H.S.Gurr, F.Reines and H.W.Sobel, University of California, Irvine
INTRODUCTION AND HISTORICAL BACKGROUND
The search f ó r the a n tin e u trin o -e le c tro n e l a s t i c s c a tte r in g process
^ + e" —* v£ + £ A/
has proceeűed since Ferm i’ s p io n eerin g paper on b éta decay appeared in 1934. I t qu ickly became evid en t th at the process was nőt going to be an easy one to observe. In 1935 Nahmias, /2/ using a n atu ral Rádium source looked f ó r the process using a g e ig e r counter as both ta rg e t
and d e te c to r . His lower l im it oh the path o f neutrinos in a i r /3 x 10'*'ocm/
was in te rp re te d by Bethe /?/ as presen tin g an upper l im it on the magnetic moment o f the neu trino at le s s than 2 x ÍO- ^ Bohr magnetons /Bm/. Unlike the in verse b éta process there was no th e o r e t ic a l b a sis on which to ca lc u la te the magnitude o f a magnetic moment. However in 1935, such an a d d itio n a l fe a tu re was nőt a p r i o r i unreasonable.
in 1950, B a rre tt used tritiu m in an experiment s im ila r to that o f Nahmias and obtained an upper l im it o f 10 ^ Bm. Alsó about th is time Wollan took advantage o f the much la r g e r neutrino flu x em itted from
x Supported by the United S ta tes Atomic Energy Commission
148
a uránium p i l e and was able to g iv e an upper lim it f ó r the in te ra c tio n c ro s s -s e c tio n o f f i s s io n neutrinos as <C ÍO- ^1 cmr" . This iraplied
that the experim ental lower l im it on the path o f neutrinos in a i r was now 2 x 1 0 ^ cm or an upper lim it on the magnetic moment o f
10
” ^ Bm.E a r lie r , Crane /7 / had used geoph ysical arguments in v o lv in g s o la r neutrinos to a r r iv e at an upper l im it o f 2 x 10“ ^ Bm.
In 1954, Cowan, Reines and Harrison working at a la rg e fis s io n re a c to r w ith a liq u id s c i n t i l l a t o r were able to set an experim ental upper l i m i t o f 10 Bm. In 1957, the same experim ental lo c a tio n bút wibh a
/ q /
la r g e r and b e tte r - s h ie ld e d d e te c to r allowed Cowan and Reines ' to set a new upper l im it o f l ó - '5 Bm.
About t h is tim e, Salam, Landau and Lee and Yang
published the two-component neutrino theory in which a th e o r e t ic a l ba sis was provided f ó r d is tin g u is h in g neutrinos and a n ti-n e u trin o s . I t was poin ted out by Salam/^0^ th at according to t h is theory the neutrino magnetic moment must be ze ro .
I t wasn’ t u n t il 1958 w ith the appearance o f the V-A theory Marshak and Sudarshan and Feynman and Gell-Mann that ex p erim en talis ts once again had a t h e o r e t ic a l p ic tu re to describ e the e l a s t i c s c a tte rin g p rocess. The success o f t h is theory in d es crib in g the in verse béta
decay process or "n on -diagon al" terms o f the weak in te r a c tio n Hamiltonian made i t n atu ral to assume the th e o ry ’ s v a l i d i t y f ó r the "d ia g o n a l,"
o r e l a s t i c s c a tte r in g , terms a ls ó .
/ q /
Analyzed in th is new l i g h t , the data o f Cowan and Reines 7 im plied th at the square o f the r a t io o f the coupling constant f ó r e l a s t i c s c a tte
-2 2 2
r in g /g
J
to the beta-decay constant /g^J
was ^ 10 .No new data were to be forthcom ing u n t il 1970. Before th a t, however, in 1969, Gell-Mann, G oldberger, K r o ll and Low /15/■" suggested that the
"d ia g o n a l" and "n on -diagon al" terms o f the weak in te ra c tio n Hamiltonian may be o f a q u ite d iff e r e n t ch a ra cter. I f th is i s true no v á lid
p re d ic tip n as to the c ro s s -s e c tio n o f the e l a s t i c s c a tte rin g process
can be made on the basis o f V-A and we would have to abandon u n iv e r s a lit y . In 1970, S toth ers /16/ using a strop h ysica l data concluded that the
0—2 /17/
c ro s s -s e c tio n was 10 times V-A p re d ic tio n s and S te in e r ' "
using CERN n eu trino experiment data gave an upper lim it o f 4-0 times V-A, f ó r the r e la te d re a c tio n
ve + e~—~ ve ♦ e . / 2 /
Alsó in 1970, Reines and Gurr, working at a re a c to r, set a new upper l im it o f <£ 4 times the V-A p r e d ic tio n .
