NEW APPLICATIONS OF ULTRA COLD NEUTRON PHYSICS IN MAGNETIC AND GRAVITY FIELDS

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NEW APPLICATIONS OF ULTRA COLD NEUTRON PHYSICS IN MAGNETIC AND GRAVITY FIELDS

M. Utsuro

To cite this version:

M. Utsuro. NEW APPLICATIONS OF ULTRA COLD NEUTRON PHYSICS IN MAGNETIC AND GRAVITY FIELDS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-269-C3-277.

�10.1051/jphyscol:1984345�. �jpa-00224062�

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Colloque C3, supplement a u n03, Tome 45, m a r s 1984 page C3-269

NEW APPLICATIONS O F ULTRA COLD NEUTRON PHYSICS IN MAGNETIC AND GRAVITY FIELDS

M. Utsuro

R e s e a r c h R e a c t o r I n s t i t u t e , K y o t o U n i v e r s i t y , Kumatori-cho, Sennan-gun, Osaka, 590-04, Japan

R6sum6 - Deux,propositions concernant des expsriences avec neutrons ultra froids (UCN) sont dgcrites. La premiere pro- position est celle d'un ralentisseur magn6tique neutrons fonctionnant sur le principe de changements de spin adiabatiques et successifs dans des champs magn6tiques forts qui produiront une haute densit6 de neutrons ultra froids polaris6s. La seconde proposition est celle d'un spectrometre bas6 sur la focalisation par le champ de gravitd offrant la possibilite d'obtenir l'angle de diffusion aussi bien qu'une trSs haute rgsolution en 6nergie des neutrons.

Abstract - Two proposals are described concerning the ultra cold neutron (UCN) experiments. The first is a magnetic neutron dece- lerator working with the principle of successive adiabatic spin flips of neutrons in strong magnetic fields which will produce high density polarized UCN through the multiple decelerations of cold or very cold neutrons. Another one is a fall focussing gra- vity spectrometer with a capability of scattering angle analysis as well as the very high energy resolution of UCN.

I. Introduction

Ultra cold neutrons (UCN) with the energy below about 1 VeV can be used in various fundamental physics experiments Such as the EDM observations, the neutron lifetime measure- ments and so on. Furthermore, the use of UCN provides the possibility of very high resolution of neutron spectro- scopy due to their low energy and the resulting intensity gain factor 1)

.

In the present paper, two possible applications of UCN physics which should serve to improve these intensity Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984345

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situations will be described. The first is the proposal of a new principle of neutron magnetic deceleration by using adiabatic spin flips in strong magnetic fields, which should open the possibility of successive slowing down of neutrons from a wide energy region into a narrow energy region of UCN. The second part concerns a new version of the gravity spectrometer 'I-i .e. the method of a fall focussing of UCN which improves the geometrical efficiency without noticeable deteriorations of the high energyresolution 3 , 4 )

11. Neutron magnetic decelerator 2-1. Principle

A neutron entering a magnetic field experiences positive or negative potential according to the polarization with the parallel or anti-parallel spin, respectively,-to the direction of the magnetic field. Therefore, if the neutron spin can be converted from the initial state to the other in the midst of the magnetic field, then neutron decelera- tion or acceleration will occur when the neutron passed through the field. Recently, Rauch et al. developed such kind of a experiment showing the neutron energy shift due to the spin flip in a magnetic field by using the magnetic resonance method5! The resonance method works well only for neutrons with a definite velocity which spend a certain time for the spin turn in the magnetic resonance coil, and thus the Rauch's method is exactly available for monochromatized neutrons.

In the present principle described below, the adiabatic method of spin flip617) takes the place of the resonance method for the purpose of the efficient spin flips of neutrons

in a wide energy region. A strong magnetic field with

a s l i g h t g r a d i e n t i n t h e magnitude i s a p p l i e d t o t h e i n s i d e of t h e n e u t r o n g u i d e t u b e , a s shown i n F i g . 1 . A h i g h

f r e q u e n c y c o i l i s a l s o p r e p a r e d a r o u n d t h e g u i d e t u b e . T h e a n g u l a r f r e q u e n c y w of t h e c o i l should s a t i s f y t h e r e l a t i o n

