Direct and resonance processes in 9Be(d, p0, 1)10Be and 9Be(d, t0)8Be at low energies

Nuclear Physics A250 (1975) 93 -- 105; ~ ) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

DIRECT AND RESONANCE PROCESSES IN 9Be(d, Po, 1) 1°Be AND 9Be(d, to)aBe AT LOW ENERGIES B. ZWIEGLII~SKI d

Institute of Nuclear Research, 00-681 Warsaw and A. SAGANEK, I. SLEDZI?4SKA t and Z. WILHELMI

Institute of Experimental Physics, University of Warsaw, 00-681 Warsaw Received 19 February 1975 (Revised 14 May 1975) Abstract: Angular distributions for 9Be(d, po)t°Be, 9Be(d, pl)l°Be and 9Be(d, to)aBe have been measured at 21 energies in the range from 0.9 to 3.1 MeV. The coefficients of expansion into Legendre polynomials of the experimental data and the DWBA cross sections calculated with energy-independent spectroscopic factors, have been compared. Arguments based on this comparison and the available data below 0.9 MeV are presented in favor of the hypothesis that DWBA falls to predict the energy dependence of the matrix elements associated with the deuteron p-wave. A broad resonance (F_~(I1B) = 17.3 MeV) strongly excited in the 9Be(d, px)X°Be reaction at Ed = 1.85 MeV is suggested to be responsible for the discrepancies. It is suggested that the narrow 16.43, 17.70 and 18.37 MeV resonances recently observed in 11B have isospin T = tE {

I

N U C L E A R REACTIONS 9Be(d, p), ((1, t), E -~ 0.9-3.1 MeV; measured or(E, 0), or(E), a. lOBe levels deduced spectroscop!c factors. Natural target.

[

I

1. Introduction The work performed on 9Be(d, po, z)l°Be and 9Be(d, to)aBe below the deuteron energy Ed = 3.5 MeV prior to 1968 has been summarized in the compilations of Ajzenberg-Seleve and Lauritsen 1). In more recent studies 2-4) the spectroscopic factors have been established by comparing the experimental angular distributions with the predictions of DWBA. Discrepancies exist 3.4) between the values of spectroscopic factors obtained in this low-energy region and those obtained at E d = 12 MeV (refs. 5,22)). This indicates that the reaction mechanism is not a pure direct transfer and that the traditionally adopted peak-fitting method of extracting spectroscopic information cannot be used. The applicability of Bowcock's 6) method is studied in the present work. In this method the coefficients of expansion into Legendre polynomials of the experimental data and the DWBA angular distributions, are compared. The structure of t Present address: Department of Physics, Warsaw Institute of Technology. 93

94

B. ZWIEGLII~ISKI et al. d

the high-order coefficients as predicted by DWBA is examined in sect. 3. It follows that the high-order coefficients are mostly determined by the peripheral partial waves which weakly penetrate the nuclear interior, provided that the kinematical matching condition between the entrance and exit channels is fulfilled. The reactions 9Be(d, Po) and 9Be(d, Pl) fulfil this condition better than the 9Be(d, to) reaction in the investigated energy range. For these two reactions we obtain, with the aid of Bowcock's method, values of the spectroscopic factors comparable to the theoretically predicted [ref. 7)] and observed values at higher energies 5,8). The low-order coefficients are used to determine the orbital momentum of the resonating partial wave.

