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Submitted on 1 Jan 1981
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PROGRESS OF THE STATE-SELECTED BEAM LOW TEMPERATURE HYDROGEN MASER
S. Crampton, J. Krupczak, S. Souza
To cite this version:
S. Crampton, J. Krupczak, S. Souza. PROGRESS OF THE STATE-SELECTED BEAM LOW TEM-
PERATURE HYDROGEN MASER. Journal de Physique Colloques, 1981, 42 (C8), pp.C8-181-C8-
184. �10.1051/jphyscol:1981820�. �jpa-00221715�
JOURNAL DE PHYSIQUE
Colloque C8, s u p p l h e n t au n012, Tome 42, de'cembre 1981 page C8-18 1
PROGRESS OF
THE
STATE-SELECTEDBEAM LOW TEMPERATURE
HYDROGENMASER
S.B. Crampton, J.J. Krupczak and S.P. Souza WilZiams College, WiZZiamstown, MA, U. S.A.
Abstract.- We describe the source of a beam of liquid helium temperature state-selected hydrogen atoms to be used in the development of a very low temperature atomic hydrogen maser frequency standard. Recent experimental results which affect the design of such a beam are presented, and future experimental plans are outlined.
Introduction.- Recently published1,2 and as yet unpublished studies by our labor- atory of hydrogen atom (H) adsorption on polycrystalline molecular hydrogen (Hz) surfaces provide the basis for designing a source of a state-selected beam of polarized hydrogen atoms (HI.) thermalized at liquid helium temperatures. Attain- able beam intensities and beam densities promise to
be one or two orders of magnitude higher than those achieved with room temperature thermal HI. beams.
Any impurities other than He atoms should be effec- tively eliminated by cryopumping. Such beams should improve the signal-to-noise of low temper- ature studies of H , ~ - ~ and they may prove useful as sources and targets of polarized protons.
Adsorption of H on H7.- Figure 1 illustrates the apparatus we have used to study the adsorption of H on H2 at temperatures from 3.2 K to 4.6 K.
H atoms are produced in a rf discharge cooled by liquid nitrogen. Atoms emerging from a 2 mm dia- meter source orifice travel down about 20 cm of
11 mm ID pyrex tubing to a 5 cm ID quartz storage bottle. Everything below the source liquid nitro- gen dewar is immersed in a liquid helium bath whose temperature is controlled by regulating the helium E gas pressure over the bath. All interior surfaces below the source are covered by solid molecular hydrogen frozen slowly from about 0.1 mole Hz vapor as the cryostat is cooled. A short pulse of micro- wave radiation near the 1420 MHz H ground state
hyperfine transition frequency induces the atoms to Fig. 1 : Schematic of the radiate a signal that decays in times of the order Adsorption Study Apparatus.
of milliseconds and whose frequency is shifted from (A) Hz inlet; (B) stainless the free space hyperfine frequency by amounts of steel liquid nitrogen dewar;
the order of hundreds of Hz. (C) dissociator rf coil; (D) From the data we are able to extract the mean orifice; (E) quartz storage phase shift @ of the hyperfine frequency radiation bottle; (F) microwave cavity;
phase per trip across the storage bottle and the (G) coupling loop; (H) quartz mean probability a that an atom is adsorbed at cavity tuning rod; (I) cylin- least once while rattling around on the surface drical capacitor; (J) temper- after a trip across the storage bottle. Figure 2 ature sensors.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981820
JOURNAL DE PHYSIQUE
Fig. 2 : Mean phase shift $ per trip across the storage bottle, plotted on a logarithmic scale against inverse temperature with lo error bars. The open circles represent results from indepen- dently prepared H2 surfaces.
Liquid Nitrogen
"Source Dewar"
Indium Seal Accomodator
Hexapole Magnet
Storage Bottle
Fig. 4 : State-selected H+ Beam
Fig. 3 : The parameter f3 = a-'-1 plotted against temperature with la error bars. Results are pre- sented for two independently prepared surfaces.
0.6-
0.5-
0.4
B
0.3
displays 6 as a log plot against inverse temperature. The mean adsorption time
<ta> per trip across the storage bottle, proportional to $, is exponential in inverse temperature with exponent 39.8(3)/T. Figure 3 displays the adsorption probability a, plotted as
= a-'-1 against temperature. a is fairly flat and of order 0.8 at the low- est temperatures studied, but begins to fall above 4 K, passing through 0.65 at about T=4.6 K. <ta> and a are so large that the atoms should be thermalized at the temperature of the surface after only a few collisions at temperatures near 4 K.
