Li7+p interaction and excited states of Be8

Nuclear Physics 36 (1962) 597--614; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

LiT+p I N T E R A C T I O N A N D E X C I T E D S T A T E S OF Be s S. CAVALLARO, R. P O T E N Z A and A. R U B B I N O

Istituto di Fisica dell' UniversitY, Sezione Siciliana dell' I . N . F . N . and

Centro Siciliano di Fisica Nucleare, Catania, Italy Received 13 April 1962

A b s t r a c t : The Li 7+ p interaction has been extensively studied and its various p r o d u c t s analysed. The results confirm the presence of the well k n o w n levels of Be a at 2.9, 17.63 and 18.15 MeV and give clear evidence for the existence of the discussed 7.56 MeV level. Three new levels of Be s h a v e been discovered a t the energies 13.91, 17.9 and 18.0 MeV. Widths, a n g u l a r m o m e n t a , parities and isobaric spins of several levels have been determined Some anomalies of the L i 7 + p interaction, previously observed by others, are explained.

1. I n t r o d u c t i o n The Li 7(p, x)He 4 reaction ~vas the first artificial nuclear reaction discovered 1) a n d a v e r y large a m o u n t of work * has been done on the interaction of protons with Li 7. The L i T + p interaction m a y involve the i n t e r m e d i a t e state of Be s and i n f o r m a t i o n on the excited states of this nucleus m a y be derived 2-10). In the cases of partial de-excitation of Be s b y emission of y radiation, the shape of the a spectra and the b e h a v i o u r of the Li7(p, 7~) cross sections allows us to obtain evidence on the excited states of Be s. In this work we examine the spectra of charged particles from the p r o t o n bomb a r d m e n t of n a t u r a l lithium, b u t we have limited the analysis to the ~ particles with e n e r g y higher t h a n ~ 3 MeV, because at lower energies the Lie(p, a) H e 3 peaks mask the alpha peaks due to the LiT(p, y~)He 4 reactions. For this reason the present m e a s u r e m e n t s on ~ particles give evidence only about energy levels in Be s for an excitation e n e r g y above 5.6 MeV. We h a v e also m e a s u r e d the energy spectra and angular distributions of the high e n e r g y y-rays in the LiT(p, y) reaction at various p r o t o n energies between 0.45 and 1.42 MeV.

2. E x p e r i m e n t a l P r o c e d u r e 2.1. B O M B A R D M E N T O F Li T A R G E T

A b e a m of m a g n e t i c a l l y deflected protons from the H.V.E.C. 2 MeV Van de G r a a f f of our l a b o r a t o r y was focused on a thin t a r g e t of n a t u r a l metallic lithium * F o r a general s u m m a r y , see refs. 2,3). 597

598

S.

C&VALLARO el lgl.

20O

~-V=4ov

/ ~

150

_£3

E I00

r-

t-

50

O

0

I

I

I

I

I

1

2

3

4

5

e=(MeV)

Fig. 1. Relative pulse-height, given b y a 256-channel analyser, as a function of t h e a-particle energy a t different values of the reverse bias.

l

~

~

I

=

!

I

~

'1 L~ (d,Ll-~

~

200 •

protons

X

deuterons

J

-

150 E E

100 E E c"

50

l

o

I

~

I

I

4

I

I

6

I

I

~

I

I

~o

I

E rM, "~12

Fig. 2. Relative pulse-height in the a n a l y s e r at 30 V of reverse bias as a f u n c t i o n of p r o t o n s , d e u t e r o n s and c~-particles energy (see text).

LiT+p INTERACTION AND EXCITED STATES OF Be~

509

(0.05 mg/cm2), evaporated on a thick brass support and quickly transferred to the accelerator extension, so that the oxidation was negligible. The Li target made an angle of 30°with respect to the direction of the incident protons. The proton energy was varied during the experiments from 0.4 MeV up to 2.2 MeV and was always stabilized to 0 . 1 % in the range 0.7-2 MeV. The stabilization outside these limits was good to m 1 % . 2.2. D E T E C T I O N O F C H A R G E D R E A C T I O N P R O D U C T S

The charged particles emitted at 90 ° from the direction of the incident protons were detected with an n-p silicon function detector x*,l~) having a sensitive area of 20 mln 2 (type C, supplied by R.C.A. Victor Co). The detector was placed 32 cm from the target, so that the angular spread for emitted particles was only 0.8 °, with a useful solid angle of about 2.10 -4 st. The voltage pulses from the detector (of the order of several millivolcs) were sent to the input of a preamplifier with a negative feed-back and a gain of 100. The pulses from the output of this preamplifier were sent to the amplifier of a 256-channel analyser. 2.3. C H A R A C T E R I S T I C S O F T H E

