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 TEMPERATURE HYDROGEN MASER. Journal de Physique Colloques, 1981, 42 (C8),
pp.C8-181-C8-184. <10.1051/jphyscol:1981820>. <jpa-00221715>
HAL Id: jpa-00221715
https://hal.archives-ouvertes.fr/jpa-00221715
Submitted on 1 Jan 1981
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JOURNAL DE PHYSIQUE
Colloque C8, s u p p l h e n t au n012, Tome 42, de'cembre 1981
PROGRESS OF
THE
STATE-SELECTED
page C8-18 1
BEAM LOW TEMPERATURE
HYDROGEN
MASER
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Introduction.- Recently published1,2 and as yet unpublished studies by our laboratory of hydrogen atom (H) adsorption on polycrystalline molecular hydrogen (Hz)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. Attainable 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 effectively eliminated by cryopumping. Such beams
should improve the signal-to-noise of low temperature 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 diameter 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 nitrogen dewar is immersed in a liquid helium bath whose
temperature is controlled by regulating the helium
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 microwave radiation near the 1420 MHz H ground state
hyperfine transition frequency induces the atoms to
radiate a signal that decays in times of the order
of milliseconds and whose frequency is shifted from
the free space hyperfine frequency by amounts of
the order of hundreds of Hz.
From the data we are able to extract the mean
phase shift @ of the hyperfine frequency radiation
phase per trip across the storage bottle and the
mean probability a that an atom is adsorbed at
least once while rattling around on the surface
after a trip across the storage bottle. Figure 2
E
Fig. 1 : Schematic of the
Adsorption Study Apparatus.
(A) Hz inlet; (B) stainless
steel liquid nitrogen dewar;
(C) dissociator rf coil; (D)
orifice; (E) quartz storage
bottle; (F) microwave cavity;
(G) coupling loop; (H) quartz
cavity tuning rod; (I) cylindrical capacitor; (J) temperature sensors.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981820
JOURNAL DE PHYSIQUE
0.6-
RUN 17
mun 19
it
0.5-
-
0.4 -
B
0.3-
t
Fig. 3 : The parameter f3 = a-'-1
plotted against temperature with
la error bars. Results are presented for two independently
prepared surfaces.
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 independently prepared H2 surfaces.
Liquid Nitrogen
"Source Dewar"
Indium Seal
Accomodator
Hexapole Magnet
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 lowest 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 proportional to atom density nH. The proportionality is approximately2 y=4x10-l6 n ~ '
y is proportional to two factors of <ta>,
so that we expect its temperature dependence to be dominated by an exponential
with exponent =80/T. Extrapolating to
T=6 K predicts y of order 1.5x10-" nH.
Storage Bottle
Figure 4 illustrates the
present configuration of our stateselected 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-
H Beam Design.-
Fig. 4 : State-selected H+ Beam
.
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 hyperfine 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 frozen H2.
They a r e d e t e c t e d
by pulsing 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 previous 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 flange. A h e a t e r wound on t h e o u t s i d e of t h e accomodator allows
i t s temperature t o be a d j u s t e d upwards. The hexapole magnet i s 10 cm long and has
a 1.3 cm diameter gap between pole 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 focus (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 possible.
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
If the density
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".
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 be about 4 x 1 0 ~sec-l.
However, measurements of H2 s u r f a c e heati n g by H recombination i n t h e s t o r a g e b o t t l e during t h e F i
1 experiment i n d i c a t e d
~
were d e l i v 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
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 tube i n which atoms made 400 surf 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
In t h e i n l e t tube
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.
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 higher 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 . Evidently, t h e elevated H2 s u r f a c e tempe 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 than would have
been expected i f t h e i n l e t tube 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 than simply t h e decrease of y
come i n t o play. 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 frozen 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 decreases 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 charact 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 near 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, but 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 kept 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 be 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 it 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
source has 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 ,
so t h a t i t should be p o s s i b l e t o open up t h e source 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 sources. I n a d d i t i o n , almost any impurity emerging from t h e
source w i l l be frozen o u t b e f o r e reaching 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 standard work.
I f t h e accomodator e x i t a p e r t u r e were very small
Anticipated Magnet Performance.compared t o t h e magnet gap and t h e magnet gap were i t s e l f small 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 focusing
However, i f some combination of t h e e f f e c t s discussed above l i m i t s
300 K atoms.
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, then 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 so
a s t o optimize focusing of o f f - a x i s atoms. We f i n d t h a t t h e magnet gap must be
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 designing s t a t e - s e l e c t i n g magnets7 7 8break down appreciably. Consequentl y , we have r e s o r t e d t o Monte Carlo techniques i n which t h e s t a r t i n g angles 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 angle f o r focusing 7 K atoms from a small 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 order of t h e 1.3
JOURNAL DE PHYSIQUE
cm magnet gap i s about 15 times 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 having 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 dens 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 speed is 6.5 t i m e s l e s s , t h e expected 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 about 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 only 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
Higher
magnet e n t r a n c e . The expected g a i n of beam d e n s i t y i s only of o r d e r 7.5.
beam d e n s i t y i n narrower beams could b e achieved 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 .
Zxperimental Plans.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 been delayed by l e a k s t o t h e l i q u i d helium b a t h through 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 have 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 envelope of t h e
a p p a r a t u s shown i n Fig. 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 experiments2 have i n d i c a t e d only t h a t a neon coated pyrex 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 Fig. 1
s t o r a g e b o t t l e t h a n H2 a t about 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 of t h e o r d e r of 10 K t h e Hz vapor p r e s s u r e should b e h i g h enough
t o remove recombination p r o d u c t s , w h i l e t h e vapor p r e s s u r e of neon should b e low
enough t o avoid 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 vapor 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 paper a t t h i s meeting.
Acknowledgements.We thank 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 Williams C o l l e g e Computer Center 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 supported by t h e O f f i c e of
Naval Research under 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.
CRAMPT TON,
S. B.,
and WEINRXB, A.,
GREYTAK, T. J . , KLEPPNER, D., PHILLIPS,
Phys. Rev. L e t t . 2 (1979) 1039.
2 ~ R A M P ~S.~ B.,
~ , J. Physique Colloq.
N~. ,
~ W.
3
41 (1980)
IJ. D . ,
SMITH, D. A.,
c7-249.
BERLINSKY,
~
~ A. J~. , and, WHITEHEAD, L. A,, Phys. Rev. L e t t .
2 (1979)
1042.
JOCHEMSEN, R., BERLINSKY, A. J . , and HARDY, W. N . ,
195 and 7 (1981) 455(E).
4 ~ ~ R R O WM.,
,
46(1981)
5
~ R.,
MORRO
~W, M . ,
BERLINSKY,
~
A.~ J . , and HARDY,
~
W. N.,
~
Phys. Rev. L e t t .
Phys. Rev.
~
Lett. ~
~
47 (1981) 852.
6
7
~ A.
C.
and
~
SMITH,
~ F. J . ,~ Atomic ~Data
~ R. L. and HAMILTON,
~
D. ~
R.,
3
(1971)
~
3 1~7 .
,
The Review
~ of S c i e n t ~
i f i c I n s t r u m e~n t s
30 (1959) 356.
*AUDOIN, C., DESAINTFUSCIEN, M. and S C H E W N , J. P . , Nuclear I n s t r u m e n t s
and Methods 3 (1969) 1 .
9
~ S. N .~, COCHRAN,
~
E.
~ L., ~BOWERS,, V. A. and JEN, C . K., J o u r n a l of
Chemical P h y s i c s 2 (1960) 963.
~