Meanwhile, evidence has been mounting that a u n ifie d theory o f weak and electrom a gn etic lep ton in te ra c tio n s proposed by Steven Weinberg in 1967 ^'*■9/ may t e re n o rm a liza b le. H.H. Chen and H.H. Chen w ith B.W. ^ee kave use(} the data o f Reines and Gurr to r e s t r i c t
p
the parameter x = sin 0 which appears in the W ein b erg,th eory. Fór the l im it g2/g^ <C 4, x is r e s t r ic t e d to be 0.45.
R ecen tly , the prospects f ó r observing re a c tio n /2/ have been empha-s iz e d at the Loempha-s Alamoempha-s Meempha-son Phyempha-sicempha-s F a c i l i t y / L A M P E w h e r e an in tense neu trino flu x is about to become a v a ila b le . The neutrinos which o r ig in a te in the LAMPF beam stop are predominantly from the decay o f jjí
and
7
r at r e s t . The muon-neutrinos from these decays are below the energy necessary f ó r muon production. The ob servation o f /2/ is th e re fo re nőt overwhelmed by in verse b éta re a c tio n s which are so p reva len t at150
hlghor energy a c c e le r a to r s . However the ex isten ce o f neutrinos from both yU and IT decay does present oth er p o s s i b i l i t i e s nőt a v a ila b le a t the re a c to r or at h igh er energy a c c e le ra to r s , e s p e c ia lly i f the
m u lt ip lic a t iv e lep to n conservation law is v a lid and / or n eu tral le p to n ic currents / e .g ., as in the Weinberg theory/ e x is t /“^b/^
The r e s u lts to be considered today are the most recen t in the con- tin u in g search f ó r the process /eq. \J at a fis s io n r e a c t o r , u s i n g an approach which has been under development fó r t h is e x p l i c i t purpose f ó r the la s t dozen yea rs.
EXPERIMENTAL C0NF1GURA.TI0N
The geometry o f t h is experiment is d ic ta te d in la rg e part by the overwhelming gamma ray background. According to V-A th eory, the 7.84 Kgm p la s t ic s c i n t i l l a t o r ta r g e t can. bo expected to experience le s s than 5 e l a s t i c s c a tte r in g events per week above our data threshold o f 3 MeV. The ta r g e t re g io n was th e re fo re segmented. in tő 16 o p t ic a lly is o la te d bars in an e f f o r t to la b e l a gamma ray by i t s c h a r a c te r is tic sequence o f s c a t t e r i n g i n the low z médium. We thus accepted as v a lid events only those in which the r e c o i l e le c tr o n was confined to a s in g let bar /about 60 7<J.
The bars were viewed through NaI s c i n t i l l a t o r s , and were surrounded by a NaI annulus. The r e s u lt was a ta rg e t e n t ir e ly surrounded by at le a s t 10.7 cm o f NaI. Advantage was taken o f the w idely d iff e r e n t pulse shapes from the p la s t ic and NaI s c i n t i l l a t o r s to determine whether the event was lo ca ted in the p l a s t ic or in the NaI l i g h t p ip e . The NaI was
in turn com pletely enclosed in a lead jack et 6.3 cm th ic k and then wrapped in a th in cadmium sh eet. This u n it was e n t ir e ly immersed in a c y lin d r ic a l 2,2oo l i t e r liq u id a n ti-co in cid en ce d e te c to r.
The c y lin d r ic a l a n ti-co in c id en ce tank was i t s e l f immediately below a la rg e rectan gu la r liq u id d e te c to r , and surrounded by water tanks and another lead s h ie ld 20 cm th ic k . More water tanks outside th is lead completed the s h ie ld in g . F ig . 1 is a schematic o f the d e te c to r u nit in sid e the c y lin d r ic a l tank.
