NEW METHOD FOR THE SEARCH FOR THE NEUTRON ELECTRIC DIPOLE MOMENT SUITABLE FOR SMALL REACTORS

(1)

HAL Id: jpa-00224061

https://hal.archives-ouvertes.fr/jpa-00224061

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

NEW METHOD FOR THE SEARCH FOR THE NEUTRON ELECTRIC DIPOLE MOMENT

SUITABLE FOR SMALL REACTORS

R. Golub

To cite this version:

R. Golub. NEW METHOD FOR THE SEARCH FOR THE NEUTRON ELECTRIC DIPOLE MO-

MENT SUITABLE FOR SMALL REACTORS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-

265-C3-267. �10.1051/jphyscol:1984344�. �jpa-00224061�

(2)

JOURNAL DE PHYSIQUE

Colloque C3, supplbment au n03, Tome

4 5 ,

mars 1984 page

C 3 - 2 6 5

NEW METHOD FOR THE SEARCH FOR THE NEUTRON ELECTRIC DIPOLE MOMENT SUITABLE FOR SMALL REACTORS

R.

Golub

Physik Department E

21,

Technische U n i v e r s i t a t Miinchen,

8046

G a r c h i n g , F.R .G.

RQsumB

-

Une s o u r c e "superthermique" de n e u t r o n s u l t r a f r o i d s contenant une s o l u t i o n d i l u e e de fIe3 p o l a r i s d dans ~e~ p e u t s e r v i r en mdme temps c o m e une s o u r c e , un p o l a r i s e u r , une c h m b r e de resonance magnetique e t un ana- l y s e u r de p o l a r i s a t i o n . On p e u t l ' u t i l i s e r pour r e c h e r c h e r l e moment Blec- t r i q u e d i p o l a i r e du n e u t r o n (edm)

.

A b s t r a c t

-

A superthermal source of Ultra-Cold Neutrons (UCN) c o n t a i n i n g a d i l u t e s o l u t i o n of p o l a r i z e d Re3 i n a e 4 can s e r v e a s a UCN s o u r c e , p o l a r i z e r , magnetic resonance chamber and p o l a r i z a t i o n a n a l y z e r . We d i s c u s s t h e proper- t i e s of such an arrangement when used t o s e a r c h f o r t h e neutron e l e c t r i c d i p o l e moment (edm).

With t h e r e c e n t demonstration

(1)

t h a t t h e s t o r a g e time of UCN i n l i q u i d Helium i s i n reasonable agreement w i t h p r e d i c t i o n s and t h a t t h e "superthermal" UCN source

i s

f e a s i b l e ,

it

now seems a p p r o p r i a t e t o c o n s i d e r new methods of applying t h e extremely high UCN d e n s i t i e s

( 2 )

made p o s s i b l e by t h i s source.

I n t h e "superthermal" UCN source n e u t r o n s w i t h wavelength around

A

-10 a r e s c a t - t e r e d by l i q u i d Eie4 i n t o t h e UCN energy r e g i o n with t h e emission of a phonon. I f t h e l i q u i d Helium

i s

c o n t a i n e d i n s i d e a v e s s e l s u i t a b l e f o r s t o r i n g UCN t h e neutron den- s i t y

w i l l

buil/d up t o a v a l u e (pU N) determined by t h e p r o d u c t i o n r a t e Q =

+

(C

i s

t h e macroscopic c r o s s s e c t i o n For UCN p r o d u c t i o n and $I

i s

t h e i n c i d e n t f l u g ) an8 t h e s t o r a g e time, T , of UCN i n t h e v e s s e l r e s u l t i n g from t h e e f f e c t s of w a l l l o s s e s ,

6

decay, ~ e " ~ s c a t t e r i n g and a b s o r p t i o n by He3.

The UCN s o c o l l e c t e d , can be t r a n s p o r t e d t o a n o t h e r v e s s e l f o r use i n experiments.

