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THE SOURCE SPECTRUM OF REACTOR ANTINEUTRINOS
K. Schreckenbach, F. von Feilitzsch, A. Hahn
To cite this version:
K. Schreckenbach, F. von Feilitzsch, A. Hahn. THE SOURCE SPECTRUM OF REAC- TOR ANTINEUTRINOS. Journal de Physique Colloques, 1984, 45 (C3), pp.C3-135-C3-138.
�10.1051/jphyscol:1984325�. �jpa-00224039�
JOURNAL DE PHYSIQUE
Colloque C3, suppi6ment au n03, Tome 45, mars 1984 page C3-135
THE
S O U R C ESPECTRUM OF REACTOR ANTINEUTRINOS
K. Schreckenbach,
F.
von ~eilitzsch* andA.A.
~ahn*"Institut Laue-Langevin, 156X, 38042 Grenoble Cedex, France
*Physik Department, T U Miinchen, 8046 Garching, F.R.G.
Ta~ifornia m t u t e of Technology, Pasadena, C A 91125, U S A
.
R6sume
-
Le spectre beta provenant des fragments de fission de 2 3 5 ~ et 239~u induit par la capture de neutrons thermiques a QtQ mesure A l'aide d'un spectrom6tre magn6- tique dans la g a m e d16nergie EB allant de 1,5 2 8 MeV. Le spectre dlCmission de-
ve en a 6tQ deduit ; il fournit une information de base pour les experiences utili- sant les ve provenant des r6acteurs-
t e l l e s q u e l a r e c h e r c h e d e s o s c i l l a t i o n s e n t r e ~ .Abstract
-
The cumulative beta spectrum of 235U and 239~u thermal neutron induced fission fragments was measured with a magnetic beta spectrometer in the range E = 1.5 to 8 MeV. The beta spectra were converted in the correlatedv
spectrum ofB
a nuclear reactor. Thus a basis is given for reactor neutrino experiments such as the search for neutrino oscillations.
The interpretation of experiments with reactor
;
such as the search for v oscilla- e'tions, depend on the knowledge of the
ue
source spectrum of the reactor. This sour- ce spectrum is composed of a superposition of v spectra of beta decaying fission-
products in the reactor core. Most of these
;
are found below Q, 10 MeV.A composition of this spectrum using experimental and calculated data for the indi- vidual fission products is found to be very difficult. Beta branches comprising several hundreds of fission products have to be taken into account and the various calculations differ considerably from each other (see comparison in refs.
/I, 21).
An experimental approach to this problem is possible by measuring the cumulated beta spectrum of the fission products. The difficulties inherent with measurements of this beta spectrum arise from the high y background, the strong decrease in inten- sity with energy and from a proper absolute calibratign (per fission) of this spectrum. Finally the conversion into the correlated v spectrum needs much care, in particular when a precision of a few percent is demanded.
In the followine we would like to discuss the ex~eriments .., on these beta-s~ectra for 2 3 5 ~ and 2 3 9 ~ ~ fission as performed at the magnetic spectrometer BILL /3/ of the High Flux Reactor, Grenoble. Since a part of this work has already been published elsewhere 11, 2/ we like to concentrate more on some possible systematic errors in this method.
The measurement of the cumulated beta-spectrum
The target arrangement is shown in fig. 1. Fig. 2 illustrates a beta spectrum mea- sured on-line after 6 hours exposure time to neutrons. The reactor power was redu- ced to limit self-heating of the target. An accuracy of \< 5 % was achieved for the main part of spectrum 11, 21. The electron beam emerging from the target is well separated from the y-background by the sequence of the two double-focussing iron- core magnets of the BILL spectrometer. Electrons arising from the absorption of +present address : University of California, Ervine, CA 92717, USA
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984325
JOURNAL DE PHYSIQUE
13 m lo
contained &wren Ni foils of- 7 mg/sm2
I
'=lo lh.
I c m t e of
I
3 I I I I I I I I I
18 MW reactor powed
-
2 3 5 ~ tarnet
1
,
Kinetic energy of betas i n MeV
Fig. 1
-
Schematic view of the Fjg. 2 - Cumulated b -spectrum of 2 3 5 ~ fis- target site at the BILL sron fragments. The "U' target determined the spectrometer background not correlated to fission eventsin the target.
fission correlated y-rays in the cover foil are o f a n i n t e n s i t y d l e s s t h a n 0 . 3 % compa- red with those from the beta decays. Another contamination of the beta spectrum may come from internal conversion electrons (ICE) competing with these y-decays. The main component stems from the (n, y) reaction competing with the (n, f) reaction
on the fissile nucleus. Above 5 1.5 MeV these ICE'S contribute by less than 1 % to the cumulated beta spectrum, as can be estimated : for instance, for
2 3 5 ~ and E = 1.5 MeV, the number of y-rays from the (n, y) reaction is roughly 0.6 y's per MeV, per absorbed neutron, given by y-ray intensity distributions
(ref. / 4 / ) and the cross section ratio o(n,y) to o(n, £1. The internal conversion coefficients for common multipolarities is less than % 10 -2 for Z = 92 and 1.5 MeV.
The resulting electron intensity gives < 6 x lob3 per MeV to compare with the intensity of the beta spectrum of 1.31 per MeV /I/. Clearly individual lines may contribute more at such low energies, in particular if they are of multipolarity EO. Above % 2.5 MeV strong individual lines become unlikely due to the strongly increasing level density with nuclear excitation energy.
