Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984346

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ILL FACILITIES FOR FUNDAMENTAL PHYSICS EXPERIMENTS

P. Ageron and W. Mampe

m t u t Laue-Langevin, ^156X,38042 Grenoble Cedex, France

R6sum6 - On pr6sente l e s caract6ristiques des faisceaux de neutrons thermiques,

froids,

trSs froids e t u l t r a f r o i d s d i s p n i b l e s pour des expgriences de physique fondamentale au rgacteur 5 haut flux de 1'ILL.

Abstract

-

A presentation i s given of the characteristics of thermal, cold, very cold and ultracold neutron beams available f o r fundamental physics experiments a t the ILL High Flux Reactor.

As we have seen i n the previous contributions, a l l reactor-based fundamental physics experiments search for extremely small quantities, i n many cases being zero within the experimental accuracy. To obtain m a x b s e n s i t i v i t y three conditions have t o be f u l f i l l e d by t h e neutqon source: (a) highest possible neutron beam intensity, (b) longest possible reaction times

(a) The High Flux Reactor produces with 1.2 x 10 n cm-Z s-

'

a t 15 cm distance from the core, one of the highest steady neutron fluxes obtained so f a r . One single f u e l element with 8.6 kg of 94% enriched uranium produces in an active core volume of 40 dm3 4x10" n/s a t 57 M i thermal power. The heavy water moderated reactor supplies 18 beam tubes with neutrons. A more detailed description of the reactor i t s e l f is given i n / I / and of the beam f a c i l i t i e s in /2,3/.

(b) A unique development of cold sources integrated into the heavy water moderator allows t o s h i f t the average neutron wavelength up. The existing cold source i s an aluminium sphere of 38 an diameter containing 25 1 of deuterium a t 25 K. This source i s located i n front of a large beam tube housing 5 neutron guides with different radius of curvature. The unperturbed flux a t the nose of the tube i s 2.2 x lo1' n an-' s-'. The present cold source gives a gain factor of 30 a t X = 6 8 and of about 60 a t X >

10 8. The modified cold source with an incorporated cavity inside the Dp w i l l be installed a t the place of the existing one during the reactor shut-down i n 1985. I t i s expected t o feed 1.5

-

1.8 times more cold neutrons i n t o the existing guides. The heavy demand of cold neutrons f o r experiments in a l l domains w i l l hopefully be covered a f t e r the integration of a second cold source into the beam hole H5 in 1986. I t w i l l be a D2 cold source as well but with a cylinder geometry 21 an i n diameter and 21 cm in t h i c h e s s located inside a beam tube of 23 CIJ diameter. The unperturbed flux a t the nose of t h i s tube i s 7 x 10" n an s ^l. I t w i l l be equipped with three guides which w i l l feed eight new instruments located i n a new neutron guide h a l l . The two main guides w i l l have a cross section of 6 x 12 an2 and 4 x 12 a n 2 and r a d i i of curvature of 5000 m and 3000 m, respectively.

A t h i r d guide of 1.5 x 12 an2 w i l l provide neutrons f o r special beam experiments.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984346

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C3-280 JOURNAL DE PHYSIQUE

I t i s further mentioned t h a t cold neutrons can e a s i l y be polarized, a feature needed f o r a wide c l a s s of experiments studying the decay pro- p e r t i e s of the neutron o r i t s interaction with external f i e l d s . The low background

of gamma radiation and f a s t neutrons both highly abun- dant i n the reactor core region i s achieved:

-

p a r t l y by t h e tangential arrangement (shown i n fig.1) of the beam tubes with respect t o the fuel element leading t o a sup- pression factor of approx. 10.

- and t o a higher extent by the use of curved neutron guides. Neutron guides transport neutrons by t o t a l r e f l e c t i o n on t h e i r nickel coated glass

surfaces with ?ow

losses f a r from FIG.1 The beam tube arrangement a t the ILL HFR t h e reactor core.

The curvature R of a guide with a width a excludes a f t e r a guide length 1 = m a the d i r e c t view of t h e gamma ray and f a s t neutron source and intro- duces a cut-off angle y =

m~

which s h i f t s the maximum of the wavelength distribution t o longer wavelengths. The guides on t h e existing cold source have a width of 3.m and a height of 20 cm which i s usually subdivised into 3 sections of 5 m. The length 1 varies between 10 and 120 m and the radius of curvature R between 25 m and 27000 m.

Table 1 shows the c h a r a c t e r i s t i c s of some neutron beams used f o r special beam experiments in contrast t o experiments on routinely scheduled instru- ment s

.

HI 7 HI 8 H2 2 IH1 HI 42 S3 554

:: monochromatic beams, TABLE 1: Special beam f a c i l i t i e s given i s the number

of neutrons per

or?2

and sec.

section (m21

3x5 3x20 3 pos.

