SEARCH FOR A NEUTRON ELECTRIC DIPOLE MOMENT

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SEARCH FOR A NEUTRON ELECTRIC DIPOLE MOMENT

J. Morse

To cite this version:

J. Morse. SEARCH FOR A NEUTRON ELECTRIC DIPOLE MOMENT. Journal de Physique

Colloques, 1984, 45 (C3), pp.C3-13-C3-16. �10.1051/jphyscol:1984303�. �jpa-00224017�

JOURNAL DE PHYSIQUE

CoJJoque C 3 , supplement au n ° 3 , Tome 45, m a r s 1984- page C3-13

SEARCH FOR A NEUTRON ELECTRIC DIPOLE MOMENT

1. Morse

Rutherford Appleton Laboratory, Chilton, Oxon 0X11 OQX, U.K.

Résumé: Pour mettre en évidence une cassure de symétrie lors d'un renverse- ment du temps de transformation, une mesure de résonance magnétique est faite afin de déceler un moment de dipole électrique (MDE) de neutrons ultra- froids placés, pour des périodes d'environ 60s, en présence d'un champ élec- trique puissant. Le MDE neutronique mesuré est (0,3 ± 4,8) x 10 ecm.

Abstract: To search for evidence of a breakdown of symmetry under the time reversal transformation, a magnetic resonance measurement is made to detect an electric dipole moment (EDM) of ultracold neutrons stored for periods =60s in the presence of a strong electric field. The measured neutron EDM is

(0.3 ± 4.8) x 1 0

ecm.

Introduction: A search for an electric dipole moment of the neutron is considered the most sensitive test to detect breakdown of symmetry under the time reversal transformation (T), as yet only observed in the weak decay modes of the neutral K-meson[l,2], Estimates of the neutron EDM based on various theoretical models have predicted dipole moments in the range 1 0

' to

1 0

ecm[3J and the interest in improving upon the present experimental limit

of (2.3 ± 2.3) x 1 0

ecm[4] is to distinguish between the various mechanisms that may be responsible for violation of T symmetry. Current studies exploit ultracold neutrons (UCN) [5], which in the present experiment are stored in a material bottle for periods of 60s, enabling magnetic resonance measurements of linewidth 8mHz.

Experimental Method: UCN, defined here as being neutrons with velocities

<, 6ms

/, are supplied to the experiment (Fig. 1) from a water converter[6]

situated within the ILL reactor of thermal neutron flux 6 x 10

cm

s

. The UCN are transported in a nickel coated glass guide then polished stainless steel tubes to a transmission polariser, consisting of a magnetically saturated Co-Fe foil of thickness =0.5u[7]. Those UCN transmitted continue along a glass guide system, the internal surface of which is covered with non-magnetic sputtered deposit of 55Cu/45Ni to maintain a high nuclear potential for neutron reflection. The neutron spins, initially transverse, are rotated to lie along the guide axis by a slowly varying magnetic field formed by coils surrounding the guide system; the UCN may thus enter and exit the storage volume within the five layer mumetal magnetic shield with minimal depolarisation.

The data acquisition is cyclic, and begins with the guide change-over system aligning to enable the storage volume to fill with polarised UCN. The neutron density within the storage volume rises to a maximum for a filling time =12s, the neutron valve then closes and the guide change-over system again actuates, disposing of those UCN remaining within the guide, which fall vertically through =lm and thus gain sufficient kinetic energy to penetrate the lOOu polished aluminium window of the He-3 proportional counter. The storage volume consists of two 25cm beryllium disc electrodes, separated 10cm by a

cylindrical, beryllium oxide ceramic insulator, these materials being optimum for UCN confinement and suitable for the application of a strong electric field E=10kVcm

.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984303

JOURNAL DE PHYSIQUE

FIVE LAYER M/IMETAL SHIELD

L_-]

7

Fig. 1. Apparatus used to observe magnetic resonance in polarised ultra-cold neutrons stored for times up to 60 seconds in magnetic

fields of 3 to 40 milligauss.

The UCIJ bounce about within the storage vessel, their spins precessing about a steady, uniform magnetic field B that is aligned with the electric field. Soon after the neutron valve has closed, a resonant, sinusoidally oscillating magnetic field is applied perpendicular to

for a period t=2s, of appropriate magnitude to rotate the neutron spin vectors perpendicular to B. After a storage time T, typically 60s, the application of a second oscillating field pulse of period t and coherent with the first, leaves the UCN polarised

antiparallel to the steady field B; the extent of this polarisation depends on the extra phase gained or lost by the precessing neutrons relative to the oscillating field phase during the interval T. The final polarisation is measured by opening the neutron valve; the UCN then diffuse along the guide to the polarising foil which now acts as an analyser, and those transmitted are counted in the detector for a time ~12s. The neutrons of opposite spin

polarisation are subsequently counted for =12s by energising an adiabatic spin- flipper coil[8] which reverses the neutron spins as they approach the analyser.

A curve showing the neutron count per cycle as a function of the oscillating magnetic field frequency is shown in Fig. 2; it has the expected Ramsey separated oscillatory field shape[3] with the linewidth equal to the

theoretical value h(T+2t/n)~z. We define a parameter a=(Cl+C2)/(Cl-CZ), where C1 and C2 are respectively the neutron counts at the peak and through at the centre of the resonance curve of Fig. 2. The value ofa may be interpreted as representing the product of polarising and analysing efficiencies, though its value is partially determined by mechanisms of depolarisation and loss of UCN occurring between polarisation and analysis. Our value of a is typically 0.7.