2. Experimental The equipment used in this experiment has been described in detail in ref. 9) and only the most salient features will be repeated here. Deuterons have been accelerated in the Van de Graaff accelerator at the Institute for Nuclear Research. The calibrated magnetic analyser with 90 ° deflection and the N M R probe allow the incident energy to be determined with a precision of ± 5 keV. A scattering chamber of 30 cm inner diameter, equipped with two silicon surface-barrier detectors, has been used. One of them, clamped to the chamber side surface, served as a beam and target monitor, while the second one, mounted on the rotary upper lid, served for angular distribution measurements. The particle spectra have been accumulated in two Intertechnique multichannel analysers. The targets were prepared by vacuum evaporation of spectroscopically pure beryllium metal onto thin organic backings. The average thickness of a beryllium layer estimated from the amount of evaporated metal and the evaporation geometry was equal to 65/~g/cm z ( ~ 15 keV deuteron energy loss at 2.0 MeV). Typical particle spectra measured at Ed = 2.4 MeV and at 0(lab) = 30° and 0(lab) = 165° are shown in fig. 1. Each line of interest has kinematical crossover points with "intruding" particles originating from the target and/or the backing material, carbon and oxygen being the most troublesome contaminants. The measurements in the backward hemisphere have been performed twice at each angle. Lowering the detector bias converts it into a d E / d x counter for 12C(d, po) 13C and t 60(d, po)l 70 protons and allows for undisturbed measurements of the 9Be(d, to) group, while operating with a full depletion thickness allows for the determination of the intensities of the 9Be(d, Po) and 9Be(d, Pl) groups. At forward angles aluminium stopping foils separated the 9Be(d, C~o.x)TLi ~-particles from the 9Be(d, Po) protons. With the stopping foil inserted, the recoiling lithium nuclei and the 9Be(d, ~2)7Li group are simultaneously eliminated from the vicinity of the 9Be(d, Pl) line. Measurements at angles smaller than 15° have been possible only up to 2.2 MeV. Foils sufficiently thick to stop deuterons with higher energies adversely affected the energy resolution of the 9Be(d, Pl) group. The 9Be(d, pl) group has a crossover point with ~60(d, pt) 170, which shifts from 102°(lab) at 0.9 MeV to 55°(lab) at 3.0 MeV. Two measure-

9Be(d, Po. 1) 1°Be, 9Be( d, to) aBe

~_~

"*.,,

'- ,.9-2.

95

. "O

.,.,..

° "1~

. *O

o "O

"O

SBe ,d

% - 2.4 ~ev

10 i

e(tob)=:~:g,,

h

e(lab)=165*

II

''"

'"*'~*

-n../'*.*

to "E :3 0 10:

I

"6 E

"~

I

%

ooO ,

,J

,0~

:7

% .;

II '10

J Jr J,

"IO D . ' I 3

al I

ID

I

2.0

I ~0

I t~0

I

5.0

I

6.0

Energy

I

7.0

I

80

[MeV]

Fig. 1. Spectra of particles measured at the deuteron energy Ea(lab) = 2.4 MeV and at 0(lab) ---- 30* (upper spectrum) and 0 ( l a b ) = 165 ° (lower spectrum). The 30 ° spectrum has been displaced upwards by one decade. Note the reversed order of the 9Be(d, Px) and 160(d, Pl) lines leading to a crossover at intermediate angles.

ments have been performed at each angle in order to determine the intensity of the 9Be(d, Pt) group in the vicinity of the crossover points. In the first measurement with a beryllium target the intensities of the composite 9Be(d, P l ) + 160(d, Pl) line, denoted as I c in eq. (I), and the 160(d, Po) line, denoted as 12, have been determined. The second measurement with the aluminium oxide (A1203) target yielded the ratio of intensities 160(d, pl)/160(d, Po), denoted as K. The net content of the 9Be(d, Pt) line in the spectrum taken with a beryllium target, denoted as 12, is found from the relation: 12 = I t - K / 2 . (1) The thickness of the A12 03 targets has been chosen so as to match the deuteron energy losses in the corresponding beryllium targets. The complete spectra including the elastic scattering peaks have been measured at

96

B. ZWIEGLII~ISKI et al. d

each energy at 120 ° and 150°(lab) and the relative angular distributions have been converted into absolute ones using published ~o) elastic scattering cross sections. The statistical errors of the measurements, including the errors of monitoring, in most cases do not exceed + 3 %. The relative errors of the 9Be(d, Pl) cross sections contain additional contributions resulting from the uncertainty in the separation of the line from the underlying background (see fig. 1) and in the region of the crossover points from errors in the quantities entering eq. (1). The accuracy of the absolute 9Be(d, d) cross sections according to the authors of ref. 10) is equal to ___5 Yo. The numerical values of the cross sections obtained in this work and the errors associated with them are tabulated in ref. 1~). The total cross sections have been extracted from Legendre polynomial fits. 3. Legendre polynomial coefficients The X2 value, defined in the usual way, has been minimized at each energy with respect to the coefficients ck(Ed) of the expansion 1 km.x

o,x(E d, O) = 4n ~ ck(Ed)Pg(cos 0),

(2)

