69 Time-of-Flight Isochronous Spectrometer for Direct Mass Measurements of Exotic Light Nuclei 1
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G. W. Butler, D. J. Vieira, J. M. Wouters, H. Wollnik, K. Vaziri , F. K. Wohn, and Daeg S. Brenner 4
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Isotope and Nuclear Chemistry Division, Los Alamos National Laboratory, Los Alamos, NM 87545 In recent years there has been a rapid increase in our knowledge of the atomic-mass surface far from the valley of β stability [BEN80,CER8l]. How ever, even with these advances there remain more than 60 neutron-rich nuclei with A < 70 for which the only known information is that they are stable with respect to neutron emission. It is clear that more detailed informa tion about these exotic nuclei is essential. Especially important are the measurements of ground state atomic masses, since the ground state mass is one of the most fundamental properties of a nucleus. The binding energy, and hence the mass, of a nucleus is dependent on the exact details of the nuclear force, and its prediction requires an understanding of these details. In a strong sense, an atomic-mass model includes everything that we know about the nuclear force and nuclear inter actions. A systematic study of accurately determined masses encompassing a wide variety of nuclei far from β stability would provide a most challenging test of current atomic-mass theories and should yield new insight into the nuclear structure of such exotic nuclei. Interest in the neutron-rich light mass nuclei increased with the dis covery by Thibault [THI75] that, beginning with Na, the sodium isotopes become strongly prolate deformed. This deformation, wnich is due to the partial filling of the f7/2 shell [CAM75], was unexpected since for Na the d3/2 neutron shell closure was predicted to occur at N=20. Mapping this new region of deformation more fully is of great interest since the deformation could possibly be driven by the odd ds/2 proton present in the sodium isotopes. This disappearance of a neutron magic number at N=20 has led to speculation that new magic numbers at N=10,14 and/or 16 might appear in such deformed nuclei [SHE76]. An experimental search for these new magic numbers and their discovery will allow more detailed investigations of the strong correlation of proton and neutron shell strengths that occurs near double shell closures and which might occur in 0. 31
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2. Time-of-Flight Isochronous (TOFI) Spectrometer TOFI is a recoil time-of-flight spectrometer that is being constructed at the Los Alamos Meson Physics Facility (LAMPF) for the purpose of making direct mass measurements of neutron-rich light nuclei that are produced via proton-induced fragmentation of U or Th targets. The design of TOFI will allow systematic mass measurements for a large number of neutron-rich nuclei below A=70 with accuracies of 30 keV to 1 MeV, depending on production rates (see Fig. 1). In this paper we outline the basic features of the TOFI spectrometer and its associated transport line, discuss the mass measurement 'Permanent address: University of Giessen, D-63 Giessen, Federal Republic of Germany Permanent address: Department of Physics, Utah State University, Logan, UT 84322 ^Permanent address: Ames Laboratory, Iowa State University, Ames, IA 50011 'Permanent address: Office of Academic Affairs, Clark University, Worcester, MA 01610 0097-6156/ 86/ 0324-0459$06.00/ 0 © 1986 American Chemical Society
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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F i g . 1. P a r t i a l chart of the nuclides showing: (1) those n u c l e i whose mass has been measured [WAP85] (black t r i a n g l e s ) and (2) the n u c l e i we propose t o measure (shaded boxes; shading i n d i c a t e s expected accuracy of mass measurements).
c a p a b i l i t i e s of the system, and h i g h l i g h t progress made on the i n s t a l l a t i o n and i n i t i a l t e s t i n g t o date. The b a s i c p r i n c i p l e of the TOFI spectrometer i s that i t i s designed t o be isochronous, which means that the t r a n s i t time of a p a r t i c l e passing through the spectrometer i s independent of i t s v e l o c i t y . Thus the measured t r a n s i t time of an i o n passing through the system provides a p r e c i s e measurement of the mass-to-charge r a t i o . Since charge i s a quantized e n t i t y , only moderately accurate measurements (at the 1-2% l e v e l ) o f the energy and v e l o c i t y , together with the known momentum acceptance of the spectrometer, are necessary t o d e f i n e uniquely the charge s t a t e o f an i o n . An accurate mass can then be determined from the mass-to-charge r a t i o as obtained from the measured t r a n s i t time o f the i o n through the spectrometer. Another important f e a t u r e of the spectrometer i s that i t i s f o c u s i n g i n both energy and angle, thus a l l o w i n g a long f l i g h t path (14 m) with a r e l a t i v e l y l a r g e s o l i d angle (2.5 msr) and momentum acceptance (±2%). Since the spectrometer i s nondispersive o v e r a l l , there i s no p h y s i c a l s e p a r a t i o n between d i f f e r e n t i o n s , which allows the simultaneous measurement o f many n u c l e i ( e s p e c i a l l y important a r e i s o b a r i c members with well-known masses that w i l l be used as i n t e r n a l c a l i b r a t i o n p o i n t s ) .
