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180°-CORRELATED EQUAL-ENERGY PHOTONS FROM 5.9 MeV/N U + Th COLLISIONS : SOME
DETAILS
W. Meyerhof
To cite this version:
W. Meyerhof. 180°-CORRELATED EQUAL-ENERGY PHOTONS FROM 5.9 MeV/N U + Th COLLISIONS : SOME DETAILS. Journal de Physique Colloques, 1987, 48 (C9), pp.C9-315-C9-319.
�10.1051/jphyscol:1987957�. �jpa-00227375�
JOURNAL DE PHYSIQUE
Colloque C9, suppl6ment au n012, Tome 48, d6cembre 1987
180'-CORRELATED EQUAL-ENERGY PHOTONS FROM 5 . 9 MeV/N U + Th COLLISIONS : SOME DETAILS
Department of Physics, Stanford University, Stanford, CA 94305, 7J.S.A
On a trouve une ligne k t r o i t (lgrgeur propre I 2.5 key) 1062 2 1 keV dans l e s spectres de d e w ~ h o t o n s correlks a 180 c.m., avec l e s energies somm&es, dans l e s collisions U + Th a 5.95 MeV/N.
Abstract
We have found a narrow l i n e (intrinsic width I 2.5 keV) a t 1062
+
1 keV i n t h e summed-energy 180"-c.m. correlated two-photon spectrum from 5.95-MeV/N U + Th collisions.1. Experimental arrangement
In t h e preceding t d k , B. MUller mentioned our experimental r e s u l t s from 5.9-MeV/N U + Th collisions. We found 180"-correlated two-photon (YY) events with a summed energy of 1062 1 keV and with an i n t r i n s i c width of l e s s than 2.5 keV.
These events appear t o come from t h e decay of a neutral system which has a mean velocity equal (within 3 0 % ) t o t h e c.m. velocity of the nuclei (Be, = 0.056) and a velocity spread equal t o approximately one-third of t h i s velocity. (We express a l l velocities B r e l a t i v e t o c).
Figure 1 sketches the experimental arrangement. Fourteen Ge detectors, each 5.0-cm dia. by 5.0-cm and surrounded by Bismuth germanate (BGO) anticompton shi- elds,' were arranged i n t o seven detector pairs, each pair i n a plane containing the U-beam axis. The forward detectors of each pair were located such t h a t four had polar angles with respect t o the beam of 65", two 43O and one 87O. The partner detectors f o r each pair were positioned a t 174O with respect t o t h e forward det- ectors, t o take into account t h e expected aberration angle f o r photons emitted from a system moving with = 0.056.
A 0.32-cm lead shield was placed i n f r o n t of each detector and another 0.32-cm lead shield w a s placed over t h e faces of t h e t a r g e t chamber, t o attenuate a background of low-energy photons. Also, each detector was provided with a 2.5-cm long hollow conical lead plug which protected the BGO shield from direct t a r g e t gamma rays and which insured an effective 8.6°-half-angle opening i n t o the Ge detector.
Beam currents up t o 50 nA U4"+ from the Lawrence Berkeley Laboratory SuperHI- LAC impinged on 1-mg/cm2 r o l l e d Th targets.= The beam current was integrated i n a 2-m long Faraday cup which included t h e t a r g e t chamber. Total accumulation time f o r the data w a s approximately 150 hours. A sorting program selected coinci- dences between any forward detector and its 174°-correlated partner, a s well as t h e mirror partner of t h e l a t t e r , r e f l e c t e d i n a plane containing t h e beam. The l a t t e r coincidences a r e called ttuncorrelatedtt below; they s a t i s f y the same f i r s t - and second-order Doppler s h i f t conditions as t h e correlated coincidences.
2. Kinematics
The kinematics of the expected YY decay of a system moving with a velocity
= t / c is indicated i n Fig. 2. I f E, and E, a r e the lab. photon energies of any two 180"-correlated, equal-energy (E,) gamma rays emitted by an isolated system trav- eling with a velocity B, the expected counts l i e i n a wedge shaped window i n the
("1n collaboration with K. Danzmann. E.C. Montenegro. Xiang-Yuan Xu. E. Dillard, H.P. HUlskotter (Stanford University). P.S. Stephens. R.M. Diamond, M.A. Deleplanque, A.O. ~acchiavelli, 3. Schweppe.