Conversion to the correlated
;
spectrumThe cumulated beta spectrum of the fission products is a very complex superposition of many branches with different C+ values, intensities and Z values (proton number).
Our conversion method starts by approximating the experimental beta spectrum by about 25 hypothetical, allowed beta-branches pi(e, E:~),
2)
with amplitude ai. TheB
mean value
2
used in the Fermi function is a function of E(~), known from the average characteristic of fission products / 2 / . The final 0 v spectrum is then the sum of the individually converted spectraThe strongly decreasing intensity of the beta spectrum well defines the strength of the ~(~kranches, since a high energy branch will not significantly influence the
6
intensity of a low energy branch.
A f u r t h e r t e s t of t h i s conversion method can b e made u s i n g t h e approximate r e l a t i o n f o r r e l a t i v i s t i c b e t a decays :
Pv (Ev) :: P6 (E tot
-
AE) (2)Etot = E + me 6
AE Coulomb term
With t h i s approximation, c a l c u l a t e d s p e c t r a Ncal NCal
B ' V can be used f o r t h e conver- s i o n , i f
Nial
a g r e e s reasonably w e l l w i t h our experimental b e t a spectrum N~~~ :exP ( E
-
AE) ~ ~ ~ BN v ( E J r Ny1(~,,)x >al (3)
B (Etot
-
AE)F i g . 3 compares N (E) and NB(Etot
-
AE) s p e c t r a . For s i m p l i c i t y AE = 0 i s chosen.The c o r r e l a t e d d e v i a t i o n f o r N and N g i v e s confidence t o our conversion method,
6
f o r which an u n c e r t a i n t y of 3 t o 4 % was estimated / 2 / .
Fig. 3
-
R e l a t i v e d i f f e r e n c e s of t h e N and N v s p e c t r a of t h e p r e s e n t work t o Bv a r i o u s c a l c u l a t i o n s : DVSM 151, VSMS 161 and KM 171. Etot denotes t h e energy i n c l u d i n g t h e r e s t mass. The d e v i a t i o n s ( N- N~ ~~ ~~~between N ) / N6 and N v ~ ~ ~ a r e t h e same w i t h i n a few p e r c e n t .
The conversion method i s based on t h e assumption of only b e t a decays w i t h t h e s t a - t i s t i c a l shape. For a f u r t h e r refinement t h e c o n t r i b u t i o n of f i r s t - f o r b i d d e n b e t a - decays and from r a d i a t i v e c o r r e c t i o n s i n t h e beta-decay (bremstrahlung) must b e d i s c u s s e d . A f i r s t e s t i m a t e of t h e e f f e c t of r a d i a t i v e c o r r e c t i o n s modified t h e deduced N v s p e c t r u m only s l i g h t l y (< 1 % a t EL, = 4 MeV, + 3 % a t 7 MeV), u s i n g t h e theory of S i r l i n 181.
It should b e noted t h a t t h e comparison of t h e p r e s e n t r e s u l t w i t h t h e r e a c t i o n r a t e s of t h e v , d e t e c t o r a t t h e Gasgen power r e a c t o r /9/ g i v e s i n f o r m a t i o n on t h e neutron l i f e - t i m e r s i n c e t h e c r o s s s e c t i o n o ( p +
qe
+ n + e+) i s p r o p o r t i o n a l t o I / r n .n '
C3-138 JOURNAL
DE
PHYSIQUEFor the case of no-oscillations of neutrinos a value of T = 882 4 1 (68 % c.1.) n
can be deduced. Allowing oscillationsanupper limit r
<
923 s is given. Inversely nthis discussion shows the importance of a precise knowledge of r for the interpre- n
tation of neutrino oscillation experiments with reactor neutrinos.
In conclusion the
ie
source spectrum of reactors can be deduced from the correlated beta spectra of fission products. A precision better than 6 % can be achieved, including the uncertainty in the conversion of Ng to Nv.The authors wish to acknowledge stimulating discussions with F. Bijhm, H.V. Klapdor, H. Kwon, R.L. Mijssbauer, P. Vogel, J.L. Vuilleumier and V. Zacek.
References
/ I / Schreckenbach K., Faust H.R., Feilitzsch F. von, Hahn A.A., Hawerkamp K. and Vuilleumier J . L . , Phys. Lett. 99B (1981) 251
/ 2 / Feilitzsch F. von, Hahn A.A. and Schreckenbach K., Phys. Lett. 118B (1982) 162 / 3 / Mampe W. et al., Nucl. Instrum. Meth. 154 (1978) 127
/ 4 / Bendt P.J. and Jurney E.T., "Neutron Capture Gamma-Ray Spectroscopy", eds.
Chrien R.E. and Kane W.R., Plenum Press 1979, p. 558 Keoling T., Nucl. Phys. A307 (1978) 239
/ 5 / Davis B.R. et al., Phys. Rev. C19 (1979) 2259 / 6 / Vogel P. et al., Phys. Rev. C24 (1981) 1543
/ 7 / Klapdor H.V. and Metzinger J., Phys. Rev. Lett. 112B (1982) 22 181 Sirlin A., Phys. Rev. 164 (1969) 1767
/ 9 / Caltech-SIN-TUM Collab., Vuilleumier J.L. et al., Phys. Lett. 114B (1982) 298
