New measurement of 8B Coulomb dissociation and E2 component

NUCLEAR PHYSICS A Nuclear Physics A616 (1997) 123c-130~

,‘lkl)t.

Physics.

Rikkyo

tIni\rersity,

3 Nish-Ikebukuro.

~l~o~l~i~~la..I’ok>.o I71

.lapa~l

Tllr (‘oulomb dissociation of ‘B in the field of “‘sl’b \~a\ ~tlltiictl at arollnd 50 .\lc\./ll incident, energy. The astrophysical ,Cfactors for t hc ‘I+(~~.-, 1’13 rcactioll. a key of t,hc 5otar nrut,rino production. were deduced. Possible mist 11rv of t iic I-2 coiiipoiie~lt s are Cxpected to be small based on our measurement of angulal. cliht ributioll ilr a wide range. This supports our earlier analysis assuming pure El t raniit ions.

1. INTRODUCTION -I‘he solar prc&ctrd i11g tile

problem is defined as t hr discrcpanc>~ l)(‘t \vwn the measured and yields from the sun [I]. Tl 1e ‘B~(p.7 )‘I3 watt ~OII is essential for estimatsolar-neutrino flux. Xmo~~g the follr vzist illg tlrvltrino measurements. [‘2] and Kamiokande [3] ex p eiiniriil b detect t IIC wlar ncilt rinos originat

neutxino

nrutrino high-energy

the Homestake ing mainly and entirely radioactive ‘Be targets.

from ‘B. respectively. There ww Their ext,racted cross scctionx

or

hi\; clircct nleasurements using a~trol)ll~xical .“-factors. Sl;.

disagree by about 30% at maximum. and t11e\ remain as 011~~01’1Iif‘ lnoit unccrtairt illput to st.andard solar models. is one of the alternative IIICI Ilorls to stlldy the radiative The (‘oulomb dissociation[l] capt urc process 7Be(p,T)8B. The projectile *B bombards a higIl-% t argel such as ““PI, and t Ile breakup process sB+‘Be+p is measured. .\ssuming t IIP tlollrinance of tllv (‘oulomb clisintegration mechanism, the rea.ction is regarded as air abwrption of a \ irtual photon: then the C’onlomb dissociat,ion yield is related to the radiat i\xt c.al)t IIW cross section. The luethod has advantages of large cross section and possibilit!, of Il\ing a thick target. High vrlerg~- cllarged particle detection and the use of the htablr “‘hPt) target are also appealing feat iire‘s in crt,racting accurate cross sections. Hence LV(~aw -t ~iclyii~g the ( ‘oulo~nt~ tlissociation of “13 at RIKEN. 199 I are reported. 2. THE FIRST

Here the results

of

OIU

1w

ezpvrilrwllt

5

pe~for~~ltd

in 199L’ and

EXPERIMENT

7‘he first experiment was performed at RlIiEN t II? L”“Pb(8B,7Be p)“OsPt, reaction was measured

cross sect ion fol [;.(i]. I Ibecoiilcidfww II radioact i1.v “u beams. which WV i1ltwac.t i011 at !)I \Ie\‘/u incident provided by the beam line RIPS [?I. The “C‘+“Rr ulerg~~ was used to produce radioactive “B beams of L’x 10’ SC’. which then bombarded a .iO nig/cmL ‘OSPb target with 99.S%# enrichment. The a\wagccI CYICT~J. of “13 in the target was 46.5 \l~~\~/u. The detector wa.s a 1 x0.96 111’ plastic ,c.ittl itlalor tiodoswpe consisting 03759474/97/$17.00 1997 - Elsevier Science B.V. PII: s0375-9474(97)00081-x

nit

124~

7: Motobayashi/Nuclear

Physics A616 (1997) 123~~130~

of trn 5-mm-thick

AE strips

GO-mmthick

and sixteen

1; Ixtrs as shown

in Fig. 1. It was

set 5 m downstream of the target and the t,ime-of-flight was ~neasuretl for each reaction product. Helium gas filled the volume between the target and detector in order to reduce background reactions compared with the case where the volume is filled with air. The time-of-flight and the position of hit,s for the breakup prodllcts. prot,on and ‘Be, were used to determine the relative energy &,I, which correspond? to t hc ccntel.-of-mass energy E,,,,, of the ‘Be(p.y)sB reaction. The scatt.ering angle 0s was also determined from the above quantities. 150

-

Figure plastic

lm-

x

n-

1. Schematic view of the scintillator hodoscope

Figure 2. The astrophy4ca.l 5’,;-factors deduced from the first experiment together with existing (p.2)

The resultant

astrophysical

S17-factors

the values measured by Vaughn demonstrated that this Coulomb the ‘Be( p,? )‘B reaction 3.