JUNCTION DETECTORS

We checked the linearity and resolution of our detectors at several reverse bias voltages, using a Pu 239 x-source and interposing a layer of air of constant thickness at variable pressure. The results are given in fig. 1. The best resolution was found at about 30 V of reverse bias. The linearity of our detectors was successively checked to about 12 MeV for x-particles and to about 2 MeV for protons and deuterons, with 30 V of reverse bias. These results are shown in fig. 2. The points for protons and deuterons were obtained with particles accelerated by the 2 MeV Van de Graaff accelerator and scattered by a thin foil of aluminium ( < 0.1 mg/cm2); the points for the alpha particles with energy up to 5.15 MeV were obtained from the Pu as9 x-source. The points in the range from about 7 MeV to about 10 MeV were obtained with the alpha particles of Li 7(p, x), and the points beyond 10 MeV were measured using the Li6(d, x) reaction. The intrinsic resolution turned out to be better than 1 % . 3. R e s u l t s 3.1. E N E R G Y

SPECTRA

We detected and analysed groups of x particles, emitted at an angle v~ = 90 ° in the laboratory system for proton energies between 0.4 and 2.2 MeV. Besides the x particles due to the reaction Li7+HX--~He4+He *,

600

S. CAVALLARO

et al.

we also observed, at some proton energies, ~ particle groups t h a t could have been produced only b y a cascade reaction of the t y p e L F + H L * B e S * - - > y + B e s~×~,

(2)

BeS~Xl-->He4+He 4.

(3)

The production of other charged particles b y proton interactions with Li T, O le and C 12 is energetically impossible. Fig. 3 shows an energy spectrum of the He groups obtained at Ep = 0.85 MeV. The 0¢- )eaks observed below about 2.8 MeV are due to the Li e (p, He3)He 4 He p a t t i c t o s a t

~"g0 °

E p = 0.85 HeY 7

L,(H,H,IH, 6

1

4

I

4

4

Li(H,H,IH,

3

¢P

L~(H', ~',H,4JH:

O p-

z o

(x4) 100

7(

,

4"I

4

,

L~t.H~2H,)H~

2.2

2A

2.6

2.8

3.6

3.8

4,0

6.7

6.9

7.1

,,

8.8

9.0

9.2

E (M v) Fig. 3. E n e r g y s p e c t r u m of t h e He g r o u p s , e m i t t e d a t 0 = 90 °, a r i s i n g from t h e i n t e r a c t i o n of p r o t o n s a t Ep = 0.85 McV w i t h t h e Li 7 a n d Li~nuclei. The v a l u e s of t h e p e a k s a t E = 3.77 MeV a n d 6.95 MeV a r e m u l t i p l i e d b y 4.

LiT+p INTERACTION AND EXCITED STATES OF Be 8

601

reaction, whose Q value is 4.022 MeV, and are of intensity comparable with that of the most energetic ~ peak of reaction (1). Other charged particles arising from the Li s plus proton interaction could not have energies higher than about 1 MeV. The spectrum in fig. 3 also gives clear evidence for two other peaks at E=I = 3.77 MeV and E=2 ---- 6.95 MeV arising from disintegrations of type (3). From the dynamics of reactions (2) and (3), when one 7 quantum is emitted, we obtain for the energy of an ~ particle emitted at 90 ° in the laboratory system the following equation: E~

E~e --

2

Ep

0.094

16 + T

hv

E~e

sin v~ cos q~+

cos va (4)

2 "M~c z

neglecting terms of the order of (h,,)~/4M,,c2. The energies are in MeV; 0.094 MeV is the energy difference between the ground state of Be s and two free ,¢ particles; va and ~ are the 7 quantum polar and azimuth angles in the laboratory system; E~e is the excitation energy of the ~-emitting state of Be 8. Assuming no polarization effects and averaging with respect to ~, we obtain: E~e : 2E~+ 1 Ep --0.094 MeV.

(5)

There is an uncertainty of the order of ½ hv~/Ep/2M~c~. By eq. (5) the energies of the two excited levels of Be 8 at Ep : 0.85 MeV are E'el = 7.56±0.05 MeV (this corresponds to the peak at E~I : 3.77 MeV) and E~e 2 : 13.91+0.03 MeV (this corresponds to the peak at E~2 : 6.95 MeV). From eq. (4) we also see that in a cascade reaction of the types (2) and (3), the effect of the recoil imparted to the Be 8 nucleus by the y radiation produces a maximum broadening in the ~ peak of the order of hv~E~/2M~c 2, when the y-rays are emitted at 90 ° from the incident protons. This effect, not very significant for broad levels, gives a maximum broadening of about 0.2 MeV for groups coming from narrow levels. For forward 7 emission with respect to the incident protons, the recoil effect is completely negligible. In our experiment the instrumental and target broadening were small with respect to the recoil broadening of the ~ groups peaked at E~I : 3.77 MeV and E~ :

6.95 MeV.

Our experimental results indicate that the width of the level at 7.56 MeV, previously estimated as 1.7 MeV and 2.0 MeV, (refs. 4,5)), isless than 0.4 MeV (fig. 3). The width of the new level at 13.91 MeV is analogously estimated to be less than 0.5 MeV (fig. 3). 3.2. D I F F E R E N T I A L

CROSS

SECTIONS

The lithium targets were bombarded with proton beams of 0.1 to 0.01 #A.

602

s. CAVALLARO el al.