The p h o to m u ltip lie r s ig n a ls rep resen tin g the ta rg e t p o rtio n o f the apparatus as w e ll as the many l i v e region s surrounding the ta r g e t were disp layed on an o s c illo s c o p e and photographed. T r ig g e r con strain ts requ ired a pulse o f the shape appropriate to thé p la s t ic s c i n t i l l a t o r w ith an energy d ep o sitio n ^ 2.7 MeV and unaccompanied by a pulse in e it h e r the rectan gu la r or c y lin d r ic a l liq u id tanks which would be c h a r a c t e r is tic o f a cosmic ray muon. The a n ti-co in cid en ce gate was 4oo y í. sec wide to exclude t r ig g e r s from neutrons produced by an
e a r l i e r muon. Fu rther screening o f events was accomplished by analyzing the f i l m reco rd . Such an in sp ectio n re vea led the lo c a tio n o f the event in the t a r g e t , the energy o f the even t, and the presence, i f any, o f NaI or liq u id s c i n t i l l a t o r p u lses. The NaI annulus was capable o f d e te c tin g an energy d ep o sitio n o f 40 KeV, w h ile the NaI li g h t pipes could revead the presence o f as l i t t l e as 200 KeV energy d e p o sitio n . S ign al lig h t s a lsó d isplayed on each frame were used to denote the occurrence o f variou s c h a r a c t e r is tic even ts.
ENERGY GÁLIBRATION AND STABILITY
The ra p id ly f a l l i n g spectrum c h a r a c te r is tic o f the background makes i t e s s e n tia l to maintain a continuous check o f the system ga in . Such
pl lL a check is provid ed by the b éta decay / 3 .2. MeV end poin t/ o f Bi
contaminant. The procedure used was to measure the ra te o f B i 214 decay as id e n t ifie d by a delayed daughter ex p a r t ie le f 7 .7 • MeV7 TTy2 = 164 yu s«
The energy a sso cia ted w ith a p re s e le c te d B i 214 ra te was taken as the re fe re n c e value and a l l runs norm alized to i t . The absolute energy sca le was s e t to w ith in - 2 % by means o f a Th^0^ gamma source f 2. 62 MeV/*
Both sources were viewed by the e n tir e p la s t ic in a n ticoin cid en ce with the surrounding NaI. The c o rr e c tio n to the o v e r a ll gain o f the system as measured by the was w ith in -2 % . F ig . 2 shows these c a lib r a tio n sp eetra .
Reactor Associated Backgrounds
— t
a. v 'j * p - * n » e
In verse béta decay produces in our d e te c to r a p o s itro n which w i l l masquerade as an e l a s t i c s c a tte r in g event i f we f a i l to d e te c t both o f the accompanying p o s itro n a n n ih ila tio n gammas and the delayed neutron capture gammas. F o rtu n a te ly , we can estim ate t h is source o f background by measurements o f the a n n ih ila tio n gamma d e te c tio n e ff ic ie n c y f ó r p o sitro n s in the p la s t ic d e te c to r and the neutron d e te c tio n e f f ic ie n c y f ó r neutrons from in verse b éta decays in the p la s t ic d e te c to r.
I f we denote the p ro b a b iity o f observing a s in g le a n n ih ila tio n gamma as and the p ro b a b iity o f seeing the neutron, hydrogen capture gamma as ti then, the p r o b a b ility o f an in verse béta decay
‘■íl
event masquerading as an e l a s t i c s c a tte rin g event is given by
p = a - \ j r i - v 2 . From observation o f p o sitro n a n n ih ila tio n in our d e te c to r , f ó r those events e x h ib itin g neutron capture, we conclude that = 0.85 ^ o’ o^ » The neutron d e te c tio n e ff ic ie n c y is estim ated to be 0.2 by comparing the r a te o f observed in verse béta decays
w ith the p re d ic te d r a te . Hence
P A - 0.27 / I - 0.857^ = 0.018 . The number o f
\Je + p re a c tio n s p er day in our p la s t ic ta rg e t capable o f depo-s i t in g between 5.6 and 5.0 MeV in a depo-s in g le p la depo-s t ic element idepo-s ca lcu la ted to be 6.2/day. This c a lc u la tio n took in tő account the fr a c tio n o f sin g le element events which we measured w ith our d e te c to r and allowed f ó r
the 1 MeV a n n ih ila tio n energy which added to the p o sitro n r e c o i l energy in these capes. The number mistaken as e l a s t i c s c a tte rin g i s , th e re fo re ,
r + .008 7 + .05
/O.018 - .005J x /6.2/dayJ = 0.11 - .05 / d
I t is amusing to observe that in searching f ó r the e l a s t i c s c a tte rin g Vg in te r a c tio n we must contend w ith th is r e l a t i v e l y la rg e
neutrino produced background.