NEW

1

data.

are shown

in Fig. 2. They

are consistent

with

ef al. [S] and Filippone ff rtl. [$I] within the errors. It dissociation provide:, a11 alternative method for studying

at low energies.

EXPERIMENT

3.1. E2 component in the ‘B Coulomb Possible E2 contribution to the Coulomb

dissociation tlissociatioll rcslllts

was discussed

bj- man?

authors [lo-121 after our first result was distributed by preprints. The ‘BF’(P.J)~B reaction is dominated by El p-emission through continuum states. The E2 amplitudes are very small, but t,hey are enhanced in the Coulomb dissociation ~~rocess. On the other hand. the Ml t.ransition is suppressed in the Couloml:, dissociation. while it might have a sizable contribution in the direct (p,~) process. Langanke and Shoppa’s estimate for E2 transition

7: Motobayashi/Nuclear

Physics A616 (1997) 123c-130~

125~

is 25% of the observed

Coulomb dissociation cross section [lo]. Gai and Bertulani. on is the other hand, proposed a smaller contribution [II]. Al.so a small E2 contribution were predicted by Type1 and Baur [la]. 0 verall effects of bot Ir I1 I and E2 transitions in the (‘oulonil~ dissociation reaction and considered in Refs [5,6]. The E2 component the hll contribution in the (p-2) reaction are theoreticall!. e~tinlated. The corrected and lmcorrected S1; values are shown respectively by closed a11tl op~w circles in Fig 2. The difference is small reflecting the fact that the hI1 and F.2 corrections are in the same order of magnitude for the continuum region, and hence the two corrections are rough11 cancelled. One of the difficulties in estimating the Ml and E2 corrections wa.s due to the fact that no experimental information wa.s available so far and different theoretical nlodels predict the amplitudes that vary by up to a factor of 4 [5] Therefore experimental determination is desirable. Fig. :I shows predictions for various components espectf,d to colttribllte to the Coulomb dissociation process. The amplitude for each component is t akrll from the prediction of I\im. Park and E;im [13]. A s seen in the figure. the differential cross section (da/do) for the E2 component increases as the scattering angle 0s increases up to the grazing angle. \vhere 61s is defined as the angle of the P-~B~ center-of-mass with respect to the beam direction. This is in contrast to the El contribution that peaks at \.ery forward angles. .l%erefore. the angular region near the grazing angle is scnsitil-rx to the (=2 components a11(1 is useful for the determination of the E2 contribution to t hc (‘oulon~b dissociation CUM section. As is also shown in Fig. 3. the nuclear excitation. which introduces an lincrrtainly in the analysis. is also important for the angular momentunl transfer (=2 besides the E2 Coulomb excitation. Therefore. the second c,zpcriment \vas tlesignrd to corer a wider angular range in order to be scnsitil-e to tl~c, i=:! angular tli~trihution. 3.2. Experiment For the second experiment [Id], the detection system was almost the same as in the firit experiment. The averaged energy of sB in thr target was 51.9 hlrL./u. slightly higher than 16.3 MeV/u used in the first experiment. ‘fhc dctcction system with a few modifications to t,he one in the first experiment was used. l‘hr, I~otloscope detectors near the hues stopper (see Fig. 1) were replaced by five AE and six F: scintillators with smaller width so as to reduce the counting rate per each strip tlur to thtx beams. The flight path was shortened to 3.1 m so that the detector systclll co\~x a larger angular range up to Hs z 10”. For comparison, the efficiency in the first cspcrilnellt \\-a\ essentially zero in the \,icinit!- of 0s = 6”. .AltZhough the shorter detector distance sl~oultl degrade the angular and 14ocitv rcsolut ions. and in turn the relative-energ!, resolution. KC obtained a sufficient Aat ive energy resolution by improving the angular resolutio11: this was achieved. in part. I,y nsillg the scintillators with sma.ller width. and ill 1)at.t. hy good timing pick-off fhlect ronics. For exaInple. the la energy resolutions arc I20 ke\. at E,.rl=.500 kc\‘ and I .?O ke\’ at E,,~=l..i I%-\‘: the la angular rrsollltion for H, i\ around 0.S” depending hliglrt 1~.on the angle. Fift!.-four identical KaI(T1) scintillators surrouiltlctl t Iif, I argrt Eat-ll crystal is of 6x6~ 12 cn? volume with a resolution of 7.3% fol, the 662 kc\’ ; ray. This D;\LI [l:‘] \flt “11 measured the cleexcit ation -, rays from tlic first czcilt~(l slate- of ‘Be at 429 kc\. pop111atrd in the “B+‘Be+p dissociation process. 111a(l(lit ion. tl1fx D.\LI arra!. was used