R.J. McDonald. B.S. Rude (Lawrence Berkeley Laboratory). and J.D. Molitoris (Lawrence Livermore Laboratory).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987957
C9-316 JOURNAL DE PHYSIQUE El, E, plane centered on t h e parametrically expressed l i n e
gere, Y = (1
-
f3@ lI2 and 8, is t h e forward photon emission angle with respect t o 6 in t h e reference frame of the emitting system. From Eq. (1) one s e e s immediately t h a t t h e sum energyis a f f e c t e d only by t h e second-order sWt and is independent of t h e detector angle o r opening. The difference energy
is d i r e c t l y proportional t o 6 and depends on t h e detector angle and opening. D i s - t r i b u t i o n s i n velocity and angle of t h e e m i t t e r a r e s t r o n g l y r e f l e c t e d i n t h e dis- t r i b u t i o n of (E,
-
E,), whereas t h e " l i n e shapen (E, + E,) is only weakly a f f e c t e d by t h e velocitv distribution of t h e emitter.3; El + E? and El
-
E2 d i s t r i b u t i o n sI n order t o i n t e r m e t t h e event d a t a i n t h e E. E, lane. we assumed t h a t t h e e m i t t e r t r a v e l s w i t h - a mean velocity
Be
i n t h e beam-direction and, aboutBe,
t h ee m i t t e r has an i s o t r o p i c x o c i t y distribution and a momentum (1 p
1
) distribution3Here, p, determines t h e width of t h e momentum distribution. The values of Be and p, can be obtained from the event data, a s indicated below.
A t f i r s t , we chose t h r e e s o r t i n g windows s o t h a t 6, = 0, Bcm, and 3Bcr?, res- pectively. The corresponding c o r r e l a t e d summed-energy s p e c t r a a r e shown i n Fig.
3(a) t o (c) and t h e uncorrelated s p e c t r a i n Figs. 3(d) t o (f ). A prominent positron annihilation YY l i n e appears a t 1022 keV i n t h e c o r r e l a t e d s p e c t r a , because t h e 87, Ge d e t e c t o r (8, = 90,) is insensitive t o t h e velocity of the emitter. I f coin- cidences from t h i s detector pair a r e omitted, t h e 1022-keV l i n e appears only i n t h e Be = 0 window. A gaussian f i t t o t h e 1022-keV l i n e gives a l i n e width of 2.8 keV which is s l i g h t l y l a r g e r than t h e summed-energy resolution of our detector system of 2.1 keV (FWHM) a s obtained with radioactive sources. I n Fig. 3(b), cor- responding t o Be = Bcm, a narrow l i n e appears a t 1062 k 1 keV, which we believe t o be a candidate f o r t h e two-gamma decay of a n e u t r a l system. No o t h e r candidate could be found i n our s ~ e c t r a which extended t o 2000 keV.
4. Results
The important c r i t e r i a f o r a t r u e YY l i n e are: (1) according t o Eq. (2), it must have a width c l o s e t o t h e detector resolution. On t h e other hand, a cascade of accidentally nearly equal-energy gamma rays from a source moving with Bcm would give a l i n e width of -7 keV a t 1000 keV, averaged over t h e Ge detector open- ing angles. Such cascade coincidences a r e found below t h e 1062-keV i n a l l the s p e ~ t r a . ~ The 1062-keV l i n e has a width of 3.4 keV; it is shown i n Fig. 4. Super- imposed on Fig. 4 is t h e l i n e shape as given by a r e l a t i v i s t i c Monte Carlo calcula- tion using an i s o t r g p i c momentum distribution of t h e emitting system according t o Eq.(4) centered on Be =
, , 3
with p, = 0.02 mc. Isotropic e m i s s i ~ n of photons i n t h e r e s t frame of t h e emitting system was a l s o assumed. (2) A t r u e YY l i n e must not appear i n t h e uncorrelated spectra. The 1062-keV l i n e does not appear i n t h e s e spectra. (3) A t r u e YY l i n e must appear with e x a c t l y t h e same energy and width i n each s e p a r a t e c o r r e l a t e d detector pair. Unfortunately, t h e s t a t i s t i c s of t h e d a t a were insufficient t o make this t e s t f u l l y meaningful. However, we were able t o divide t h e detector p a i r s i n t o two groups, one consisting of t h e four pairs with forward d e t e c t o r s a t 65, and t h e o t h e r group composed of t h e t h r e e remaining det- e c t o r pairs. The 1062-keV l i n e appears i n each group although with reduced s t a - t i s t i c a l significance. (4) The l i n e should not appear i n t h e Be = 0 window, other- wise it could be due t o a s t r o n g l y c o r r e l a t e d cascade from a radioactive o rslowly moving nucleus. A 0'-0+ t r a n s i t i o n from a nucleus a t r e s t would appgar a s a 2.1-keV wide l i n e i n all Be windows; i f the nucleus were moving with Be
>