k=0

where o,.(Ed, O) denotes the expelimental differential cross section at energy Ed and angle 0, P~(cos 0) is the kth order Legendre polynomial, and k,.a. is the order of the highest, statistically significant coefficient in this expansion. The zeroth-order coefficient coincides with the total experimental cross section. For the DWBA angular distributions an identical form of expansion is used with the corresponding coefficients denoted by bk(Ed). The following formula (3) relates the coefficients bk(Ea) and the DWBA matrix elementsf~,~b:

bk(Ed) = 4rCND(Ed)(-- 1)'+~7-1SoW x

~,

£a£'.£b£~,

LaLbL'sL" b

x
(3)

where S o is the spectroscopic factor and the coefficient W = 1.5 ft/fi for (d, p) and W = 3.33 for (d, t) reactions; Je and J, are the spins of the residual and initial nuclei, respectively. By :~ we denote the statistical factor 2x + 1 and Re stands for the real part of the expression in parentheses. The La, L~ are the incident and L b, L~, the outgoing orbital momenta; I, j are the orbital and total momentum transfers, respectively. The term ND(Ed) denotes the normalization factor standing before the sum over the transferred momentum projections in eq. (21) of ref. 2i). The definition of the matrix element f~L~ is identical to eq. (17) from that work. In the reactions studied the transfer of a neutron with the orbital momentum l = 1 is involved. In this case the outgoing orbital momenta L b = L~ + 1 are allowed only for a given incident momen-

9Be(d, Po, x)l°Be, 9Be(d, to) 8Be

97

turn La. The potential parameters used to calculate the DWBA matrix elementsj~Lb are taken from table 2 of ref. 12). The total cross sections are shown in fig. 2. A strong resonance occurs in the 9Be( d, Pl) reaction at Ed = 1.85 MeV (Ex(I~B) = 17.3 MeV). The lower limit of the width has been estimated as Ft.=. >~ 1.0 MeV. The resonance effects are not so pronounced in the total cross sections of the 9Be(d,Po) and 9Be(d, to) reactions. However, we will see below that the anomalies in the remaining Legendre coefficients are correlated with its position. The resonance at Ed = 1.98--+0.05 MeV, with F = 225_+ 50 keV, has been previously reported by Battleson and McDaniels ~7) in the 9Be(d, ?) reaction. This also occurs in the 9Be(d, Po) but not in the 9Be(d,to) reaction, as may be seen from fig. 2. In the former two reactions the isospins in the exit channel may add up to T = ½, contrary to 9Be(d, to) for which only T = ½ is an allowed isopin value. The possibility that this and other narrow resonances recently observed in 9Be(d, p) studies 4,18) are the isobaric analogs is further discussed in sect. 4. In fig. 3 the energy dependences of the experimental and DWBA Legendre coefficients for the 9Be(d,Po) reaction are compared. Solid lines represent the results of calculations using eq. (3) with the energy-independent spectroscopic factor S = 2.26 ( S / S t = 0.96), where St is the predicted 7) value of the spectroscopic factor. This factor normalizes the calculated and experimental coefficients of sixth order at 2.4 MeV. One may note that the high-order (k = 5, 6 and 7) coefficients follow in a satisfactory way the DWBA predictions. Contrary to the DWBA predictions c~ falls off rapidly with decreasing energy, from positive values exceeding the theory at i

~

i

i

J

~- ~ -.

40

i

i

J

t

i

see (d,to)

30

./ /~ 2O

>-" I0

/

~ _....~ -

ix y._-s-- .~.~

~td,p°)

14

12

I0

E d [MeY]

F]g. 2. Total cross sections for the 9Be(d, Po. 1) lOBe and 9Be(d, to)SBe reactions. The background under the narrow 17.46 MeV resonance in the 9Be(d, po)l°Be reaction is indicated with the dotted line. Dash-dot curves serve to guide the eye.

98

B. ZWIEOLI]~$KuIet

aL

9 Be ( d~pO) rOBe '

I

'

I

'

I

i

I

'

I

'

I

mb

=

|

I

|

I

I

..:

-S S

-Ic

-.....

/-b,

0

"lOi

~

.

,'

c

-S

.'''"

.

.

CI •

•

.