7: Motobayashi/Nuclear

126~

I’,

‘I

1,.

‘,

I ”

”

I

Physics A616 (1997)

“‘I’

Erel= 1 (MeV)

123c-130~

- (4

’

.

target

with

I”“I~“‘I”“I~“’ 208pb(8B.7Be

p)‘“‘Pb

.

Em=51 9 Me”,u

5000

0

2

4

8

8

10

0 0

0.5

%,(ded

Figure

3.

Predict,ed

angular

1

1.5

2

2.5

3

Er&+‘)

distrihu-

tion for the Coulomb dissociation ‘The El (dashed curve), E2 (dotted components and the coherent sum and i=‘L nuclear components (solid

of “B. curve) of E2 curve)

are shown. They are calculat,ed quantum mechanically. The E:! component predicted by the semi classical formula [1] is also shown for comparison.

Figure 1. (a 1 E:rperiment.al C’oulomb dissociation \-ieltl as a function of relative energy. Thr three histograms are the yields with and without the ‘“sPb target and the subtracted ~+4tl. (1~) Doppler-corrected energ!. spectrum of -, rays measured bJ the D.\I,I detector froln NaI scintillators detected

to measure the -, rays associated with the inelastic excitation of tlte target nucleus ‘OsPb during the dissociation process. No significant yield \vas ol)\er\-ed above O..i MeY. .L\ la upper limit for the target excitation for the first excited state of “‘“Pb at 2.61 MeV was extracted to be O.% of the total breakup yield. Figure 4(a) shows the dissociation yield due to the target. which is obtained by subtracting the dat,a wkhout the target from those with the target. The yield without t,arget is mainly due to the dissociation reactions in the hrlittttt gas. 7’he dissociation yield is further corrected for the loss of fragments due to nuclear reactions in the detector materials and by the population of the 4%ke\’ state of ’ Be. \rhich was clearly observed as shown in Fig. l(b). The yield for the 429 lie\.-peak corwsponds the “B+‘Be+p dissociation events observed in the eslwitnent 3.3.

to approximately

5% of

Angular distribution and E2 component Experimental angular distributions are shown in Fig. .? as a function of the scattering angle Hs for three relative energy bins. The yield labelled as t da/d@ is related to the original cross section do/d0 through the respottsc fttttction that accounts for the detection efficiency, angular spread of the bratrt. mttlt il)le sc,at tering in the t,arget. and uncertainties in the determination of time-of-fligltt and posit iott of hit for each particle. ?-lie response function was obtained by a lZIonte-( ‘arlo sitttttlat iott and usrd in calculating the theoretical El and C=2 curves shown in Fig. 1s. l‘hr nuclear contribution introduces possible uncertainties in the fitted E2 amplitude. brcarw it dominates the f=2 distribu-

T. Motobayashi/Nuclear

tiou aud drpmds

rat her strongly

Physics A616 (1997) 123~~130~

12lc

on the choice of t lw opt ical Imtrllt inls.

amplitudes are determined by \’ fits WIMYY~ a11 O~I t hc “O+2”“Ph scattering [lci] is adopted.

it11t1 E_’

ic,ai

I)otclllt

I‘lle mwall

ICI

ial clr~trrmiurd

1)~

100

10-l

500-750

10-2

keV

G T

10-l

e % \ $

10-2 1250-

1500

keV

10-S 10-l

10-2

2000-2250

10-Z 0

2

keV

4

6

6

10

68

Figure .5. Observed cross sections tdu/d8 as a function of H, for t I~rec dative energy bills of .iOO-730 lieV (a). 1250-1500 lirLy (1I ) and 2000-Z?iO lit%\. t c). Tire cross sectioli is the product of the detection efficiency 6 and diffewllt ial cm\< WC-~~OII drr/dhJ. l:or the ~IITYPS. see iri t Iir test.