For t h e s e a r c h f o r t h e neutron edm by t h e "conventional" method t h e n e u t r o n s must p a s s through a p o l a r i z e r b e f o r e e n t e r i n g t h e experimental chamber where t h e y a r e s u b j e c t e d t o e l e c t r i c and magnetic f i e l d s a r r a n g e d s o a s t o a l l o w t h e d e t e c t i o n o f t h e edm ( 3 ) .

The p r e s e n t super-thermal source

i s

mounted on t h e end of a c o l d neutron quide ex- t e r n a l t o t h e ILL r e a c t o r . However p r e v i o u s work ( 2 ) h a s i n d i c a t e d t h a t

it

should be p o s s i b l e t o o p e r a t e such

a

source i n s i d e a neutron moderator where t h e e f f e c t i v e f l u x averaged over t h e Helium v e s s e l

i s

s e v e r a l t i m e s g r e a t e r t h a n 10'' cm-' s e c - l . This can produce UCN d e n s i t i e s on t h e o r d e r of

10'

U C N / C ~ ~ . A "conventional" way of u s i n g t h e s e UCN would be t o t r a n s p o r t them t o an a p p a r a t u s s i m i l a r t o t h e p r e s e n t one ( 3 ) l o c a t e d o u t s i d e t h e b i o l o g i c a l s h i e l d i n g , through a p o l a r i z e r . However,

it

h a s been suggested

( 4 )

t h a t t h e e n t i r e edm experiment can be performed " i n - s i t u " i n t h e Helium v e s s e l l o c a t e d i n s i d e t h e moderator.

P o l a r i z e d ~ e " s e l e c t i v e l y absorbs neutrons of o n l y one s p i n ( d i r e c t e d o p p o s i t e t o t h e He3 s p i n ) and h a s long been c o n s i d e r e d a s a p o s s i b l e neutron p o l a r i z e r

( 5 ) .

A

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984344

(3)

C3-266 JOURNAL DE PHYSIQUE

superthermal UCN source c o n t a i n i n g p o l a r i z e d He3

w i l l

have d i f f e r e n t s t o r a g e times

r + , r -

( T + / T - = y-/y = Ic>l) f o r t h e two neutron p o l a r i z a t i o n s and hence t h e two p o l a r - i s a t i o n s

w i l l

b u i l d

zp

t o d i f f e r e n t d e n s i t i e s according t o equ.

1.

I f d u r i n g t h e UCN b u i l d up t i m e , a d-c magnetic f i e l d , B o , p a r a l l e l t o t h e ~e~ p o l a r i z a t i o n

i s

a p p l i e d t o g e t h e r with a p e r p e n d i c u l a r r o t a t i n g magnetic f i e l d , B , ( f r e q u e n c y

w )

t h e t o t a l number of UCN i n t h e chamber

w i l l

s e r v e a s an i n d i c a t o r l o f t h e neutron magnetic r e s - onance and one can s e a r c h f o r t h e edm by l o o k i n g f o r t h e e f f e c t of an e l e c t r i c f i e l d on t h e magnetic resonance p r o b a b i l i t y .

hXile t h e a b s o r p t i o n r a t e of UCN by t h e p o l a r i z e d He3

i s

a d i r e c t measure of t h e p o p u l a t i o n o f UCN w i t h a n t i - p a r a l l e l s p i n s ( - s t a t e ) and can b e observed by t h e s c i n - t i l l a t i o n s i n t h e l i q u i d Helium following a neutron a b s o r p t i o n ,

it

seems more prac- t i c a l t o p e r i o d i c a l l y measure t h e t o t a l number of UCN p r e s e n t by i n s e r t i n g a d e t e c - t o r i n t o t h e Helium f i l l e d chamber while s h u t t i n g o f f t h e i n c i d e n t neutron f l u x . The t o t a l number of UCN p r e s e n t can be c a l c u l a t e d from t h e Schroedinger e q u a t i o n and i s shown v s . U =

(w-w

⁾/y+ I n f i g . 1 f o r K = 4.24 and 52 = 2.12

( 5 i s

t h e s t r e n g t h of t h e r o t a t i n g fieldOmeasured i n u n i t s of y )