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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The spectrometer c o n s i s t s of four i d e n t i c a l , mirror symmetric u n i t c e l l s that contain a 81 sector magnet l o c a t e d between i d e n t i c a l d r i f t lengths [W0U85]. The symmetry of the system r e s u l t s i n a u n i t y , f i r s t order t r a n s f e r matrix i n which only the time term dependent on the mass-to-charge r a t i o remains. Furthermore, a l l higher order aberrations are s m a l l , r e s u l t i n g i n a t o t a l time d e v i a t i o n of -80 ps f o r d i f f e r e n t t r a j e c t o r i e s through the spectrometer. Assuming timing u n c e r t a i n t i e s of -200 ps from detector time j i t t e r , -200 ps from magnetic f i e l d inhomogeneities and -80 ps from o p t i c a l aberrations f o r a t o t a l of -300 ps, the r e s o l v i n g power of the spectrometer i s c o n s e r v a t i v e l y estimated to be m/M = 2000. Extensive magnetic f i e l d mapping and o p t i m i z a t i o n of the four spectrometer d i p o l e magnets i s c u r r e n t l y nearing completion. I n s t a l l a t i o n of the d i p o l e s i n t h e i r f i n a l c o n f i g u r a t i o n w i l l begin soon, and i n i t i a l t e s t i n g with alpha sources i s scheduled to begin i n October 1985. The f i r s t e x p e r i ment, i n which we w i l l measure the masses of heavy Ne, Na and Mg i s o t o p e s , w i l l begin i n November 1985.
3. TOFI Transport
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The spectrometer i s connected v i a a secondary beam t r a n s p o r t l i n e to the production t a r g e t (see F i g . 2 ) . This transport l i n e c o n s i s t s of 4 quadrupole t r i p l e t s that are arranged as two p a i r s with an intermediate
F i g . 2. Schematic of the TOFI spectrometer and i t s a s s o c i a t e d secondary beam transport l i n e .
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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focus between the p a i r s . The transport l i n e i s magnifying o v e r a l l with a v e r t i c a l and h o r i z o n t a l magnification of -2 so that a l a r g e r s o l i d angle could be matched t o the acceptance of the spectrometer. In a d d i t i o n , the l i n e i n c l u d e s a crude mass-to-charge f i l t e r (an e l e c t r o s t a t i c d e f l e c t o r followed by a d i p o l e magnet) that eliminates the h i g h - y i e l d l i g h t mass r e a c t i o n products such as protons, deuterons, alphas and a l l n e u t r a l r e c o i l s . The t r a n s p o r t l i n e provides a very important advantage i n that the spectrometer can be operated independently of the h i g h - i n t e n s i t y LAMPF beam operation. A l l of the components of the transport l i n e have been i n s t a l l e d and t e s t s with alpha p a r t i c l e s and l i g h t r e a c t i o n products are nearing comple tion. In order t o c h a r a c t e r i z e i t s operation, the t r a n s p o r t l i n e has been tuned with various alpha sources and low Ζ r e a c t i o n products from 800-MeV protons on thorium. A multiwire p r o p o r t i o n a l counter was used to obtain a focus, and a good image (1.0 χ 1.4 cm ) was produced at the f i r s t f o c a l point of the t r a n s p o r t l i n e from a A m source (0.6 cm diam.) l o c a t e d 30 cm behind the normal t a r g e t p o s i t i o n i n the s c a t t e r i n g chamber. To determine the energy-transmission c h a r a c t e r i s t i c s of the transport l i n e , an S i detec tor was used i n conjunction with a m u l t i p l e energy alpha source (a combina t i o n of G d and T h ) . Through a comparison of alpha i n t e n s i t i e s measured at d i f f e r e n t energy or momentum s e t t i n g s of the transport l i n e , the momentum transmission of the f i r s t h a l f of the transport l i n e was measured to be 6p/p = 26% (FWHM), which i s much l a r g e r than r e q u i r e d f o r the spec trometer (δρ/ρ = 4 % ) . The mass f i l t e r was c h a r a c t e r i z e d by using Z=l and 2 nuclides produced i n 800-MeV protons on a thorium t a r g e t . For t h i s t e s t , the transport l i n e was set f o r a p a r t i c u l a r momentum-to-charge value, and then the mass f i l t e r was tuned f o r various mass-to-charge r a t i o s . By com paring the i n t e n s i t y of the alphas and t r i t o n s at various mass-to-charge s e t t i n g s , we obtained the mass-to-charge transmission f o r the f i r s t h a l f of the transport l i n e (see F i g . 3). The r e s u l t s showed that a t a s e t t i n g of 2
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Mass-to-Charge Ratio Setting (amu/Q) F i g . 3. The mass-to-charge transmission of the f i r s t h a l f of the t r a n s p o r t l i n e f o r alphas (dashed l i n e ) and t r i t o n s ( s o l i d l i n e ) , as emitted from a thorium t a r g e t . The t r a n s port l i n e was set f o r a c e n t r a l momentum-to-charge value of 190 MeV/c/Q.