For the same reason recombination losses are dominated by two body collisions be.- tween atoms while adsorbed. By measuring the surface heating due to recombination at high atom fluxes, we find that the probability y of recombination per trip across the storage bottle is indeed pro- portional to atom density nH. The propor- tionality is approximately2 y=4x10-l6 n ~ ' y is proportional to two factors of <ta>, so that we expect its temperature depen- dence to be dominated by an exponential with exponent =80/T. Extrapolating to T=6 K predicts y of order 1.5x10-" nH.
RUN 17
mun 19 -
i t
--
t
H Beam Design.- Figure 4 illustrates the present configuration of our state- selected H+ beam. Atoms emerging from a source like that depicted in Fig. 1 pass through a 5 mm ID by 3.2 cm long cylindrical "accomodator
.
" Two semi-c i r c u l a r b a f f l e s e l i m i n a t e l i n e of s i g h t p a r t i c l e s and r a d i a t i o n . Atoms i n t h e upper two (F=l,m=l) and (F=l,m=O) ground s t a t e H h y p e r f i n e l e v e l s a r e focused by a hexapole magnet i n t o a s t o r a g e b o t t l e coated w i t h f r o z e n H2. They a r e d e t e c t e d by p u l s i n g t h e h y p e r f i n e t r a n s i t i o n , a s i n t h e p r e v i o u s experiment.
The accomodator i s cooled by conduction t o t h e l i q u i d helium b a t h through a 1 cm t h i c k b r a s s f l a n g e . A h e a t e r wound on t h e o u t s i d e of t h e accomodator a l l o w s i t s temperature t o b e a d j u s t e d upwards. The hexapole magnet i s 10 cm long and h a s a 1.3 cm diameter gap between p o l e p i e c e s . The f i e l d a t t h e p o l e p i e c e s i s about 2700 gauss. The magnet parameters have been chosen t o Optimally f o c u s (F=l,m=O) atoms having thermal speeds a t 7 K and s t a r t i n g a s f a r o f f a x i s a s p o s s i b l e .
The average number of s u r f a c e c o l l i s t o n s made by an atom a s s i n g through t h e accomodator i s Ac=lOO, s o t h a t a t T = 4 K , yTTc=,;I a t nH=Z.5 x 1011 cm". I f t h e d e n s i t y a t t h e accomodator were t o s a t u r a t e a t about t h a t v a l u e , t h e maximum f l u x from t h e accomodator would b e about 4 x 1 0 ~ ~ s e c - l . However, measurements of H2 s u r f a c e h e a t - i n g by H recombination i n t h e s t o r a g e b o t t l e d u r i n g t h e F i 1 experiment i n d i c a t e d t h a t a t optimum d i s s o c i a t o r p r e s s u r e and power about 4 ~ 1 0 ~ " s e c - ~ atoms were d e l i v - ered t o t h e s t o r a g e b o t t l e a t t h e end of an i n l e t t u b e i n which atoms made 400 s u r - f a c e c o l l i s i o n s on t h e average. A t t h o s e high atom f l u x e s t h e temperature of t h e s t o r a g e b o t t l e s u r f a c e was observed t o r i s e by about 0.13 K. I n t h e i n l e t t u b e above t h e s t o r a g e b o t t l e t h e d e n s i t y was much h i g h e r and t h e recombination h e a t i n g p e r u n i t s u r f a c e a r e a t h a t much g r e a t e r . E v i d e n t l y , t h e e l e v a t e d H2 s u r f a c e temp- e r a t u r e l e d t o decreased y and consequently t o h i g h e r atom d e l i v e r y t h a n would have been expected i f t h e i n l e t t u b e s u r f a c e temperature had been only 4.2 K.
A s t h e s u r f a c e temperature r i s e s , e f f e c t s o t h e r t h a n simply t h e d e c r e a s e of y come i n t o p l a y . From t h e measurements of Hardy e t . a1. we know t h a t t h e mean f r e e p a t h of H atoms i n t h e s a t u r a t e d vapor of H2 a t 6 K i s of t h e o r d e r of 0 . 2 mm.