We used these low beam intensities in order to avoid appreciable variations of the target during the bombardment. Measurements were repeated using several targets and the experimental results were in good agreement. Analysing the yields of the ~ peaks observed in the energy spectra as a function of proton energy, we can obtain the differential cross sections at 0 = 90 ° 0.2

0.2

0.5

0.I

0.4

0.3

io 02

u~

0.1 r0 .o

°o II

I

t I

I

/.~, p ~J '~Vl

8

Be IZ253

IZ63

I 18.15

18.9 1 9 1 1 9 . 2

t 18.0

Fig. 4. D i f f e r e n t i a l cross s e c t i o n s a t 0 = 90°: a) for t h e Li ~(p, c~)He4 r e a c t i o n , b) for t he Li ~(p, ~ ~) }Ie 4 r e a c t i o n , w i t h Y2 t r a n s i t i o n from t h e e x c i t e d i n i t i a l s t a t e t o t h e f i n a l e x c i t e d s t a t e of Be 8 a t 13.91 MeV b r e a k i n g u p in t w o ~-particles, c) for t h e LiT(p, ~ l ~ ) H e ~ r e a c t i o n w i t h ~1 t r a n s i t i o n f r o m t h e e x c i t e d i n i t i a l s t a t e t o t h e f i n a l e x c i t e d s t a t e of Be 8 a t 7.56 MeV b r e a k i n g u p in t w o or-particles.

L i T + p INTERACTION AND EXCITED STATES OF Be 8

603

for the LiT(p, a ) H e 4 reaction and for the Li7(p, y~)He* reactions involving the 7.56 MeV and 13.91 MeV excited states of Be 8. Fig. 4 shows the differential cross sections for the three observed processes in the L i 7 + p interaction, as a function of the p r o t o n e n e r g y Ep a n d of the corresponding excitation e n e r g y of Be s. The LiT(p, ~)He 4 reaction cross section (fig. 4 ( a ) ) d o e s not show sharp resonances for Ep up to 2.2 MeV. The yield at 90 ° for this reaction was m e a s u r e d b y H e y d e n b u r g et al. le) for Ep from 1 to 3.75 MeV and shows a b r o a d p e a k at E~ = 3 MeV, which is i n t e r p r e t e d in terms of a level at 19.9 MeV in Be 8 with j = 2+. The absolute differential cross section for the LiT(p, a ) H e 4 reaction at 90 ° was m e a s u r e d for Ep between 1 and 1.5 MeV b y F r e e m a n et a117). Fig. 4 (b) is the Li 7(p, Y2 ~) He4 reaction cross section for Y2 transitions from the initial excitation state to the excited Be s level of E~e = 13.91 MeV, involving the disintegration into two alpha particles from this e n e r g y level. The cross section presents a small resonance a r o u n d Ep = 1 MeV, a step between Ep = 1.3 MeV to Ep = 1.6 MeV and a continuous rise at higher proton energies. The Li7(p, Yl a) He* reaction cross section, for Yl transitions to the level of Be s at E~e = 7.56 MeV followed b y the distintegration into two alphas, is shown in fig. 4(c). This cross section presents a v e r y s h a r p resonance a r o u n d Ep = 0.85 MeV, corresponding to an excitation e n e r g y in Be s of a b o u t 18 MeV. The a s y m m e t r i c shape of this resonance can be explained b y t a k i n g into acc o u n t the 17.9 MeV e n e r g y level in Be s we found in the LiT(p, y) reaction (see subsect. 3.4). 3.3. THE SOFT

y

RADIATION

In order to check the p r o t o n energies, especially for the new observed resonances in the L i 7 + p reaction, we have carried out a m e a s u r e m e n t of the well k n o w n ~1,22) excitation curve for the 478 keV Y rays coming from the LiV(p, p' y) process. T h e y rays were d e t e c t e d with a 3.8 c m × 3.8 cm NaI(T1) c r y s t a l m o u n t e d on a RCA 6342 p h o t o m u l t i p l i e r t u b e and the o u t p u t was fed into an R C L 256channel analyser. T h e spectra exhibit a peak at 478 keV and the excitation c u r v e at ~91ab ---120 °, if not corrected for G a m o w factors, is v e r y similar (as shown in fig. 5) to the usual one 21,22). Similar m e a s u r e m e n t s were m a d e on fluorine in order to check the p r o t o n energies. 3.4. THE HARD y RADIATIONS FROM LiT(p,y) REACTIONS The excitation c u r v e for h a r d y r a d i a t i o n from p r o t o n b o m b a r d m e n t of lithium was m e a s u r e d b y m a n y a u t h o r s 21-2s) and shows a v e r y s h a r p resonance

604

s. CAVALLAROet

al.

at Ep = 0.441 MeV and a smaller and wider resonance a r o u n d Ep = 1.03 MeV. These m e a s u r e m e n t s were all m a d e at fixed angles a n d w i t h o u t discriminating between the various h a r d energies (often for E~ > 5 MeV). The angular distributions for y-rays of e n e r g y higher t h a n 5 MeV were given b y K r a u s Jr. 21). The v a r i a t i o n of a s y m m e t r y with p r o t o n e n e r g y in the y radiation was studied b y several a u t h o r s 2o, 21, 26). Tile two h a r d y - r a y lines for transition to the g r o u n d state and to the first excited state of Be s were resolved using y - r a y pair spectrometers 26,27) and y - r a y i

i

--"" ,, o

c

i

i

1

i

i

L:(p 'P'9

(22)

Pres.