T. Motobayashi/Nuclear Physics A616 (1997) 123c-130~

128~

The best fit values and upper limits (la) of the (=2 mixture are ext.ra.cted using the ‘.A” [16] and .*B” [17]. The correspondcalculations with two different optical potentials. ing E2 ,S-factors, S17(E2), are shown in Fig. 6 toget.her with the predictions by Type1 a.nd largest E’S and Baur [12] and Kim, Park and Kim [1:3]. which yield the smallest amplitudes, respectively, among the existing models. The best fits yield S1;(E2)=0 for I$.,. <1.6 MeV and the lu upper limits are lower than the predictions for E,,l <2 MeV. Maximum corrections to the S1T-factors obt,ained by assuming pure El Coulomb dissociation mechanism are calculated from these upper limits. They are. for example, 2’%, 4%) and 37% for the potential “A” or 1%. 2% and 33% fol the potent.ial “B”, at. L&1=0.6 MeV, 1.4 MeV and 2.1 MeV, respectively.

I

’ ‘-““‘I

’ “““‘I

b+=F%; Kim, Park and Kim (x l/4)

800

1200 Ecm (kev)

I

1600

SUMMARY

’

“7

keV

7

“\

2000

Figure 6. Extracted E2 S-factors. The shaded area indicates allowed region at la limits for the case of analysis with the optical potential “B” (see in the text). For comparison. the E2 predictions based on the nuc1ea.r structure models of 8B by Kim. Park and Kim [13] (divided by a factor of four) and Type1 and Baur [12] are also shown at a few energies (the lines are just to guide eyes).

4.

~=KOO

Figure 7. \‘irtual phot.on numbers plotted as a function of the incident ‘B energy for Ml. El and E2 transitions.

and DISCUSSIONS

The Coulomb dissociation of the 8B nucleus was studied to extract the ast.rophpsicad Sfactors for the 7Be(p.y)7Be reaction relevant to the high-energy solar-neutrino production. The results demonstrates the usefulness of the method. Esperiment,al angular distributions are decomposed by C=l (El) and E=2 (EP+nuclear) components with the help of x2 fits. The extracted amplitudes for the f?=2 transition are considerably smaller than any supports t,heoretical predictions (at least for I&l < 2 MeV). This small r=:! component

129~

i? Motobayashi/Nuclear Physics A616 (1997) 123c-130~

our first analysis

with

pure

El

transition

in estractillg

tlw .5,;-factors

[i.cj]. if the

.\ll

is small as expected from t.he small E2 component ‘1’llc.w rwults encourages our furt,her effort for IIIOI’Paccarwtc tleterminat ion of Xl; 11~. t IIC (‘oi~lor~rl~dissociation method. .A new experimciit Ilit\ 1wei1 carrifd 0111 n-ith vacutim \rlGrll rrplacetl the helium ga.s. to yield a better sigllal-to~l~acl;gi~o~intl ratio. These data arc’ ~LO\Vheillg evalliatrd and are expected to allow us to e’\;tm~d t Ilrx (‘oulomh dissociation

cx)lnponcnt

w\ulth illrt her t.o lower energies and to provide an accurate \-~IIIv of .\‘I:. I,‘inall!-. a remark 5honld be made on the 111 contribution to t II~ ‘Be(p.: )“H reaction. lo l~‘or accurate dctcrniinatiori of $7 by the ~‘ouloml~ tliswciar ioil. it i\ also tlvsirahlr IIICHSUITthis XI1 component. although the preseilt small 1~2 aml~lit II&~ ma!. leatl to xrnall predicted 11 I alItI F2 c~ot~c~t ioIls are wrwiatcd eac.11 11 I lllixt IIW Iwcause the theoretically otllcr a11d tcv~ci to be compensated [.;I. \Ve started all r~sl,t~~illl(‘lltal atutl! at (:Sl at a11 5 10~11 iir I-ig. ‘T. t Iif’ ixllat i\-f> iniJwrtauc_c of t lit’ iilc~irlwit “IS energy of 2.30 YIF\~/u [lS]. :I.,5 :l \I 1 i’oirll)oiiw~t increases as the energ>- increases. l~lir~~~~lorc~ t 110 C:Sl mrawlwiwiit wo~iltl allol~ for a clrtermination of the Ml component Tllt~ Inwent \vork was performed in collaboration wit IL T. I
.,/lJ/j. \I. (;ai. R. France

III. k.1. Hahn.

Z. Zhao ( litlc

l uirr ~..%ity).Th. Delhar.