,

t h e parameter v a l u e s which g i v e t h e maximum s l o p e of s i g n a l s t r e n g t h v s . a p p l l e d frequency i . e . t h e maximum s e n s i t i v i t y t f o r t h e edm s e a r c h ( 4 ) . Also shown

i s

t h e comparable curve f o r t h e "conventional"

method where t h e r o t a t i n g f i e l d

i s

a p p l i e d f o r s h o r t p e r i o d s a t t h e beginning and end of the measurement time. The c u r v e s a r e f o r s o u r c e s l o c a t e d a t t h e same p o s i t i o n allowing a f a c t o r of 3 f o r volume d i l u t i o n i n t h e c o n v e n t i o n a l method and assuming

100% ~e~ p o l a r i z a t i o n , and t h e same UCN s t o r a g e time i n both v e s s e l s which a r e t a k e n t o be e q u a l i n volume. It i s seen t h a t t h e maximum s l o p e ( p o s i t i o n shown by arrows)

i s

s l i g h t l y l a r g e r f o r t h e c o n v e n t i o n a l method b u t t h i s comparison does n o t allow f o r t r a n s m i s s i o n l o s s e s through t h e n e c e s s a r y UCM g u i d e , p o l a r i z e r s and windows which

w i l l

be n e c e s s a r y i n t h e c o n v e n t i o n a l system. The number of UCN remaining i n t h e v e s s e l

i s

r e l a t e d t o p ( a ) ~ / 2 y

+'

CONVENTIONAL

2 4 6 8 1 0 a

^m

Fig. 1- S i g n a l s t r e n q t h vs. a p p l i e d frequency f o r t h e optimized " i n - s i t u " and

"conventional" methods. Arrows i n d i c a t e p o s i t i o n s of maximum s l o p e .

(4)

He3 p o l a r i z a t i o n s on t h e o r d e r of 70-80% have a l r e a d y been o b t a i n e d by l a s e r - o p t i c a l pumping and i n s o l i d ~e~ and

it

seems t h a t

s t i l l

h i g h e r p o l a r i z a t i o n s can be o b t a i n e d by cryogenic t e c h n i q u e s (6). A ~e~ p o l a r i z a t i o n of 90%

w i l l

r e s u l t i n an i n c r e a s e o f

Y+

by 25% over t h a t due t o t h e w a l l l o s s e s a l o n e f o r K = 4.24.

A s shown p r e v i o u s l y

( 2 )

t h e a p p a r a t u s

w i l l

have t o be c a r e f u l l y s h i e l d e d by -30 cm o f cooled Bismuth t o reduce t h e h e a t i n p u t t o a c c e p t a b l e l e v e l s

i n

t h e c a s e o f a h i g h - f l u x source. Under t h e s e c o n d i t i o n s i f we assume t h e s t r u c t u r e surroundinq t h e l i q u i d Helium c o n s i s t s of 4 mm o f Beryllium ( o r Beryllium Oxide) t h e

6

decay r a t e a f t e r 1 y e a r of exposure t o t h e neutron f l u x

w i l l

be about

lo-'

of t h e UCN counting r a t e f o r a 20 l i t r e c y l i n d r i c a l Helium v e s s e l and 10 second counting time. I n addi- t i o n i f t h e d e t e c t o r

i s

i n s e r t e d near t h e c e n t e r of t h e Helium v e s s e l t h e s e

6

r a y s

(which have a range o f a few m i l l i m e t e r s i n l i q u i d Helium) w i l l n o t even r e a c h t h e d e t e c t o r . A much h i g h e r decay r a t e would

be

caused by a c t i v a t i o n of t h e d e t e c t o r

it-

s e l f . T h i s can be d e a l t w i t h by keeping t h e d e t e c t o r i n a s h i e l d e d a r e a while t h e neutron f l u x

i s

a p p l i e d .