I

Data ~

•

c

o

>

..J

// J 0,6

0.8

I 1.0

I

I 1.2

I

I 1.4

Eo (.ov) Fig. 5. Excitation function for the 478 key y-rays from the LiT(p, p'y) process. scintillation spectrometers2S,29), b u t the pulse-height distributions of these two lines were studied at a few p r o t o n energies and at some ~ angles only. The energies of the two lithium h a r d y-rays increase with p r o t o n e n e r g y 27). As the a n g u l a r distributions t o g e t h e r with the t o t a l cross sections for these two resolved h a r d y radiations can give useful i n f o r m a t i o n on the resonant states of Be 8, we decided to e x t e n d our s t u d y to include the h a r d v-rays from the Li 7 (p, y) reaction. W e detected the h a r d y-rays b y means of a scintillation spectrometer. The scintillator was a cylindrical N a I (T1) crystal 5.1 cm x 5.1 cm coupled to an RCA 6342-A p h o t o m u l t i p l i e r o p e r a t i n g at 700 V. The pulses were analysed b y an R C L 256-channel amplifier. E a c h m e a s u r e m e n t of the y - r a y spectra was r e p e a t e d twice and was carried out for a fixed n u m b e r of m o n i t o r i n g counts (106) of x-particles p r o d u c e d in the LiT(p, ~)He 4 reaction. The ~ particles were d e t e c t e d b y a silicon n-p junction detector. At Ep = 0.45 MeV the n u m b e r of m o n i t o r i n g a particles was 105. The thickness of the lithium t a r g e t in these m e a s u r e m e n t s was 0.1 m g / c m 2.

L i T + p INTERACTION AND EXCITED STATES OF Be 8

605

The measured background, obtained by proton bombardment of the thick brass support without lithium during several hours, was completely negligible for Ez > 3 MeV. We used the mono-energetic y lines from Cs137(0.669 MeV), Coe°(1.17 and 1.33 MeV) and Po-Be source (4.43 MeV) to test the spectrometer response also at lower energies. .....J~ - - ~ - -

L~+p

Ep = 0.45 MeV

II

"a'f,,.,Jp'9.*

~=0°

2+

a 0

0

////1/ii/~

__-----:;Y--"",,i

'l I

\

0 0

I 5

I 10

I 15

x 20

Fig. 6. Pulse-height distribution produced b y the y-rays from the Li'(p, ~) reaction and emitted a t 0 = 0 ° and Ep = 0.45 MeV. The experimental y - s p e c t r u m is represented b y the solid curve. The dashed curves are the e x t r a p o l a t i o n s for the two hard y - r a y separated groups. The s y m b o l 7K.8. represents the transition to the g r o u n d state of Be s and Yr.e.B. represents the transition to the first excited state of Be 8.

We obtained the ratio of the intensities of two hard y radiations from the areas of the spectra which give the pulse-height distribution. As one must take into account the imperfect resolution due to the small size of the crystal, use was made of an approximate extrapolation procedure similar to that described in ref. 2s). We preferred to avoid lengthy Monte Carlo calculations. Fig. 6 shows the experimental ~ spectrum obtained at 0 = 0 ° and at Ep ----

606

S.

eL a / , .

CAVALLARO

J(P,~') ~-;l'gj.to the g¢ounCl state OFeBe

Ep

~- ~(festo

the lit'st e x c L t e d s t a t e ol:eBe

([MeV~

|.02

a =0.30 b=0.30

b=CL20 i

a

A

=

=

i

=

t5 0

0 +

~,.=

1

b=0.30

O5

i

l

I

T _ ~

0.9

2

O ~= 3 0 ~ b=0.25

I

J

L5

=

=

=

I

'

'

i

i

0.5

0 0 4.-

1.51~

0.87

II

b =0.30

05

'

I

I 0.'3

LL~ L5 1.5 ~ I

T

T

1 0.82 I

b =0.20

0.5

I

J

i

k

/ =

I

0.5

l

a =0.25 b =0.05

Iit5~

0.7;~

~ b=0"15

(ZS I

0o

=

'

'

9o"

'

'

'

zso"

0.5

o*

IvY'(degrees)

L5

!

i

i so"

~

=

I I

0.5

ioo °

Fig. 7. E x p e r i m e n t a l a n g u l a r d i s t r i b u t i o n s of t h e t w o h a r d e s t 7 - r a y lines in t h e LiT(p, F) r e a c t i o n for Ep b e t w e e n 0.72 a n d 1.02 MeV. T h e c o n t i n u o u s c u r v e s f i t t i n g t h e e x p e r i m e n t a l figures a r e of t h e f o r m o ( 0 ) = l + a cos O + b cosSO.