1’. Lipnil;

and (‘. 1iicliotte (Louw7in lu .Yf urf). ‘This work ~vas ~~~pport~~~l fr01n Ilir \linistr!. of l~~clucatio~~. Science. Sports and (‘nlturr I,!, Grant-Ill-:1itl t’ol, Scielltific Rrw~1rc11on Priorit!- .1was nndrr the program number O.j%310:3. alit1 some t 1.a\x4eqmisc’s aw 5npportc4 i:~ 1)art I)\. C:rant-iw.\id for Int.ernational Sciellt ific l~cwarc~l~ (.Jc)illt Research) r111t1rrthey 1)rograin ‘~~r~inlx~ 060-41203.

REFERENCES 1 2.

.ktrophysics, ~‘aml~ritlgr I~iii\xwit~- l’rcss. T~K 1ork. IW. 7th \ITorkshop on Grand ITnificat ioIl. .I O,I.~III~. .JapaIt I%6. cd. .J.

.1.X. Uahc-all. Neutrino I?. l)a\-is.

I’wc.

.\~~af’r~iic(\I.orld Scientific. Singapore): l<. Laiiclc. t31ill. .\irw. Phy~;. Sot. 35 ( 1993) IT!);: R. F. (‘Loveland. Nucl. Phys. R 95 (1995) 17. 11. 1i.S. Hirata rf 01.. Phys. Rev. Lett. 63 (1989) 16: I<. IIIOIIV. llt\.itrd Talk. 2Sth Kmcom t iw tic, Aloriond “Electra \$:eak Interactiou aild I~ilific~l Ihwrics”. Les .I\i-cs. Sa\-oir. l.‘raliw. Ylarcti 13~20. 1993: 1’. Suzuki. X11(.1. Ph>.>. I3 :IS j I!FJi) 51. 1. (i. f.\. A458. ISS (I!)S(i). .i. .I-. \lotohayashi ff nl., Phys. Rev. Lctt. ‘i3 (I!)!) 1) XSO. (i. S. Iwasa rt nl.. .J. Phys. Sot. Jpn. 6.5 (1996) l?i(i. 7. I. Iiul~o ft ;4’. F..l. \.allghn.

!I.

trl..

SIIC~. Instr. Sleth. I3 70 (1992) :W). R..1. (‘hahners. I). I
I fi.JT. I%.\\‘. lCilippone, / l!NI)

I IT: Phys.

S..J. Elwyn. (‘..\r. David\. Rev. (’ ‘8 (1983) “‘2.

alit1 Ij.1).

Pl1~5. Kfs\-. (’ 2 (1970)

Itt. ,50

13oc

10. K. Langanke 11.

12. 13. 14. 15.

T Motobayashi/Nuclear Physics A616 (1997) 123c-130~

and T.D.

Shoppa,

Phps.

Rev. (’ 49 (1!1!44) RlXl:

Errat.um,

Phys.

Rev.

C 51 (1995) 2844; Phys. Rev. C 52 (199.5) 1709. >I. Gai and C.A. Bertulani, Phys. Rev. C’ 52 (199.5) I TM. S. Type1 a.nd G. Baur, Phys. Rev. C 50 (1994) 2101. 1C.H. Kim. M.H. Park, and B.T. Kim. Phys. Rev. (’ :%i (1987) 323. T. Kikuchi tt nl., submitted to Phys. Lett. B. T. Nishio tt al.. in preparation; T. hlotobayashi rt nl.. Phys. Lett. B 346 (199.5) 9.

16. J. Barrette et al.. Phys. Lett. B 209 (19X8) IS?. 17. 11. Buenerd rf al., Kucl. Phys. A 424 (1954) 313. 1s. E. Grosse tf nl., .Coulomb dissociat,ion of “B into (unpublished).

‘Brtp”.

GSI proposal

(1994)