B. Neutrons
I t can be imagined th at neutrons produce background events by d ir e c t r e c o i l from protons in the p la s t ic . However, th is process
154
is discrim inaüed again.rt by the poor se i n t i l l a t ion e ff ic ie n c y o f the p la s t ic f ó r r e c o i l prolons in the re le v a n t energy rangé, / I t is ^ 0.5 that f ó r e le c tro n s at 6 MeV J Neutrons o f the r e q u is it e energy,
"> 6 MeV cannot a r is e from natu ral photo processes.
A t e s t o f maximum p o s s ib le neutron associa ted background was made using a Pu-Be source. I t produced a co\mt ra te in the NaI d e te c to r o f 92.9/min, /J\ ► 10 MeV7 and a s in g le element ra te o f 0.002/min in the p la s t ic d e te c to r /3.6 —► 5.0 MeV/. The re a c to r associated
background in the NaI was raeasured to be Á 0.15/min. Assuming. the neutron speetrum from the re a c to r to be as e n e rg e tic as that associa ted w ith the source and fu r th e r assuming that a l l the annulus up-down d iffe r e n c e is due to neutrons, we fin d that the maximum re a c to r associated background in the p la s t ic a r is in g from neutrons is
x /Ó.002/min J /1 4 4 -0 min/day J - 0.005/day, /3.6 —► 5.0 MeY7
C . Gammas
Although re a c to r a sso cia ted gammas probably a ris e in la rg e measure from neutron capture, we make a separate l im it f ó r completeness.
Deductions as to the maximum background c o n trib u tio n from these gammas
can be made by comparing re a c to r associated ra tes in the NaI anticoincidenc<
d e te c to r b efo re the a d d itio n o f 6.3 cm Pb outside the NaI /l2/mi]^7 and s c a lin g the pre-Pb p la s t ic ra te C 6/day J
C 6/day J =
0
.075
/day*
Since 1/3 o f these w i l l he s in g le element even ts, we obtain a maximum background due to re a c to r associa ted gammas o f
0.025/day /3.6 —* 5.0 MeV J
R E S U L T S
F ig . 3 shows the observed p la s t ic d e te c to r events in the e l a s t ic s c a tte r in g mode in the energy rangé 3 .0 —* 5 MeV f ó r re a c to r "onu
days7 and " o f f ” ,/52 days7 • The ra te s and run times have been co rrected f ó r dead time £ ' —' 22 %7 • Alsó shown are d e te c to r ra tes p re d icted by the V-A theory /13»14,25/ ^ These p re d ic tio n s take in tő account the in crea se in ra te due to d e te c to r energy re s o lu tio n /12 % h a lf width at h a lf maximum/, and a decrease in ra te C ’~~* 40 %J due to e le c tro n s escaping a s in g le p la s t ic elem ent. Alsó included in the pre
d ic te d ra te i s a statement o f e r ro rs based on the u n c e rta in tie s in the f i s s io n V4 spectrum and in the fi decay coupling constants.