Li~-~p INTERACTION AND EXCITED STATES OF Be 8

607

0.45 MeV (full line) together with the two extrapolated curves (dashed lines) corresponding to the ~ 17.6 and ~ 14.8 MeV y rays. The ratio of the areas bounded by the two dashed lines was equal to 1.7 in agreement with ref. 2s), which gives a value of 1.7+0.2. We think that the uncertainty in the determination of these ratios according to the adopted procedure was certainly less than 10 %. We analysed the angular distributions of the hard 7-rays to the ground state

t

,.t=30 o

%

'3 rt3 .L. 0

,,i-. t/)

.it_

'-

23

2

,,t-.. ..J

>,~ .t_.

o

.b

r'~

r'0 • l~.. ~

0 a.-

C3

0.5

1.0

1.5

I I q--on" : __/~-r r ~.d-_T_-E._V-7_

6.t-

"_i t " " i...a..~

7-

4

S-

I

J

I

2

, O.S

,

,I

,

h

,1 , 1.0

,

,

.

/ t 1.5

i

O.S

i 1.0

1.5

Ep (M 9 Fig. 8. E x c i t a t i o n f u n c t i o n s o f t h e L i T(p, F) r e a c t i o n f o r t h e t w o h a r d e s t 7 - r a y l i n e s s e p a r a t e l y r e p o r t e d f o r Ep b e t w e e n 0 . 4 5 a n d 1.42 M e V , a n d a t t h e 0 a n g l e s o f 0 °, 30 °, 90 ° a n d 150 °.

608

S. CAVALLARO et al.

and to the first excited state of Be s from Ep = 0.45 MeV up to Ep ~- 1.42 MeV. Fig. 7 shows these angular distributions at some proton energies between 0.72 and 1.02 MeV. The curves of fig. 7 are expressed as a(v~) ~- W(v~)/W(90°) l + a cos v~+b cos 2 v~. The values of the a and b coefficients are shown in fig. 7. In subsect. 4.4., we also analyse the angular distributions by a linear combination of Legendre polynomials. The excitation functions of LiT(p, ~) reaction for the two hard y-rays groups are separately reported in fig. 8 as a function of proton energy. These differential cross sections, given at four different angles, show the systematic presence of three peaks at about 0.72, 0.87 and 1.02 MeV proton energies, corresponding to resonant states of Be s at about 17.9, 18.0 and 18.15 MeV. The present results show a more marked decrease in the LiT(p, y)excitation functions than that found by other authors 2~,22,26) at proton energies above 1.1 MeV. We found for Ep ~ 1.1 MeV other hard y-groups of energy lower than 9 MeV. probably due to LiT(p, 7) as well as Li6(p, ~) processes. No attempt was made to estimate the intensities of these groups. 3.5. A L P H A

PARTICLES

FROM

FLUORINE

The Li7(p, y) resonance at Ep = 0.85 MeV was attributed 23,24) to a target contamination by fluorine. We also detected the ~ particles from the F19(p, e) reaction emitted at ~ = 90 ° in order to exclude influences from fluorine in our experimental results. A CaF2 target of 0.03 mg/cm 2, evaporated on a thick brass support identical to that used for the lithium targets, was bombarded with protons from 0.7 to 1.1 MeV. We observed the ~0 particles emitted in the transition to the ground state of the 016 residual nucleus, with energy varying from 7 to 7.25 MeV while the proton energy increased. The relative intensities of ~o particles varied greatly with bombarding energy (see ref. 13) for general summary). The 0% e2 and % particles, emitted for transitions to excited states of O le having energies lower than 2.33 MeV, showed different resonance 3) and the 0cenergies increased with the proton energies. As these characteristics of the F~9(p, ~) reaction are very different from those of the Li 7(p, 7 0~) reactions we conclude that contaminations from fluorine in the present experiment m a y be excluded. 4. D i s c u s s i o n 4.1. T H E LiT(p, ~ ) H e ~ R E A C T I O N

The shape of the e x c i t a t i o n curve and the angular distribution expressed in the form Y(v~) = Y(90°)(l+Acos2v~+Bcos4v ~) indicate that t w o levels of Be ~ are i n v o l v e d ~, 16, is) in this reaction.

LiT+p INTERACTION AND EXCITED STATES OF

Be 8

609

Besides, for low proton energies (up to some MeV) it is assumed that only p and f wave protons are interacting, because the nuclei of Be s break up into two ~ particles and can only form in states with even J and even parity. The experimental Y(zT) distribution does not show odd powers of cos ~ as expected for two interfering states of the same parity. The th eo r y is) is based on the assumption that the reaction in this energy region proceeds via interference between 0 + and 2+ states of the compound nucleus. The 2 + state at 19.9 MeV with width about 1 MeV corresponds to the broad peak for Ep ~ 3 MeV in the excitation curve me), while the 0 + level is only required to be v er y wide 2,1s). The behaviour of our experimental cross section for the Li T~ , ~)He 4 reaction (fig. 4(a)) does not show sharp resonances for Ep between 0.4 and 2.2 MeV. This indicates, and confirmsls) (for Ep > 1 MeV) that in the energy range of Be 8 excitation from 17.6 to 19.2 MeV there are no narrow states having both even J and parity. 4.2. T H E Li~(p, 72~)t{e 4 R E A C T I O N