.

e o e

.,,(

s

~' ,"

°

'

°

~ •

0j

C~ ,}.}..½"

• ½ °

½

•

1

"~°t

s

~

~

~cs i

~s

i

I

tz

r

I

ts

~

I

z0

,

I

z,~

,

I

z.s

Ed(lob) [MeV]

0.8

1.2

h6

2,O

[

2A

I

I

2.8

I

32

Ed(t~ ) [MeV]

Fig. 3. Comparison of the experimental (dots) and DWBA Legendre coet~cients (solid lines) for the 9Be(d,po)t°Be reaction. Black dots denote the experimental coefficients of second order. The D W B A coefficientshave been calculated using an energy-independent spectroscopic factor S / S t = 0.96. The theoretical spectroscopic factor (St) is due to Cohen and Kurath 7).

3.1 MeV towards negative values at the lowest energies with a zero-crossing point around 2.2 MeV. The sign reversal of this coefficient causes a rapid fall-offoftheexperimental cross sections at forward angles and a rise at backward angles. A strong energy dependence of spectroscopic factors obtained in ref. 3) by the peak-fitting method results from this effect. The coefficient c2 shows a non-monotonic energy dependence. It starts at low energies with the magnitude and sign nearly coinciding with the DWBA coefficient, then decreases towards a minimum around 2.3 MeV, and above this energy rises again towards the values predicted by DWBA. Our Legendre polynomial fits to the data of Bertrand et al. 13) in the range from 0.5 to 1.0 MeV show that c2 is the highest statistically significant coefficient below 0.8 MeV, and thus only the deuteron s- and p-waves are contributing at the lowest energies. Since the negative sigaa of c 1 is observed both below and above 0.8 MeV, where d-waves already contribute, one can conclude that the origin of the observed discrepancies rests in the incorrect description of the matrix elements associated with the deuteron s- and p-

9Be( d, Po. 1) '°Be, 9B¢(d, to) aBe

99

mb 5L

2L o.Q-

I t

)o

bl

b4

9Be ( d,Po)~°Be A- 7r.,d - ~ 9 l ~ V •-

*-

I ~

1.9 M e V 3.1 M e V

I

0.3 :

I

O,2

t

,IT

-~2 -0.3 -0.4 : -O,ll "~

-OJ -1.0 -2 -3 .4

op

li2 2p 3t~ ~ls oil 112 23 34 ~s 01 ~2 23 31~ , s s6 11o 21~ 312 ~p io 21 32 43 sl~ 110 2~ ~12 413 s~

~ 2~ aN 1ha 2h a2 ~

4s sI s14

Fig. 4. Contributions o f the individual matrix elements to the DWBA Legendre coefficients at three bombarding energies: 0.9 MeV -- triangles, 1.9 MeV -- black dots and 3.1 MeV -- open dots, calculated for the 9Be(d, po)l°Be reaction. The pairs o f numbers along the abscissa label the incident (first number) and the outgoing (second number) partial waves.

waves in the DWBA formalism. The non-monotonic energy dependence of c2 around the 1.85 MeV resonance indicates that the latter possibility is more probable. The contributions of the individual matrix elements to the selected coett~cients (k = 0, 1, 4 and 6) for the 9Be(d, Po) reaction are shown in fig. 4. We have obtained them by summing in eq. (3) over the primed orbital momenta, while the unprimed momenta were kept fixed. The pairs of numbers along the abscissa label the orbital momenta LaL b of the fixed matrix element. Contributions with absolute value smaller than 0.1 mb have been omitted. The matrix elements 10 and 12 are associated with the deuteron p-wave, which is suggested to deviate in experiment from the DWBA description. The p-wave contribution to the k = 6 coemcient is thus determined by the interference of the matrix elements 10-56, 12-54 and 12-56. Momenta L,,L b = 54 and 56 are the lowest orbital momenta which can fulfil together with 10 and 12 the triangular conditions imposed in eq. (3). Calculations performed with only the Coulomb terms left in the distorting potentials indicate that the matrix elements in which partial waves L~, L b > 3 are involved are equal in these two cases. Thus the width of the "window" occuring in the high-order Legendre coe~cients is determined by the model-independent factors, Coulomb matrix elements being essentially determined by the bound-state asymptotics. Among the partial waves which penetrate the