The sum o f the above re a c to r associa ted backgrounds /^0.14/day7 is approxim ately the same as our observed re a c to r o n - o ff d iffe r e n c e / fó r E . = 3 . 6 MeV7 and the data s u ffe r from a la rg e s t a t i s t i c a l
mm
u n c e rta in ty . We chose the r e s u lts f ó r Emin = 3.6 MeV as a compromise
between m inim izing S maximizing V-A
— tra r no f f
Based on the data in Table I , we fin d a one standard d e v ia tio n upper l i m i t on th is e l a s t i c s c a tte r in g s ig n a l to be
✓ + ir7 in + .05 C 0 . 4 + V / . 1 7 / 2 + /. 05/ 2
< .15 - .17 - .11 - #q3 < 0.22/day
156
In obfcaining th is l i m i t , we have c o n s e rv a tiv e ly nőt subtracted o f f our small re a c to r a sso cia ted neutron and gamma background. This corresponds bo an e l a s t i c s c a tte r in g c ro s s -s e c tio n lim it ^~expt
< 6.6 x 1CT47 cm2 f i s s io n fó r the produqtion o f r e c o i l e le c tro n s in the energy rangé 3 .6 —* 5.0 MeV.
I t is apparent from F ig . 3 th a t we are searching f ó r a small e l a s t i c s c a tte r in g s ig n a l superimposed on a very ra p id ly f a l l i n g back
ground. Thus gain s h if t s which may be — - 1 % could produce counting ra te s h ift s * *iq day- ^. Comb in in g the s t a t i s t i c a l and gain
u n c e rta in tie s g iv e s an u n certain ty in the measured ra té o f - .23. In t h is way we a r r iv e at an upper l i m i t , r e l a t i v e to the V-A p r e d ic tio n , o f
6 " expt ^ 0 . 2 ^ 7
— 0. 16
V-A
< 1.7
I f we, in stead o f takin g the Central value and the maximum d e v ia tio n we allow ed fó r the asymmetry in the í j t h e l im it would be the same i . e .
,21— < 1.7 /RT
The most recen t model presented by Weinberg ^ takes the parameter X = 0.25 0 = 30° . This would imply a ra te in our d e te c to r in the energy rangé 3 . 6 —*5 *0 MeV o f about 1.3 time/2^ the ra te p red icted by V-A th eory. Our lim it s in th is case are th e re fo re
& expt
^ W ein b erg
< 1 . 3
0 = 30°
In terms o f a m ignetic moment th is im p lies 2 x 10- ^° Bm.
F i g . 4 in d ic a te s the lim it s which these data piacé on various th e o r e tic a l p o s s i b i l i t i e s . /19,^1,22/ note cro ss -s ectio n in the case f ó r x = 0 C 0 = 0ci7 is ^*V-A This i s at the lim it
----T ~
o f a c c e s s ib ilit y f ó r the improved experiment which we now d escrib e.
Improved Experiment
G onsideration o f the data in Table I in d ica te s that the system used in these measurements was s e r io u s ly lim ite d by the small ta rg e t mass and a high background which placed a prémium on instrumental s t a b i l i t y . An estim ate shows th at the primary source o f background was from unlabeled Bi 214 decay. A cco rd in g ly , we have r e b u ilt the d e te c to r, doubling the
s iz e o f the p la s t ic ta r g e t and thin n ing the aluminized mylar o p tic a l
—Zt P
d iv id e rs to 5.3 x 10 g/cm so as to make v i s i b l e the otherwise
absorbed alpha p a r t i e l e which fo llo w s the Bi 214 decay. In a d d itio n , we have lowered the unlabled in verse béta background by removing the MgO packing m a te ria l, which separated the NaI and p la s t ic s c i n t i l l a t o r s . Prelim in ary runs w ith the m odified system in d ic a te a marked reduction
21 Ll
in the background from unlabeled B . The o v e r a ll increase in the p red icted V-A s ig n a l to background appears to be g r e a te r than a fa c t o r o f 3 / F ig . 57.
Although the e l a s t i c s c a tte r in g data in th is improved system are s t i l l too sketchy to be discussed h ere, we can re p o rt that the inverse béta decay process has already been observed in the m odified d e te c to r.
