Nuclear Instruments and Methods in Physics Research A320 (1992) 606-607North-Holland

tter to the Editor

method for pleasvoit store ltraco

A.D. StoicaInstitute ofPhysics and Technology of Materials, Bucharest, POB MG-7, Romania

Received 20 February 1992 and in revised form 19 May 1992

A method for measuring the neutron lifetime is proposed, using simultaneous detection of electrons and upscattered neutronsgenerated during the ultracold neutron storage .

Direct measurements of the neutron ß-decay expo-nent can be performed by counting the number ofultracold neutrons (NUCN) remaining in a materialcontainer as function of the duration of the storage (t)[1,2] :

NUCN(t, = No exp(-t/Tsc)-

(1)The time of UCN storage rs, is determined by the(ß-decay lifetime Tn and by the time of losses Tlos atthe container walls :

7-st

,'_n

+Tlos

where the losses probability T-s =

-1 -Tlos -

2

fl'limf0

(S~(L~UCN)L'UCNn(r, LIUCN , Í) dS dL'UCN

l'I~ro

4f0

~12n(r, vucN, t) d12 dv ucN

ring the neutron lifetimeeutrons

is an average over the spatial and velocity distributionof the UCN gas density, (g(CUCN ) is the loss probabil-ity at one collision, u ii n the UCN limit velocity, t thetime, n(r, vUCN, t ) the UCN gas density, S the innersurface of the storage container, f2 the UCN storagevolume).

The neutron lifetime was obtained by extrapolatingthe storage time at infinite container dimensions (rive- 0) [1,21 . The evaluation of the neutron lifetime bythis method require the accurate experimental expo-nential decay law and the calculation of the lossesprobability. Consequently in these experiments UCNspectrum determination [1] or adequate time scaling [2]are necessary .

The object of this letter is the description of analternative method for the neutron lifetime determina-tion, suitable even in the case of a single filling . Thesalient features of this method are the use of a wide

0168-9002/92/$05 .00 (o 1992 -- Elsevier Science Publishers B.V. All rights reserved

energy spectrum at filling in order to have a strongdeviation from the exponential decay law and the si-multaneous detection, during storage of the electronsgenerated by neutron ß-decay and of the thermal neu-trons generated by the upscattering of UCN at the wallcollisions [3] . The electron count NR(t) in a shortinterval At at time t, is proportional to the number ofUCN remaining in the container :

Nß(t) = ( £ßlTn) OtNUCN(t) ,where £R is the detection efficiency. The count ofinelastically scattered neutrons Nie(t) in the same in-terval is :

Nie(t) = ( £ie/T los) AtNUCN(t)1,

(where £ie is the detection efficiency taking into accountwall losses by absorbtion . The total number of UCNleaving the container is a linear combination of theintegral counts detected by the electron and neutrondetectors :

NUCN(0) - NUCN(t) = (£pAt)-1f0Nß(t') dt'

+ ( £ i,At ) -1 f'Nie(t') dt' .0

In fact both time spectra

NR(i) and

Nie(i) (i = 0,1, - - - n) are experimentally obtained . Then the rela-tion (6) can be made discrete and each of the terms bemultiplied by £ß0t/Tn to give :

®(i) = (®t/TO E NR(J)

+(£R/£ie)(®t/Tn) Nie(i) ,

(7)j=0

where:

Nß(0) In [Nß(0)/Nß(1)í1 -Nß(1)/Nß(0)

Nß(i) In[ Nß(t - 1)1Nß(i)]NR(i - 1)/Nß(i) - 1

The expression (7 ') includes adjustment factors fordistinguishing between counts accumulated over finitecounting times At and counting rates at the instantwhen the interval At begins for j = 0 and ends forj = i . In this way the neutron lifetime can be evaluatedby fitting the experimental spectra obtained in a singlecontainer filling, without opening . A strong change ofthe ratio NR(i)/Nie (i) during the storage is the maincondition to complete such a procedure successfully.

The new dynamical methods proposed to extractUCN at the pulsed reactors [4] make our proceduremore attractive. This methods give the possibility toobtain a number of 108 stored UCN. Considering thetheoretical velocity dependence of the loss probability[5]:

î~(UUCN) =2711arcsin(y) -y(1 -y2)1/21/y2 ;

y =VUCN/Ulim

(8)

and the Maxwellian filling spectrum we can estimatethe statistical uncertainty of the lifetime . The optimumprecision is obtained for : SU1im?71412 =Tn 1 when arelative precision of 5 x 10-3 is reached in one filling

A.D. Stoica /Methodfor measuring the neutron lifetime

only. The influence of the gravity was not taken intoaccount .

Possible systematic errors in this kind of measure-ment could be generated by variation of the efficien-cies during the storage . This can occur if there aredifferent velocity dependences for the loss probabilityby upscattering and by absorbtion, or as a consequenceof the change during the storage of the upscatteredUCN spectrum. Additional experiments are necessaryto clarify this point .

Acknowledgement

The author wishes to thank to Dr. AN. Strelkov,Dr. V.N. Shevtsov and Dr. Yu.N. Pokotilovsky forstimulating disscutions.

References

607

[1] A.G . Kharitonov et al., Nucl. Instr. and Meth . A284 (1989)98.

[2] W. Mampe et al., Nucl . Instr. and Meth . A284 (1989) 111 .[3] A.D . Stoica, AN. Strelkov and M. Hetzelt, Z. Phys . B29

(1978) 349.[4] A.D . Stoica, AN. Strelkov and V.N . Shvetsov, JINR Com-

munication, to be published.[5] A. Steyerl, Springer Tracts in Modern Physics 80 (1977)

57.