@ 10 3x5 2x2 1x1 guide

guide guide beam hole

guide guide guide

cold cold them.

cold cold cold cold

maximum at

('1

9 20

1.5 3 7 8 5

@c (n/m2s)

4.5x109 4.5x108 lo9 10l0 5x109 5x106 ::

3x106 ::

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given by the entrance windows of the beam tube and f o r short wavelength by the curvature of the following guide. For some experiments very monochromatic beams are obtained by putting a single c r y s t a l into a normal beam which r e f l e c t s out neutrons of one energy.

Polarized beams :

Actually there e x i s t s one polarized beam (SN7) fed by the existing cold source.

A second one w i l l be i n s t a l l e d a t the new cold source with a larger cross section and a higher flux.

(a)

E :

The cold guide with a beam s i z e of 3 x 5 an2 transports over a length of 120 m t h e neutrons t o the f a r end of the existing neutron guide h a l l . The flux i s peaked a t about 5-!.

Tp

capture f l u x a t the guide e x i t has a density of (4-5) x lo9 n an s

.

The modified cold syurce w i l l hope- f u l l y bring t h i s flux up t o a value of 8 x 10' n an s

.

The actual experimental area i s situated outside the guide h a l l allowing the i n s t a l - l a t i o n of dangerous target material l i k e liquid-hydrogen. The capture flux a t t h i s position i s actually 1.4 x lo9 n cm s and w i l l be increased in future by the factor 1.5

-

1.8.

The beam is equipped with a spin flipper and a curved soller-type super- mirror polarizer developped a t the ILL by 0. Scharpf. This polarizer with

size 3 x 5 cm2 has a transmission of 52% and a poiariqation product of 94.5%. A polarized flux density of 4.8 x 10' n an s

'

peaked a t ^'1.5.5 A i s obtained. This flux w i l l be increased by the modified cold source.

(b) The new polarized beam on the second cold source: This f a c i l i t y ^{w i l l}be available i n 1986 together with the i n s t a l l a t i o n of the second cold source.

The experimental area outside the second guide h a l l w i l l be connected with the cold source by a 6 x 12 an2 guide, 80 m long and with a radius of curvature of 5000 m which leads t o a cut-off a t h = 3

a.

²⁰^m^up-stream

from the experimental area the guide i s widened t o 6 x 18 m2 due t o the i n s t a l l a t i o n of a backscattering spectrometer. This leads t o a flux dilution factor which varies over the height of the guide and with wavelength between 0.56 and 1. The calculated capture flux i s , before dilution, 14 x 1 09, n q s which has t o be compared with the capture flux of 5 x l o 9 n c m s-' of the present SN7 beam, whereas the t o t a l neutron flow of the new 6 x 12 cm2 beam w i l l be 2.4 x 1011 n s-I compared t o 2.2 x 10" n s a t the present 3 x 5 an2 beam.

Ultra cold neutron (UCN) f a c i l i t i e s : There are three UCN f a c i l i t i e s of different type a t the ILL, one of which being only in the project phase: (a) PN5, (bj superthennal He source, (c) v e r t i c a l guide f o r very cold neutrons combined with a Steyerl turbine.

(a) PN5 i s in operation since 1977 and served as a t e s t f a c i l i t y f o r the development of the related techno- logies as well a s f o r f u l l scale experiments l i k e the EDM experiment and neutron optics experiments pre- sented a t t h i s workshop. PN5 (fig.2) uses an in-pile converter (H20) a t room temperature s i t t i n

thermal flux of 6

x

10"

?z-2

^{s - l .}

This converter is separated from a 35' inclined electropolished stain-

FIG.2 UCN Source PN5

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C3-282 JOURNAL DE PHYSIQUE

l e s s s t e e l guide of 68 mm i.d. by a 0.6 mm zircaloy window. Outside the reactor walls follows a 7 x 7 an2 nickel coated glass guide with a radius of curvature of about 10 m with the function of eliminating the direct radiation coming from the reactor, introducing even a more severe cut-off f o r neutrons with velocities v > 100 m s

'

and bending the remaining neutron beam into the horizontal plane. The beam e x i t i s 3 m above the converter. A beam distribution box can direct the UCN beam to three different experiments. The original fluxes reported /4/ were about 100 UCN K 2 s - ' (with v(ms-l)<6) and 2 x 105VCN cm 's-l (6<v(ms ')<loo). In the meantime the UCN flux has decreased by approx. a factor of 10 due t o degradation of the guide transmission. The guide seems t o be mainly damaged i n the section which captures the thermal neutron flux. A replace- ment of t h i s section i s forseen i n the near future.