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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A/Q = 3 ( t r i t o n s ) those species with mass-to-charge r a t i o s of 2 or l e s s were reduced by more than 2 orders of magnitude. (Scattered background events l i m i t e d the measurement of t h i s r e d u c t i o n f a c t o r . ) T h i s mass-tocharge f i l t e r i n g i s p a r t i c u l a r l y important i n our future experiments because the protons, deuterons, and alpha p a r t i c l e s are produced with y i e l d s that are, r e s p e c t i v e l y , Ι Ο , 1 0 , and 1 0 times l a r g e r than those o f the ions f o r which measurements a r e planned. r
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4. Summary The o v e r a l l goal o f the TOFI spectrometer p r o j e c t i s t o perform, i n a systematic f a s h i o n , d i r e c t mass measurements o f l i g h t n u c l e i that are f a r from s t a b i l i t y and thus i d e n t i f y general, as w e l l as i s o l a t e d , trends i n nuclear s t r u c t u r e . This isochronous, nondispersive spectrometer w i l l be the f i r s t high r e s o l u t i o n r e c o i l spectrometer t o use the t i m e - o f - f l i g h t t e c h nique f o r d i r e c t mass measurements. The TOFI spectrometer w i l l be able t o measure the masses o f -60 n u c l e i , two-thirds o f which w i l l be measured f o r the f i r s t time. T h i s experimental program w i l l begin during the f a l l o f 1985 with the completion o f the i n s t a l l a t i o n of the spectrometer.
Acknowledgments We would l i k e t o thank both the Isotope and Nuclear Chemistry and Meson Physics d i v i s i o n s o f the Los Alamos National Laboratory f o r the support they have provided towards the development and c o n s t r u c t i o n o f the TOFI spectrometer. T h i s work was conducted under the auspices o f the U.S. Department o f Energy.
References [BEN80] W. Benenson and J. Nolen, Eds., "Proceedings of the Sixth Inter national Atomic Mass Conference", East Lansing, Michigan, September 1979 (Plenum Press, New York, 1980). [CAM75] X. Campi, H. Flocard, A. Kerman, and S. Koonin, Nucl. Phys. A251 193 (1975). [CER81] "Proceedings of the Fourth International Conference on Nuclei Far From Stability", Helsingor, Denmark, June 1981, CERN Report 81-09 (1981). [SHE76] R. K. Sheline, Proceedings of the Third International Conference on Nuclei Far From Stability, Cargese, Corsica, May 1976, CERN Report 76-13, 351 (1976). [THI75] C. Thibault, R. Klapisch, C. Rigaud, A. M. Poskanzer, R. Prieels, L. Lessard, and W. Reisdorf, Phys. Rev. C12 644 (1975). [WAP85] A. H. Wapstra and G. Audi, Nucl. Phys. A432 1 (1985). [WOU85] J. Wouters, D. Vieira, H. Wollnik, H. Enge, S. Kowalski and K. Brown, accepted for publication in Nucl. Instr. and Meth. in Phys., (1985). RECEIVED July 15, 1986
Meyer and Brenner; Nuclei Off the Line of Stability ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