The behaviour of the excitation curve we obtained (fig. 4(b)) for this reaction presents a mean trend increasing with the proton e n e r g y , a small resonance peaked around Ep ~ 1 MeV and a minimum at Ep ~- 1.1 MeV. The rise of this cross section is more marked at proton energies above 1.6 MeV and does not show sharp resonances up to Ep ~- 2 MeV. These results indicate the influence of the levels around 17.9, 18.0 and 18.15 MeV. At Ep above m 1.3 MeV, the suggested 2,1s,~s) very broad 0 + and 2 + states m a y be of importance. The LiT(p, 72 ~) He4 reaction proceeds via 7 transitions from the initial excited states to the excited level of Be s at 13.91 MeV. This level, which breaks up into two alpha particles, is a state of even pa ri t y and total angular momentum 4.3. T H E LiT(p, Yl °QHe4 R E A C T I O N

In our experiments this reaction exhibits a marked and sharp resonance peaked at a proton energy of 0.85 MeV; we do not find any evidence of this type of reaction at Ep above 1 MeV (fig. 4(c)) up to 1.5 MeV. The resonance indicates a state of Be s at 18.00±0.03 MeV not observed previously. Additional evidence for this state is provided by the present results on the LiT(p, 7) reaction, as it shows two resonance states of Be S at 17. 89i 0. 03 and 18.02-60.03 MeV. The presence of this new lower state around 17.9 MeV could explain the asymmetric shape of the resonance. The LiT(p, 71 ~) He4 reaction, proceeding via 71 transition to partial de excitation of Be s, gives clear evidence of the much discussed 7.56 MeV excited state, breaking up into two alpha particles.

610

S. CAVALLARO et a l .

4.4. T H E LiT(p, ~) REACTIONS

The known results on the Li 7(p, 7) reaction for 7 radiation to the first two states of t3es show a sharp resonance at a proton energy 19,2o,23,25) of 0.441 MeV and a less sharp resonance 21,22,2s) at Ep ----- 1.03 MeV. The yields of elastically and inelastically scattered protons also exhibit also resonances at proton energies 19,22,34-3e) of 0.441 and 1.03 MeV. These resonances have suggested the corresponding energy levels in Be s a t 17.63 and 18.15 MeV. Analysis of experimental data at these resonance energies indicates that the two quoted excited states are formed by p-wave protons and are both of the 1+ ty p e 20,22,32, 3 5 , 3 6 ) . Theoretical interpretation of the resonance at 0.441 MeV requires a state of odd parity, non-resonant for the elasting scattering 22,34-36), at an excitation energy higher than 17.63 MeV. The influence of this state is felt in the resonance at 0.441 MeV. The known results at Ep -= 1.03 MeV for the LiT(p, p' 7) resonance and for the LiT(p, 7) resonance, superimposed to a slowly rising yield, usually called background, indicate an interference between the 1+ state at 18.15 MeV in Be 3 and a non-resonant state of odd-parity. The present results well confirm these resonant levels at 17.63 and 18.15 MeV and for the first time give evidence of two other resonant states with excitation energy around 17.9 and 18.0 MeV. The presence of these two other resonant states in proximity to the known level at 18.15 MeV, corresponding to a proton energy of 1.03 MeV, could explain the rising of the yield, in the Li7(p, 7) reaction between ~ 0.7 MeV and 1 MeV, usually called background. We confirm the presence of a strong interference term, varying with the proton energy, in the angular distributions of the two y-rays groups for transitions to the ground state and to the first excited state of Be s. We attribute this situation to the interference among the four Be s resonant states of 17.63, 17.9, 18.0 and 18.15 MeV excitation energy. The interpretation of the different levels is difficult as the half-widths are comparable with the level distances. In order to t r y at least some qualitative conclusions, we first put the differential cross section in the form: 2

W(~9) = X A~V~(cos ~9).

(6)

/o=0

The values of these coefficients of the Legendre polynomial (derived from the a and b coefficients of subsect. 3.4) are reported in fig..~, for the two hard 7 groups separately. The A 0 coefficients represent the integrated cross sectio~ls tor the two 7 emis-

611

L i ~ + p INTERACTION AND EXCITED STATES OF Be 8

sions, if we consider the azimuthal distribution to be uniform. The A o show peaks at proton energies around 0.44, 0.72, 0.87 and 1.02 MeV, as was expected from the excitation functions at various angles (fig. 8). The variations of the At coefficients with proton energies are very i ni'eresting. • t--A/~ to

i

for

the

i

ground

i

-,4~

ttan".ition

i

state

of Be 8

I

,

'

i

for

transition

excited ,

,

,

,

,

to I ' h e

of

stare

first

Be8

I i

,40

Ao

I I I I I I 4

%-.