A t t h e low temperatures under c o n s i d e r a t i o n h e r e

it

seems u n l i k e l y t h a t

a

convention- a l UCN d e t e c t o r can be used. However bolometers based on b o t h superconducting g r a i n s

( 7 )

and S i l i c o n r e s i s t o r s (8) a r e s e n s i t i v e enough t o d e t e c t n e u t r o n s when covered

w i t h

a Boron o r Uranium containirxg l a y e r . I n t h e c a s e of a B" l a y e r on a S i r e s i s t o r 3x4x.3mm3 a s i n g l e n e u t r o n would produce a p u l s e of about 10 F v o l t s which

i s

about t e n times h i g h e r t h a n t h e thermal n o i s e . I n t h e c a s e of t h e r a t h e r high c o u n t i n g r a t e s t o be expected i n a 20 l i t r e v e s s e l c o n t a i n i n g around

lo3

UCN/cmbith a 10 s e c emptying time

-

2 x 1 0 ~ counts/sec

-

t h e n o i s e

w i l l

be determined by t h e s t a t i s t i c a l f l u c t u a t i o n s i n t h e neutron c o u n t s r a t h e r t h a n by t h e t h e r m a l n o i s e Ln t h e r e s i s t o r even i n t h e c a s e of analogue measurements.

I n

this

p r e l i m i n a r y f e a s i b i l i t y s t u d y we have d e s c r i b e d a method f o r s e a r c h i n g f o r t h e neutron edm which r e p r e s e n t s t h e most s e n s i t i v e technique t h a t one can p r e s e n t l y imagine while o f f e r i n g t h e p o s s i b i l i t y of c a r r y i n g o u t su& experiments a t r e l a - t i v e l y weak neutron sources. I n a d d i t i o n t o t h e problems d i s c u s s e d h e r e t h e r e a r e o t h e r s t h a t

w i l l

have t o be addressed (e.g. t h e e f f e c t s of t h e r a d i a t i o n on t h e i n s u l a t i n g a b i l i t y o f t h e l i q u i d Helium) and s e v e r a l t e c h n i c a l developments

w i l l

be n e c e s s a r y b e f o r e t h e " i n - s i t u " t e c h n i q u e f o r s e a r c h i n g f o r t h e neutron edm can be p u t i n t o p r a c t i c e .

REFERENCES

GOLUB R . , JEWELL C., AGERON P., MAMF'E

W.,

HECKEL B . , KILVINTON I.

Z . ~ h y s .

B

(1983) 187

GOLUB R., B~NING K . , WEBER H. " R e a l i s i e r u n g s s t u d i e zur S p a l l a t i o n s Neutronenquelle, T e i l 111, Ann. C , ed. G.S. Bauer e t

a1 K !

( J i i l i c h ) , KFZ (Karlsruhe) June 1981 PENDLEBURY J . M . , SMITH K.F., GOLUB R., BYRNE J . , MCCOMB T.J.L., S U m R T . , BURNETT S . M . , TAYLOR A.R., HECKEL B., RAMSEY N.F., GREEN K . , MORSE J . , KILVINGTON A . I . , BAKER C.A., CLARKE S.A., MAMPE W . , AGERON P. ( t h i s conference)

GOLUB R., J . d e Physique-Lettres

2

(1983) L-321

JENKIN J.T., "The F u t u r e o f p o l a r i z i n g f i l t e r s a t t h e I.L.L., (unpublished) and P r i v a t e Communication

FROSSATI G . , J . Physique CollOq.

41

(1980) (27-95 See a d d i t i o n a l r e f e r e n c e s i n r e f e r e n c e 4 ) .

DRUKIER A.K., IGALSON J., SNIADAUER L. Nuc. I n s t . Meth.

15$

(1978)91

FROSSATI G .

,

IMBERT D.

,

PECCOUD L.

,

THOULOUZE D.

,

WAKSMAN B. PrOC

.

¹ ⁴^{I n t}^~

.

^~Conf.

on Low Temp. Physics Otaenimi, Finland (North Iiolland) Aug. 1975 p. 80