- ¹

where y = 29/5 = 18210 s-' G i s t h e gyromagnetic r a t i o w i t h p t h e magnetic m o m e n t o f a n e u t r o n , and H c t h e m a g n e t i c f i e l d a t a n a r b i t r a r y p o i n t w i t h i n t h e r e g i o n o f t h e s l i g h t g r a d i e n t i n t h e magnitude. Then, a s p i n f l i p o c c u r s f o r t h e p a s s a g e o f n e u t r o n s i n

6 1 a w i d e v e l o c i t y r e g i o n s a t i s f y i n g t h e a d i a b a t i c c o n d i t i o n ,

y ~ a / 2 (vl d ~ c / d Z I 1 >> 1 , ( 2 1

where H a i s t h e a m p l i t u d e of t h e h i g h f r e q u e n c y f i e l d perpen- d i c u l a r t o Hc.

>

1 ,

Net

T

F i g . 1 - A s c h e m a t i c s t r u c t u r e and t h e magnetic p o t e n t i a l v a r i a t i o n i n a n e u t r o n magnetic d e c e l e r a t o r .

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As the result, the initially parallel spin neutrons are decelerated with the amount of an energy difference of 2pHc , while the initially anti-parallel ones accelerated with the same amount of the energy difference. When

the neutrons are going out of the field, there comes another point which satisfies again the relation (11, but making the field gradient at that point much steeper than the main point with the slight gradient, the occurrence of the double flips can be avoided for the neutrons within the velocity region of our interest. The same result will, of course, come out in a reversed arrangements of both the field gradient and the coil.

After passing through the deceleration (or acceleration) stage in the main magnet, neutrons should enter the auxiliary stage consisting of a much weaker magnet and a coil for a lower frequency in order to recover the initial spin state for the realization of successive decelerations (or accelera- tions). Due to a much smaller acceleration(ar deceleration) effect in the auxiliary stage, we can obtain the net effect of the deceleration (or acceleration) per the unit process through the coupled stages, and we can further repeat the same processes until theneutrons arrive at the desired energy region.

2-2. UCN production

It will be possible to apply the present principle for the production of polarized U C N from initially cold or very cold neutrons through the multiple decelerations in a device with a number of units mentioned above, or by the cyclic passages of neutrons in a looped guide tube with a single or a few deceleration units. In the limit of

t h e slowingdown, theneutronvelocityshouldapproachto t h e v a l u e of a l i m i t v e l o c i t y which s a t i s f i e s t h e a d i a b a t i c c o n d i t i o n i n t h e r e g i o n of t h e s t e e p e r f i e l d g r a d i e n t a l s o . Then, t h e e f f e c t i v e d e c e l e r a t i o n w i l l n o t o c c u r any more, and a s t h e r e s u l t a f i n a l d i s t r i b u t i o n of t h e n e u t r o n v e l o c i t y w i l l b e b u i l t up i n t h e d e v i c e .

I n t h e extreme c a s e t h a t t h e e n e r g y of t h e n e u t r o n s w i t h such a l i m i t v e l o c i t y i s s m a l l e r t h a n t h e maximum v a l u e of t h e magnetic p o t e n t i a l f e l t by t h e n e u t r o n s , t h e n t h e deceleratingneutronswillbe r e p e l l e d a t l a s t b e f o r e t h e a p p e a r a n c e o f t h e above-mentioned s i t u a t i o n o f t h e d o u b l e f l i p s . Anyway, a b a l a n c i n g d e n s i t y o f t h e d e c e l e r a t e d n e u t r o n s w i l l be r e a l i z e d a t t h e l o w e s t e n e r g y r e g i o n i n a n a c t u a l c o n d i t i o n of a f i n i t e f e e d i n t e n s i t y and a f i n i t e s t o r a g e t i m e of n e u t r o n s i n t h e d e v i c e .

2-3. A p r e l i m i n a r y experiment

A p r e l i m i n a r y experiment i s now b e i n g c a r r i e d o u t by u s i n g a s i m p l e d e v i c e and t h e UCN o u t p u t produced by t h e s u p e r m i r r o r n e u t r o n t u r b i n e a t KUR 8 r 9 ) , a s shown i n F i g . 2.