This p ro c e s s , : seen a t a ra te o f about 15 day-1 w i l l enable us to make a d r e c t determ in ation o f the f i s s io n an tin eu trin o speetrum. As mentioned
IS8
e a r l i e r , i t w i l l in a d d itio n , provid e valu able evidence that the d e te c to r is fu n ctio n in g p ro p e rly ; the re a c tio n can alsó be used to experim en tally determine our e f f ic i e n c y f ó r observing the a n n ih ila tio n gamma rays and the eventual neutron capture. Whereas the neutron d e te c tio n e f f ic ie n c y remains at about 20 %, the .51 MeV a n n ih ila tio n gamma d e te c tio n
e ff ic ie n c y has been re is e d to
.96
- .04 . The r e s u lt is that the in verse b éta background between 3.6 and 5.0 MeV which was p re v io u s ly .11/. 16=*70 o f the "ex p ected " s ig n a l, i s now .< 15 % o f the V-A p red icted ra te in the same energy rangé.I t should alsó be noted, th a t the -1 % gain u n certain ty we chose increased our upper l im it from to or by .5 /V-A/. That
same
1
% gain s h i f t w ith our improved s ig n a l to background corresponds to 0.1/V-A/at the same energy. A cco rd in gly, the improved d e te c to r should y ie ld s p e c tra l in form ation i f the s ig n a l i s at the V-A l e v e l .Fór a l l o f the ábove reasons, we f e e l co n fid en t th at our m odified d e te c to r w i l l a llow us to improve s ig n ific a n t ly the r e s u lts we rep ort to you here today, and w i l l make p o s s ib le an experim ental statement as to the v a l i d i t y o f V-A f ó r the e l a s t i c s c a tte r in g re a c tio n and in
g&neral provid e somé fu r th e r co n stra in ts on t h e o r e t ic a l c o n je ctu re.
Acknowledp;ements
We wish to thank the members o f the Ir v in e neutrino group, in
p a r t ic u la r , H.H. Ohen f ó r d iscu ssion and A.A. Hruschka f ó r h is invaluable assista n ce in re b u ild in g the d e te c to r and s h ie ld in g array. The continued h o s p it a lit y o f the E . I . Dupont de Nemours Company a t the Savannah R iv e r Reactor is g r a t e fu lly noted.
REFERENCES
1. E. Fermi, Nuovo Cimento, 11, 1 /193V and Z e it s c h r ift fü r Physik
88
, 161 /1934/2. M.E.Nahmias, P roc. Cambridge P h il. Soc. 31. 99 /1935/
3. H.A. Bethe, P roc. Cambridge P h il. Soc. 31* I
08
/1935/4. J.H. B a r r e tt, Phys.Rév. 9o7 /195o/
5. E.O. W ollan, Phys.Rév. £2, 445 /1947/
6
. F.G. Houtermans and W. T h ir r in g , H e lv e tic a Physica Acta 2£, 81 /1954/7. H.R. Crane, Revs. Modern Phys. 2o, 278 /1948/
8
. C.L. Cowan, J r . , F. R ein es, and F.B. H arrison, Phys.Rév. %6 1294 /1954/9. C.L. Cowan, J r . , F. Reines, Phys.Rév. lo 7 . 528 /1957/
10. A. Salam, Nuovo Cimento 299 /1957/
11. L. Landau, Nuclear Phys. 2 t 1^7 /1957/
12. T.D. Lee and C.N. Yang, Phys.Rév. lo 5 . 1671 /1957/
13. R.E. Marshak and E.C.G. Sudarshan, S o c ie ta It a lia n a d i F is ic a , Padua, I t a l y , /1958/ and Phys.Rév. lo 9 . I
860
/1958/14. R.P. Feynman and M. Gell-Mann, Phys.Rév. I
09
, 193 /1958/15. M. Gell-Mann, M. G oldberger, N. K r o ll and F. Low, Phys,.Rév. 179.
1518 /1969/
16. R.B. S to th e rs , P h y s .R é v .L e tte rs 24 , 538 /197o/
17. H.J. S te in e r , P h y s .R é v .L e tte rs 24 , 746 /197o/
18. F. Reines and H^S. Gurr, P h y s .R év .L e tters 24, 1448 /197o/
19. S. Weinberg, P h y s .R é v .L e tte rs 1^, 1264 /1967/
20. G. t 'H o o f t , Nucl. Phys. BJ£, 167 /1971/, B.W. Lee, Phys.Rév. D£, 823, /1972/, S. Weinberg, Phys.Rév. L e tte r s 2]_, 1688 /1971/,
S .Y . L ee, UCSD P r e p rin t 10P10-97, S .Y. Lee, UCSD P rep rin t 10P10-99
S .Y . L ee, UCSD P r e p rin t 10P10-97, S .Y. Lee, UCSD P rep rin t 10P10-99