The main drawbacks of t h i s source are the absence of a cold converter and the high transmission losses (550) of the long guide system for UCN which are piped up from the core region a l l the way t o the experiments. These drawbacks are avoided with two alternative source types discussed i n the following section.

(b) Superthermal He source: This source type has been proposed by Golub and Pendlebury /5/ and realized for the f i r s t time a t the ILL /6/ as shavn on fig.3. This novel UCN source type uses 10

a

neutrons which are scattered down t o r e s t i n liquid ~e~ a f t e r creation of a phonon with the right energy and momentum. The probability t h a t a UCN produced i n t h i s way is l o s t by absorption of an appropriate phonon o r by scattering of a quasi-particle is very small a t the operating temperature of 0.5 K of the ILL source.

Therefore, UCN densities can build up i n an accumulation mode. With the existing source on the cold beam H I 7 densities of the order of 10' UCN are expected inside the source provided that losses on the container walls and due t o He3 impurities are kept low. The actual He container is a 3 m long internally electropolished stainless s t e e l tube of 67 mm inner diameter which l e t s the 10

a

neutrons enter a t one end through an UCN t o t a l l y refelcting 0.5 mm stainless s t e e l window. I t i s closed a t i t s other end during the a c c m l a t i o n phase by a f l a p valve which can be operated from- outside. The He4 a t 0.5 K is purified from ~e~ t o a fraction of about I0 l o .

This source is clearly best adapted f o r experiments working i n a storage mode l i k e the E M experiment. Up t o now a density of only 7 UCN has been actually detected mainly because of the low transmission of the cryo- genic A 1 windows and the related gaps. I t is foreseen t o improve i n the near future the UCN extraction system and the storage time of the He container by a coating with Beryllium.

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cal guide w i l l be installed (fig.4) which extracts neutrons directly from the cold converter with an expected gain factor of about 50 compared t o a thermal conver- t e r . A guide with a radius of curvature of 13 m traverses the swimming pool and feeds partly a Steyerl turbine with

50 m s

'

where they are transformed into ^,.v*..^'^.^c⁽^.^.^"

UCN, partly it supplies an intense beam of very cold neutrons. The transmission

d..n - 4 *- losses w i l l be by a factor of 3

-

5 less

severe using a turbine in combination 7-

with a 50 m s-' guide. Therefore the

d

overall gainfactor compared t o the ^3.0 ^W'O original PN5 performances w i l l amount t o

about 200. This system makes f u l l profit of the HFR and, in addition, is much less sensitive t o deterioration with time. I t i s based on a long r e a l ex- perience. The guide and the turbine are designed and manufactured by A. Steyerl's group i n Munich and w i l l be installed a t

ILL i n 1985. FIG.4 Vertical UCN guide and

turbine Table 2 gives a comparative information on

the three source types.

TABLE 2: Comparison of the different UCN sources a t ILL Unperturbed thermal flux

a t the nosefof the thim- ble n s

'

Velocity of useful neutrons m s

steady UQi2

measured flux n un s potential measured stored UCN

density in g

Last but not least it should be mentioned that the reactor can be seen as a special f a c i l i t y for electron antineutrino experiments and that the neutron interferometer is an ILL f a c i l i t y in the sense of t h i s report. Both subjects have been discussed i n d e t a i l earlier.

Vertical guide on the modified cold source and turbine (1 985)

4.5~10"

50 4.5 2 PN5 (1 977)

room temperature converter

6x10"

5 45

Helium source on HI 7

(1982)

2 . 2 ~ 1 0 ' ~

500 3

$1 00 0.2-0.3

10

$100 expected

1 with -teff = 50s 100-200 times the 10 with T = 50s values of

100 with

{E

^=5OOs ^PN5

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JOURNAL DE PHYSIQUE

References

/ I / Commissariat B 13Energie Atomique 1971 BIST No 165, December, and 1972 BIST No 166, January.

/2/ Inst. Laue-Langevin 1983, Neutron Bem F a c i l i t i e s a t the High Flwr Reactor available f o r Users; ILL, i n t e r n a l report.

/3/ Mampe (W.) and Ageron (P.), I n s t . of Phys. Conf. Ser. (1978)

42,

148.

/4/ Ageron (P.)

,

Hetzelt (M.)

,

Mampe (W.)

,

Golub (R.)

,

Pendlebury (M.)

,

Smith (K.) and Robson (J.)

,

IAEA-SM-219/58 (1978), 53

/5/ Golub (R.) and Pendlebury (J.M.)

,

Phys. Lett. (1975) 53 A, 133 Golub (R.) and Pendlebury (J.M.)

,

Phys. Lett. (1977)

62,

337 /6/ Golub (R.)

,

Jewel1 (C.)

,

Ageron (P.)

,

Mampe (W.)

,

Heckel (B.) and K i l -

vington ( I . ) , Z. Phys. B (1983)

51,

187