectrum of the reaction 147Sm (~2C,xn) taken with a wide transmission range of 150_+ 10 amu. In the upper portion the mass filter was tuned for mass 152 and unit resolution, a52Ho and ~52Er are now the major peaks. There is a slight inter-

ference from ~53Er. The counting rate for 152Er is about 2 counts/min. Assuming a cross section of 10 mb for the reaction 147Sm (12C, 7n) 152Er, a 12C beam of 2 pA, and a target with 1 mg/cm 2 147Sm, the formation rate for 152Er is 4 x 105 particles/min. This corresponds to a total transmission of 0.5 x 10 -5, and compares favorably with the calculated transmission, if one considers a certain amount of absorption in the target itself. Figs. 34, 35 and 36 show the complete ion source, the target with the mass marker oven, and a microphotograph of the graphite felt used for stopping the recoils.

10. Summary and conclusions It has been shown in this paper, that the unusual combination of recoil target, efficient electron bombardment source and a Paul mass filter can be used as an on-line isotope separator. Diffusion times in hot solid catchers are short enough to study sub-second isotopes. Without altering the principles, by mere changes in geometry a factor 100 could be gained in over all

J

Fig. 34. Exploded view of the magnetron ion source (with "old" ion optics!).

ON-LINE ISOTOPE SEPARATION AT THE HILAC

69

transmission. These changes w o u l d include all m a i n parts o f the M a r k II separator, ion source, transfer q u a d r u p o l e , a n d mass filters. 1 wish to express m y a p p r e c i a t i o n to Dr. A. G h i o r s o for his c o n t i n u e d interest in this w o r k and the inspiring and stimulating a t m o s p h e r e he and his g r o u p have provided. Only a small fraction of w h a t was a c c o m p l i s h e d would have been possible w i t h o u t the ingenuity o f C. A. C o r u m and his unconventional, efficient approach to mechanical designing. I am very grateful for his help. I wish to t h a n k R. G r a z i e r for the design o f the rf p o w e r supply for the q u a d r u p o l e . The splendid c o o p e r a t i o n o f the accelerator technicians and the H I L A C crew is greatly a p p r e c i a t e d . The a u t h o r was the recipient o f a G e r m a n A c a d e m i c

Fig. 36. Micro photograph of graphite felt catcher. Exchange Service ( D A A D ) fellowship d u r i n g p a r t o f the research.

Fig. 35. Target holder; left: beam side, right: ion source side with heat shield and mass marker oven.

References 1) I. Bergstr6m, Nucl. Instr. and Meth. 43 (1966) 116. 2) G. Rudstam, Nucl. Instr. and Meth. 38 (1965) 282. 3) A. Ghiorso, private communication. 4) W. Paul and M. Raether, Z. Physik 140 (1955) 262. 5) j. M. Alexander and L. Winsberg, UCRL-8997 (1960). ~) R. E. Honig, RCA Rev. 23, no. 4 (1962) 567. 7) A. W. Hull, Phys. Rev. 18 (1921) 31. s) L. Brillouin, Proc. IRE 32 (1944) 216. ~) R. D. MacFarlane, Phys. Rev. 136 (1964) B941. 10) p. F. Donovan, B. G. Harvey and W. H. Wade, UCRL-9060 (1964). 1i) See ref. G).