Unless t h e f r o z e n H2 above a few l a y e r s t i g h t l y bound t o t h e s u b s t r a t e i s pumped away t o c o l d e r r e g i o n s , t h e d e n s i t y of H2 vapor above t h e H2 s u r f a c e w i l l impede t h e H atoms and thereby ilncrease t h e number of s u r f a c e c o l l i s i o n s they make i n t h e accomodator. A s y d e c r e a s e s and t h e d e n s i t y of H t h a t can b e maintained above t h e s u r f a c e i n c r e a s e s , t h e mean f r e e p a t h of H i n t h e H gas w i l l f a l l below t h e charac- t e r i s t i c dimensions of t h e accomodator and magnet. From t h e c a l c u l a t i o n s of A l l i s o n and smith6 we e s t i m a t e t h a t t h e mean f r e e p a t h of H i n H a t nH=1015 i s of o r d e r 1 mm n e a r 4 K. Not only w i l l H-H s c a t t e r i n g i n c r e a s e t h e number of s u r f a c e c o l l i s i o n s i n t h e accomodator, b u t s c a t t e r i n g i n t h e magnet w i l l i n t e r f e r e w i t h s t a t e s e l e c t i o n . I f t h e H2 d e n s i t y above t h e s u r f a c e can b e k e p t low by pumping away t o c o l d e r r e g i o n s , t h e u s e f u l accomodator H d e n s i t y w i l l b e l i m i t e d by s c a t t e r i n g i n t h e HI. beam, a s i t i s i n t h e c a s e of room temperature H beams, t o about h a l f t h e s a t u r a t i o n d e n s i t y of room temperature beams. The low temperature s o u r c e h a s t h e advantage of r e l a t i v e l y e f f i c i e n t pumping by recombination on a warm, uncoated magnet and crypumping of t h e r e s u l t a n t H2 by t h e magnet can w a l l s , s o t h a t i t should b e p o s s i b l e t o open up t h e s o u r c e a p e r t u r e more than i s p o s s i b l e f o r room temperature s o u r c e s . I n a d d i t i o n , almost any i m p u r i t y emerging from t h e s o u r c e w i l l b e f r o z e n o u t b e f o r e r e a c h i n g t h e s t o r a g e b o t t l e s u r f a c e , an important c o n s i d e r a t i o n i n frequency s t a n d a r d work.
A n t i c i p a t e d Magnet Performance.- I f t h e accomodator e x i t a p e r t u r e were very s m a l l compared t o t h e magnet gap and t h e magnet gap were i t s e l f s m a l l enough t h a t t h e magnetic moment of t h e (F=l,m=O) s t a t e were e f f e c t i v e l y s a t u r a t e d , t h e e f f e c t i v e s o l i d a n g l e of t h e 7 K magnet would be about 30017 t h a t of a magnet f o r f o c u s i n g 300 K atoms. However, i f some combination of t h e e f f e c t s d i s c u s s e d above l i m i t s t h e accomodator d e n s i t y t o some s a t u r a t i o n v a l u e and i f t h a t s a t u r a t i o n v a l u e i t s e l f depends on t h e number of s u r f a c e c o l l i s i o n s made i n t h e accomodator, t h e n i t i s u s e f u l t o open up t h e accomodator e x i t a p e r t u r e and choose magnet parameters s o a s t o o p t i m i z e f o c u s i n g of o f f - a x i s atoms. We f i n d t h a t t h e magnet gap must b e about t h r e e times t h e accomodator e x i t a p e r t u r e diameter. For a l a r g e magnet gap t h e magnetic moment of t h e (F=l,m=O) s t a t e i s f a r from s a t u r a t i o n , and t h e u s u a l methods f o r d e s i g n i n g s t a t e - s e l e c t i n g magnets7 7 8break down a p p r e c i a b l y . Consequent- l y , we have r e s o r t e d t o Monte Carlo t e c h n i q u e s i n which t h e s t a r t i n g a n g l e s i n f r o n t of t h e magnet a r e randomly s e l e c t e d and t h e t r a j e c t o r i e s a r e c a l c u l a t e d numerically.