Fitted/ " }",~\

tO .D

0

J

I

o

,g O O

.

l

,

l

0.5

@1

l

i

i

l

i

f

1.0 '

'

'

'

:-CaLc~!ate~d

I

l

'

'

'

'

I

',/5/. ,

i

i

L

'

'

' I}

'

J 05

I

~

I

J

curves. The A 1 points

i

i

,

'

I

'

'

I.S '

r

./~L

I 1.5

I

1.0

0.5

are compared

1.0

I.S

,42

t

. . . . 1.5

0.5

I.O

1.5

4(MoV)

F i g . 9. A ~ c o e f f i c i e n t s o f e q . (6} a s a f u n c t i o n dashed

i

\ 1.0

,I~ I ,

[_

A~

A2 0

|

Calculated

4 I

l

ID '

t 1

A,

T

2

0.5

i

0.5

of proton

e n e r g y . T h e .d o p o i n t s

with the calculated text).

solid curves

are :

~ed b y

t!~,

(see e q . (7) i n t h e

They never have negative values, as expected when the levels of the compound nucleus have half-widths comparable with the distances from one another. If we suppose that only the interference between these four levels of compound nucleus contributes essentially to the behaviour of A1, tile form of A t can be

612

s. CAVALLARO et al.

represented as a linear combination of cross terms in a Wigner m a n y levels formula. Thus, we have:

A1 __ ~, Ki j

i=

(E--Ei) ( E - - E s ) + F*I~J 4

(7)

1 , 2 , 3 , 4 ; / ' = 1,2,3,4,

where E~ and E~. are the experimental resonance energies, F~ and Fj are the corresponding half-widths, E is the excitation energy and Kij are the coefficients, the values of which depend on the reduced widths for the transitions from the various levels and on the Z and Zv coefficientsS7,38). We assume all terms, with the exception of E, are independent of the energy. There are some selection rules on the Kij, depending on the parities of the various levels. We see, in fact, that there are no contributions to the A 1 term by two levels of the same parity. In the actual case, the two levels at 17.63 and 18.15 MeV, being both of the type 2,s) 1+, have the same parity. Because of the non-vanishing of A 1, one or both of the other levels at 17.9 and 18.0 MeV must have odd parity. We indicate the resonant states of Be s at the energies of 17.63, 17.9, 18.0 and 18.15 MeV with the numbers 1, 2, 3, and 4 given to the i and j indices appearing in K , . We use relation (7) in order to find the K~ values and obtain the curves which provide a good fit of the experimental A1 points (fig. 2). For our calculations we use energy steps ~Ep = 0.05 MeV and put K14 -- 0, / ' 1 ~ 0.01 MeV, F 2 = 0.11 MeV, F 3 = 0 . 1 3 MeV and F 4 = 0 . 1 9 MeV. The results are not sensitive to variations of about 20 °/o of the assumed half-widths. In table 1 are reported theK,~values that give A~ and A'~ the shapes shown in fig. 9. TABLE 1 R e l a t i v e v a l u e s of K,~ for t h e r e l a t i o n (8) of Be s Transition to the ground 0+ state KI~ K1 a K~. K24 Ka4

Transition to the first 2 + s t a t e a t 2.9 MeV

-- 59

~

100 ~ 0 30 55

42 ~ 0 65 100

0

The coefficient A 1 refers to the v-transition to the 0+ g[ound state of Be s .and A'I refers to the y-transition to *_he 2.9 MeV excited ~tate of Be s.

LiT-kp I N T E R A C T I O N AND E X C I T E D STATES OF Be 8

613

From the relative K,~ values it appears that only K~3 is always about zero, while the other Kij values give finite contributions to the A I coefficients. From these results we can see that the two levels at 17.9 and 18.0 3leV both have odd parity. Besides, we expect that the total angular momenta of the 17.9 and 18.0 MeV excited states are both 1, because the existence of the relatively strong resonances in the LiT÷p reactions observed by us does not seem compatible with magnetic quadrupole transitions when 0 + final states are involved. Furthermore, the cross-sections at the resonances of these two new levels are in agreement with the hypothesis of electric dipole transition s which are attenuated b y the isobaric spin slection rules. In fact, if the two levels at 17.9 and 18.0 MeV have both T ~- 0, as expected on the basis of the analogy with the Li s and B s states 2,3) the presence of a very small amount of T = 1 impurities 39-41) could explain the intensities of the resonances in the LiT(p, ),) reaction.

5. Conclusions The results of the present experiments give evidence of four excited states in Be s at energies of 7.56, 13.91, 17.9 and 18.0 MeV, besides the well,known levels at 2.9, 17.63 and 18.15 MeV. The level at 7.56 MeV of the type 0 + has not been considered 3,9,10) or has been completely excluded 11-13) by m a n y authors. The present experimental results definitely establish the existence of this level. Present experiments give the first evidence of the level at 13.91 MeV, having even J and p ar ity and proves the existence of the levels at 17.9 and 18.0 MeV, certainly of odd p a r i t y and probably of the 1- type. Analysis of the angular distributions of the two hardest ),-rays from the Li 7 (p,),) reaction shows strong interferences among the four states at 17.63, 17.9, 18.0 and 18.15 MeV in Be 8. Our interpretations are in agreement with the assignment of T = 1 value for the level at 17.63 MeV and T = 0 values for the levels at 17.9, 18.0 and 18.15 MeV. Present results indicate also the absence of narrow states having even J and p ar ity in the energy region between 17.6 and 19.2 MeV in Be 8. The presence of the two new levels at 17.9 and 18.0 MeV allows us to state t h a t the Li7(p, y) yield between Ep ~ 0.6 and Ep ~ 1 MeV, usually called background 21,22,25), is actually due to the presence of these resonant levels. At proton energies between m 1.1 MeV and ~ 1.5 MeV, we do not confirm the relatively high yield in the Li 7(p,),) reaction for ),-transitions to the ground state and to the 2.9 MeV state in Be s found b y some authors21,22,25).