I n t h e p r e s e n t e x p e r i m e n t , t h e t u r b i n e i s m o d i f i e d t o produce p u l s e d UCN o u t p u t and we observed t h e t i m e - o f - f l i g h t s p e c t r a of n e u t r o n s t r a n s m i t t e d t h r o u g h a s t r a i g h t r e c t a n g u l a r g u i d e t u b e w i t h t h e i n n e r s i z e of 5 cm x 2 cm and t h e i n s i d e c o a t e d w i t h c o p p e r .

A t y p i c a l r e s u l t o f t h e c o m p a r i s o n b e t w e e n t h e s p e c t r a f o r t h e h i g h frequencycoilcurrentalternativelychangedoff and on s o o f t e n i s s h o w n i n F i g . 3 f o r t h e e x p e r i m e n t a l c o n d i t i o n s o f t h e f i e l d H c = 2 . 1 6 kG ( i - e . v=w/2~r=6.30MHz),the f i e l d g r a d i e n t d H / d z = 5 3 . 3 G / c m r t h e h i g h f r e q u e n c y c u r r e n t I p p = 1 5 A a n d t h e e f f e c t i v e f l i g h t p a t h l e n g t h of a b o u t 2 8 cm. A s f o r t h e p o l a r i z e r and t h e a n a l y z e r , Co 92% and

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Turbine b l a d e

Polarirer

Fig.2 - Set up of a preliminary expriment for a neutron magnetic decelerator.

Fe 8% alloyevaporatedonthinaluminiumfoilswere used.

Although the statistics are not sufficiently good in the Figure, the result shows the effects of the adiabatic spin flip occurring for neutrons in the jelocity region

below about 6 m/s. The expected deceleration or acceleration effects could not be discussed definitely in the present

result because of the low countings and the moderate strength of the magnetic field. Further measurements are carried out in order to make the deceleration effects more clear, and furthermore, an advanced experimental set-up with a much stronger magnetic field of % 10 kG is also in preparation.

6 Ws 3.5 % N e u t r o n velocity

- ~ 1 Y c

9

2 'b,

S E

Simple theoretical e x p e c t a t i o r ~

I

3 ^u

-

5

E

2 100-

4 3 0 U

2 u

9 So-

Tcme of f 1 ; j h t C m s )

I I I I I I I L

30 20 .I0 8 6 5 4 35

N e u t r o n velocity ( W S )

Fig.3 - Preliminary result showing the comparison between the repeated alternative measurements of the high frequency current off and on.

111. Gravity spectrometer with fall focussing of UCN

The gravity brings significant effects on the trajectory of UCN, and therefore an ultra high resolution spectrometer can be developed for UCN with the principle utilizing the gravity effects. Actually, Steyerl's reach focussing gravity spectrometer named NESSIE~' is now working at FRM

'I,

reactor with the highest energy resolution of As = 17 neV.

Now, we are proposing another type of a gravity spectrometer working in the principle of fall focussing of UCN which provides a higher geometrical efficiency for the neutrons ' ? ⁾

With the present principle of the fall focussing, it will become possible to analyse the scattered neutron energy distributions for several scattering angles at once by

using the device as shown in Fig.4. Some numerical studies are now carried out for a typical case of the proposed structure.

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\I

Tube

Fig.4 - ^A schematic design of the fall focussing gravity spectrometer with scattering angle analyses.

The present principle of the fall focussing of UCN requires the preparation of a specially designed curved mirror system, but the preparation of such mirrors with required sizes and with the satisfactory surface accuracy was found to be possible. A test experiment by using a small part of such a mirror is starting at the site of the supermirror turbine mentioned above.

IV. Concluding remarks

Employment of the adiabatic spin flip gets rid of the problems ofthehighmagnetic fielduniformityandthe high stabilitywhichshouldbecome severe requirementsin thecase of the resonance method. Further, by using the principle of the magnetic deceleration, it can also be thought possible

. . - - - - -

to use fully the source neutrons for multiple UCN productions.

For the gravity spectrometer proposed, shielding from the possible background neutrons due to muLtiple scattering and diffuse reflections will be required to carry out an accurate

e x p e r i m e n t . A n y w a y , f o r b o t h proposals described above, e x p e r i m e n t a l v e r i f i c a t i o n s of t h e a p p l i c a b i l i t y w i l l be t h e m o s t i m p o r t a n t s u b j e c t , and t h e y are n o w under progress.

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