Bcm, t h i s l i n e would be a t l e a s t 7 keV broad. (5) I n s o r t i n g t h e d a t a on El-E,, we s h i f t e d t h e origin of E,-
E, s o t h a t f o r each d e t e c t o r El-
E, =+O corresponds t o t h e energy difference expected f o r a system moving with e x a c t l y Be along t h e beam direction. We then chose various values of Be. A symmetric El-
E, distribution was obtained only f o r Be = (1.1 i: 0.3) Bcm. The difference energy distribution is shown i n Fig. 5. To improve t h e s t a t i s t i c s , t h e d a t a f o r El-
E, positive and neg- a t i v e were averaged. Superimposed we show r e s u l t s of t h e above mentioned Monte Carlo calculation with p, = 0.02 and 0.05 mc. If p,<
0.01 o r p,>
0.04 mc, a f i t can no longer be obtained with t h e experimental data. (6) The l i n e must be s t a - t i s t i c a l l y significant.Figure 5 indicates t h a t t h e number of counts i n t h e 1062-keV l i n e increases as the [El
-
E,] window is opened. The maximum number of counts is 200, but t h e optimum peak t o background r a t i o is 167/680. Hence, t h e l i n e appears t o be s t a - t i s t i c a l l y significant. Including possible systematic uncertainties i n t h e detec- t i o n efficiency, we f i n d t h e t o t a l production cross section of the 1062-keV YY l i n e t o be (50 k 25) pb averaged over t h e I-mg/cm2 t a r g e t thickness o r , equiva- l e n t l y , over t h e p r o j e c t i l e energy i n t e r v a l from 5.75 t o 5.95 MeV/N. We a l s o f i n d t h a t any higher-energy 2.8-keV wide YY l i n e has a production c r o s s section of less than 3 ub. Possible YY l i n e s below 1022 keV would be masked by t h e l a r g e back- ground of nuclear cascades.' None of t h e narrow excursions i n t h e s p e c t r a below 1022 keV have a s t a t i s t i c a l significance g r e a t e r than 30.E, F i g . 2
Fig. 1. Arrangement of Ge detectors. Dashed locations have detectors below
and'
above plane of figure. A c o r r e l a t e d d e t e c t o r pair (1,2) and an uncorrelated p a i r (1,2') a r e indicated.
Fig. 2. Location of expected events f o r a t r u e YY decay on an E l , E, diagram, where E l and E, a r e t h e lab. photon energies [Eq. (111.
JOURNAL DE PHYSIQUE
Fig. 3. Summed-energy photon s p e c t r a i n all Ge d e t e c t o r pairs. I n t h e i n s e r t s , t h e v e r t i c a l s c a l e is compressed by a f a c t o r of ten. The prominent 1022-keV l i n e is due t o positron annihilation. The 1062-keV l i n e , appearing i n spectrum (b), meets a l l t h e c r i t e r i a f o r c o r r e l a t e d two-photon decay of t h e n e u t r a l system.
Fig. 4. Summed-energy photon spectrum corresponding t o t h e s o r t i n g window f o r Be
= Bo. Dashed l i n e is t h e l i n e shape,on a l i n e a r background,given by a r e l a t i v i s - t i c Monte Carlo c a l c u l a t i o n with p, = 0.02 [Eq. (4)]. The r e s o l u t i o n of t h e detec- t o r s has been taken i n t o account.
Fig. 5. Yield per keV of photons within t h e 1062-keV peak shown i n Fig. 3(b) a s a f u n c t i o n Of t h e energy d i f f e r e n c e E,
-
E, assuming t h e e m i t t e r moves with a mean v e l o c i t y 6, = Bern. E,-
E, = 0 corresponds t o t h e energy d i f f e r e n c e expected f o r an emitting system moving with e x a c t l y Be along t h e beam direction. Superimposed a r e t h e r e s u l t s of a Monte Carlo c a l c u l a t i o n with p, = 0.02 mc (dashed l i n e ) and p, = 0.05 mc (long dash-
d o t t e d l i n e ) [Eq. (4)l.This work was supported i n p a r t by t h e National Science Foundation Grants NO.
PHY-86-14650 and INT-84-14671 and by t h e Director, Office of Energy Research, Divi- s i o n of Nuclear Physics, of t h e Office of High-Energy and Nuclear Physics of t h e U.
S. Department of Energy under Contract No. DE-AC03-76SF00098.
1. D e t a i l s of t h e s e d e t e c t o r s a r e given i n R.M.Diamond, i n Nuclear Science Research Conference S e r i e s , Vol
.
7, Instrumentation f o r Heavy-Ion Research, e d i t e d by D. Shapira (Harwood Acad. Pub., New York, 1985), p. 259.2. Produced by MicroMatter, 123 Madrona Lane, Deer Harbor, WA 98243.
3. T.E. Cowan, J.S. Greenberg, and EPOS collaboration, invited l e c t u r e a t NATO Advanced Studies I n s t i t u t e on Physics of Strong Fields, Maratea, I t a l y , June 1986, Yale p r e p r i n t 3074-927, Nov. 1986, A.W. Wright Nuclear S t r u c t u r e Laboratory, Yale University, New Haven, CN; T.E. Cowan, p r i v a t e communication.
4. W.E. Meyerhof e t dl, Phys. Rev. Lett.