614

s. CAVALAAROet al.

We are greatly indebted to Professor R. Ricamo for his constant interest and active support in this work. Thanks are due also to Professor A. Agodi, Dr. G. Papini and Dr. G. Schiffrer for many helpful discussions. Finally, we should like to express our thanks to Dr. G. Calvi, Dr. D. Zubke and Mr. G. Caruso for their valuable assistance in doing the experiments. References 1) J. D. Cockcroft and E. T. S. Walton, Proc. Phys. Soc. 137 (1932) 229 2) W. E. Burcham, in Handbuch der Physik 40 (Springer-Verlag, Berlin, 1957) p. 1 3) F. Ajzenberg-Selove and T. Lauritsen, Nuclear Physics 11 (1959) 1, Ann. Rev. Nucl. Sci. 10 (1960) 409 4) H. T. Richards, Phys. Rev. 59 (1941) 796 5) L. L. Green and W. M. Gibson, Proc. Phys. Soc. 62 (1949) 407 6) W. F. Hornyak, T. Lauritsen, P. Morrison and W, Fowler, Rev. Mod. Phys. 22 (1950) 291 7) P. ErdSs, P. Scherrer and P. Stoll, Helv. Phys. Acta 26 (1953) 207 8) E. K. Inall, and A. J. F. Boyle, Phil. Mag. 44 (1953) 1081; E. K. Inall, Phil. Mag. 45 (1954) 768 9) R. Malm and D. R. Inglis, Phys. Rev. 92 (1953) 1326 10) E. ~V. Titterton, Phys. Rcv. 94 (1954) 206 11) R. E. Holland, D. R. lnglis, R. E. Malm and F. P. Mooring, Phys. Rev. 99 (1955) 92 12) E. C. Lavier, S. S. Hanna and R. W. Gelinas, Phys. Rev. 103 0956) 143 13) D. F. Gemmell, Austral. J. Phys. 13 (1960) 116 14) G. Dearnaley and A. B. Whitehead, U. K. A. E. A. report, Harwell, Aug. 1960 15) S. S. Friedland, J. W. Mayer and J. S. Wiggins, Nucleonics 18 (1960) 54 16) N. P. Heydenburg, C. M. Hudson, D. R. Inglis and W. D. Whitehead, Jr., Phys. Rev. 73 (1948) 241, 74 (1948) 405 17) J. M. Freeman, R. C. Hanna and J. H. Montagne, Nuclear Physics 5 (1958) 148 18) D. R. Inglis, Phys. Rev. 74 (1948) 21 19) W. A. Fowler and C. C. Lauritsen, Phys. Rev. 76 (1949) 314 20) S. Devons and M. G. N. Hine, Proc. Roy. Soc. 199 (1949) 56, 73 21) A. A. Kraus, Jr., Phys. Rev. 93 (1954) 1308 22) A. B. Brown, C. W. Snyder, W. A. Fowler and C. C. Lauritsen, Phys. Rev. 82 (1951) 159 23) L. R. Hafstad and M. A. Tuve, Phys. Rev. 48 (1935) 306" 24) L. R. Hafstad, N. P. Heydenburg and M. A. Tuve, Phys. Rev. 50 (1936) 504 25) R. G. Herb, D. ~V. Kerst and J. L. McKibben, Phys. Rev. 51 (1937) 691 26) IV[. B. Stearns and B. D. McDaniel, Phys. Rev. 82 (1951) 450 27) R. L. Walker and B. D. McDaniel, Phys. Rev. 74 (1948) 315 28) R. S. Foote and H. W. Koch, Rev. Sci. Instr. 25 (1954) 746 29) J. P. Longequeve, J. Phys. de Rad. 20 (1959) 37 30) W. D. Whitehead, Phys. Rev. 79 (1950) 393 31) H. Staub and W. E. Stephens, Phys. Roy. 55 (1939) 845 32) F. E. Steigert and M. B. Sampson, Phys. Rev. 92 (1953) 660 33) R. F. Christy and R. Latter, Rev. Mod. Phys. 20 (1948) 185 34) F. Mozer, W. A. Fowler and C. C. Lauritsen, Phys. Rev. 93 (1954) 829 35) W. D. Warters, W. A. Fowler and C. C. Lauritsen, Phys. Rev. 91 (1953) 917 36) E. R. Cohen, Phys. Rev. 75 (1949) 1463 37) L. C. Biedenharn, J. M. Blatt and M. E. Rose, Rev. Mod. Phys. 24 (1952) 249 38) S. Devons and L. J. B. Goldfarb, in Handbuch der Physik 42 (Springer-Verlag, Berlin, 1957) p. 362 39) L. A Radicati, Phys. Rev. 87 (1952) 521, Proc. Phys. Soc. 66 (1953) 139, 67 (1954) 39 40) W. M. MacDonald, Phys. Rev. 98 (1955) 60, 100 (1955) 51 41) G. Morpurgo, Phys. Rev. 110 (1958) 721, 114 (1959) 1075