The uniformity of the magnetic field over the UCN storage volume is such that no significant reduction in the value of a from processes of spin relaxation is observed, even for storage times up to 100s. Following a-c demagnetisation of the five layer mumetal shield, magnetic field scans made using rubidium vapour magnetometers[9] indicate field gradients

3x10-~ gausscm-l over the region of the UCN storage volume. These gradients are measured superimposed on the steady field B, typically 1x10-~ gauss, generated by a uniformly wound coil situated just inside the innermost layer of the shield. The coil axis is orientated perpendicularly to the cylindrical shield axis, since the shielding to external influences along such a transverse axis exceeds that along the cylindrical axis by a factor "5 [9]. In the direction of the steady B field, the measured shielding factor is -lo5 for the typical variations of gauss which occur in the experimental reactor hall. As measured by the

magnetometers the field B is observed to drift slowly gauss per hour,

attributed primarily to external temperature changes =0.5'C.

mCLLATlNG MAGNETIC FELU F R E M N C Y

(MI-

0

29,5 29.6 29.7 29,8

Fig. 2. A neutron magnetic resonance curve obtained with polarised UCN stored for 40 seconds in a magnetic field of

10 milligauss when an oscillating magnetic field is applied for 4 seconds at the beginning and end of the storage period. The solid line is a

theoretical curve fitted using the Ramsey theory which predicts a linewidth at the centre of the pattern of 0.012 Hz.

Maximum sensitivity to a neutron EDM is achieved by setting the oscillating field frequency to correspond to the half-height of the central valley of the resonance curve, and detecting any change in neutron count per cycle which correlates with the periodic reversal of the electric field direction relative to the magnetic field. The minimum value for one standard deviation

uncertainty in the neutron dipole length D is then determined by counting statistics to be U ~ = H / ~ ~ U E T J N , where N is the total number (both spin states) of polarised neutrons detected. For most of the data accumulated to date, the neutron count was -30 per cycle of period =100s, and using the previously mentioned values, the above expression gives a sensitivity

= l x l ~ - ~ ~ c n per month, Recent introduction of the Cu-Ni coated neutron guide section and baking clean the beryllium oxide cylinder resulted in an increased neutron count of =85 per cycle for the last few data runs. The E field has also been raised to 14kV cm-I with the frequency of sparking reduced by the controlled entry of nitrogen into the apparatus, raising the base vacuum pressure of 2x10-~ to 1x10-~ torr. These improvements unfortunately coincided with a sudden reduction of the available UCN density caused by radiation damage of a section of the ILL nickel coated neutron guide; renewal of this guide section should raise the sensitivity of the experiment to uD = 2 . 5 ~ 1 0 - ~ ~ c m per

month.

Possible sources of spurious EDM signals at the present level of sensitivity include:

1 Leakage currents over the storage volume insulator, generating local magnetic fields. These currents are constantly monitored and kept S O n A , corresponding to a worst case spurious signal (10-~~ecm.

2 Other small changes in magnetic field which correlate with the periodic reversal of the electric field. Three rubidium magnetometers are mounted close to the UCN storage volume, and their signals processed in the same manner as the neutron counts per cycle to look for "magnetometer EDMs".

For the last 106 data runs, these signals were equivalent to (-1.2f1.2), (0.0?1.8), and (l.lf2.9)~10-~~ecm.

3 The Eg effect, by which the moving neutrons see a relativistic

netic

field, has halted the progress of neutron beam EDM measurements [lOy, but

C3-16 JOURNAL DE PHYSIQUE

i s c o n s i d e r e d i n s i g n i f i c a n t a t t h e 1 0 - ~ ~ e c m s e n s i t i v i t y of t h e p r e s e n t experiment a s t h e a l r e a d y s m a l l n e u t r o n v e l o c i t i e s a r e e f f e c t i v e l y reduced f u r t h e r by a v e r a g i n g o v e r t h e long s t o r a g e p e r i o d s . Only a n e x t r e m e l y u n l i k e l y , l a r g e c i r c u l a t i o n of t h e s t o r e d UCN i n t h e r a d i a l E f i e l d c r e a t e d by t h e non-uniform c h a r g i n g of t h e i n s u l a t o r would r e s u l t i n a s i g n i f i c a n t e f f e c t .

R e s u l t s : To d a t e , 136 d a t a r u n s have been a n a l y s e d , each r u n l a s t i n g =23 h o u r s and of -800 n e u t r o n count c y c l e s . The E f i e l d was t y p i c a l l y r e v e r s e d e v e r y 32 c y c l e s . The o s c i l l a t i n g magnetic f i e l d f r e q u e n c y was s t e p p e d each c y c l e t o form a r e p e a t e d sequence of f o u r measurements on each s i d e of t h e c e n t r a l r e s o n a n c e v a l l e y ; t h e c o r r e s p o n d i n g n e u t r o n c o u n t s a r e t h e n used t o d e t e r m i n e b o t h t h e s l o p e of t h e r e s o n a n c e , and hence a, and t h e r e s o n a n t n e u t r o n

f r e q u e n c y i t s e l f . The d i f f e r e n c e between a v e r a g e n e u t r o n f r e q u e n c i e s

c o r r e s p o n d i n g t o p o s i t i v e and n e g a t i v e E f i e l d s f o r a d a t a run was c a l c u l a t e d u s i n g a n e x p r e s s i o n chosen t o remove slow f r e q u e n c y d r i f t t e r m s up t o t h i r d o r d e r i n time, y i e l d i n g a f i n a l 'EDM v a l u e ( 0 . 3 * 4 . 8 ) ~ 1 0 - ~ ~ e c m , of normalised X2=0.93 f o r 67 d e g r e e s of f r e e d o m [ l l ] .

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S c h u b e r t

R e t a l , Phys. L e t t . 31B (1970) 662.

A r e c e n t review of c a l c u l a t e d EDMs i s g i v e n i n Ramsey N F, Rep. Prog.

Phys. 45 (1982) 95.

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