We f i n d t h a t t h e s o l i d a n g l e f o r focusing 7 K atoms from a s m a l l a p e r t u r e through a c i r c u l a r h o l e 20 cm downstream having a diameter of t h e o r d e r of t h e 1 . 3
JOURNAL DE PHYSIQUE
cm magnet gap i s a b o u t 1 5 t i m e s t h e s o l i d a n g l e f o r f o c u s i n g 300 K atoms through a h o l e 20 cm downstream h a v i n g a d i a m e t e r of t h e o r d e r of t h e p r o p o r t i o n a t e l y s m a l l e r magnet gap. I f t h e s a t u r a t i o n d e n s i t y i s of t h e o r d e r of h a l f t h e s a t u r a t i o n den- s i t y f o r room t e m p e r a t u r e s o u r c e s , b u t t h e u s a b l e s o u r c e a p e r t u r e d i a m e t e r i s f o u r t i m e s a s l a r g e w h i l e t h e mean s p e e d is 6.5 t i m e s l e s s , t h e e x p e c t e d g a i n of H+ beam i n t e n s i t y a t 7 K compared t o 300 K i s a b o u t 20. T h i s improvement w i l l b e r e a l i z e d i n p r a c t i c e o n l y i f t h e r e i s e f f i c i e n t pumping of unwanted H and H2 away from t h e magnet e n t r a n c e . The e x p e c t e d g a i n of beam d e n s i t y i s o n l y of o r d e r 7.5. Higher beam d e n s i t y i n n a r r o w e r beams c o u l d b e a c h i e v e d u s i n g s h o r t e r magnets w i t h s m a l l e r gaps a t some c o s t of t o t a l beam i n t e n s i t y .
Z x p e r i m e n t a l P l a n s . - P r a c t i c a l r e a l i z a t i o n of improved H beam i n t e n s i t y a t low t e m p e r a t u r e s h a s b e e n d e l a y e d by l e a k s t o t h e l i q u i d h e l i u m b a t h t h r o u g h t h e f a r t o o many indium s e a l e d j o i n t s i n t h e p r e s e n t a p p a r a t u s . We h a v e been a b l e t o v e r i f y t h a t t h e i n t e n s i t y i s improved a t c o n s t a n t i n p u t from t h e d i s s o c i a t o r by h e a t i n g t h e accomodator t o 6 t o 8 K. A f t e r r e b u i l d i n g t h e vacuum e n v e l o p e of t h e a p p a r a t u s shown i n F i g . 4 , we p l a n t o t r y a few d i f f e r e n t accomodator and magnet g e o m e t r i e s , i n o r d e r t o f i n d o u t what t h e l i m i t i n g f a c t o r s are and how b e s t t o circumvent them. C o n c u r r e n t l y , we p l a n t o u s e t h e beam t o i n v e s t i g a t e t h e u s e of f r o z e n neon as a hydrogen s t a n d a r d s t o r a g e s u r f a c e . Our own e x p e r i m e n t s 2 have i n d i - c a t e d o n l y t h a t a neon c o a t e d p y r e x t u b e seems t o d e l i v e r more H f l u x t o t h e F i g . 1 s t o r a g e b o t t l e t h a n H2 a t a b o u t 4.2 K. Foner e t . a l . a t t r i b u t e d 9 t h e i r i n a b i l i t y t o s t a b i l i z e H i n a neon m a t r i x t o i n e f f i c i e n t c o n d e n s a t i o n of H on a neon s u r f a c e . At t e m p e r a t u r e s o f t h e o r d e r of 1 0 K t h e Hz v a p o r p r e s s u r e s h o u l d b e h i g h enough t o remove r e c o m b i n a t i o n p r o d u c t s , w h i l e t h e v a p o r p r e s s u r e of neon s h o u l d b e low enough t o a v o i d t h e d i f f i c u l t i e s a s s o c i a t e d w i t h h i g h s u b s t r a t e v a p o r p r e s s u r e d i s c u s s e d i n t h e p r e v i o u s p a p e r a t t h i s m e e t i n g .
Acknowledgements.- We t h a n k P e t e r Kramer f o r h e l p i n t h e e a r l y s t a g e s of t h e magnet c a l c u l a t i o n s and t h e W i l l i a m s C o l l e g e Computer C e n t e r f o r generous a l l o c a t i o n s of computer time. T h i s r e s e a r c h was s u p p o r t e d by t h e O f f i c e of
Naval Research u n d e r c o n t r a c t #N00014-80-C-0240, by t h e NSF under g r a n t PHY79 10967 and by t h e Jet P r o p u l s i o n L a b o r a t o r y under c o n t r a c t 6955441.
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