100

B. ZWIEGLINSKIet al.

nuclear interior, only the deuteron d-wave has strong influence on the magnitude of b6; note in fig. 4 the marked enhancement of the contribution of the matrix element 43 over that of 34 due to the constructive interference of 23 with 43. The agreement seen in the three highest Legendre coefficients thus tends to indicate that the matrix elements associated with the deuteron d-wave, i.e. 21 and 23, do not deviate strongly in experiment from the DWBA predictions. The bl coefficient reflects the cumulative effect of the interference of the matrix elements differing by unity with respect to their orbital indices. The positive sign of b t at 0.9 MeV and lower results from the predominantly constructive s-p interference. On the other hand the experimental data (cl in fig. 3) show the interference pattern changing from destructive below 2.2 MeV to constructive above this energy. In fig. 5 the comparison of Legendre coefficients for the 9Be(d, Pl) reaction is presented. Solid lines represent the results of calculations using eq. (3) with S = 0.182 (S/St = 0.805). The highest three experimental Legendre coefficients follow closely the DWBA predictions. Strong discrepancies are again observed between the coeffi9Be(d,Pl)~Be

mt

12

ts

2,o

zA

2.a

~2

Ed (Iobl EMeV]

o,s

t2

ts

2.0

2~

zs

32

Ed(tob~[MeV]

Fig. 5. Comparison of the experimental (dots) and DWBA Legendre coe~cients (solid |ines) for the 9Be(d, pl)l°Bc reaction. An energy-independent spectroscopic factor S/St = 0.805 has ?coon

used. Dash-dot curves serve to guide the eye.

9Be( d, PO, 1) a°Be, 9Bc(d, to) sBe

I01

cients of first and third order. The maximum for ca is observed at 2.1 MeV. while a deep minimum occurs for Cl at 1.8 MeV, somewhat below the resonance energy established on the basis of the total cross sections for this reaction. The results of calculations for the aBe(d, to) reaction are compared with experimental coefficients in fig. 6. The DWBA results have been normalized to the value of e4 at 2.95 MeV with S = 0.70 (S/St = 1.206). For this reaction the signs of all statistically significant coefficients are positive, in agreement with the DWBA predictions. One may observe, however, that DWBA tends to underestimate the rate of decrease of the first two coefficients. The fits to the data from ref. 13) below 0.9 MeV revealed that ct changes sign to negative values at 0.6 MeV and cz at 0.8 MeV, where the calculated b I and b 2 remain positive. Strong enhancement of c 3 over the DWBA predictions is seen with maximum deviations occuring around 1.9-2.0 MeV. The width of the "window" and therefore the efficiency of suppression of the lower partial waves strongly depends on the momentum matching between the entrance and exit channels. With increasing momentum of the outgoing particle the barrier effects for the higher partial waves become less effective in the exit channels and the )

°Be( d,t mb

'

'

=

/'(

mb

= " " "{'

t_

C

eBe I

I

j

I

l

l

l

3c 20 •

•

~,

°

o 0

//,"

r~.4. •a

i

I

L2

i

I

t6

,

I

2~

t

I

2~ Ed

i

I

zs

i

(Lob} [ M e V ]

3.z

~

°

t

t6

o

4-'4 i

I

2.O

,

t

,

I

2.~ 2.8 Ea{t,~) [ M e V ]

Fig. 6. Comparison of the experimental (dots) and DWBA Legendre coefficients (solid lines) for the 9Be(d, to)sBe reaction. An energy-independent spectroscopic factor S/St = 1.206 has been used.

B. ZWIEGLINSKI et aL

102

TABLE 1 Spectroscopic factors for the 9Be(d, Po. t) t°Be and °Be(d, to)eBe reactions

Nucleus

Level

St a)

S

Ed (MeV)

Ref.

10Be

0 +(g.s.)

2.36

1.67

12. 0

s)

2.356 1.78 2.43

5.0 2.8 3.1

s) 2) 3)

2.26

0.9-3.1

b)

10Be

SBe

2 +(3.37)

0 + (g.s.)

0.226

0.58

0.24

12.0

s)

0.274 0.20 0.18

5.0 2.8 3.1

s) 2) 3)

0.18

0.9-3.1

b)

0.51

11.8

22)

0.77

2.5-3.1

3)

•) Theoretical, ref. 7). b) Present work, high-order Legendre coefficients. "window" widens, allowing the low partial waves to contribute. The increased momentum of the outgoing triton in 9Be(d, to) gives the p-waves a chance to contribute destructively to high-order Legendre coefficients. This is marked by the downward deviation of cs and c6 in fig. 6 from the DWBA predictions around the resonance energy. When one goes off resonance towards higher energies the matching condition is gradually restored and the contribution of the resonance is diminished. In table 1 the available information on spectroscopic factors is summarised. The spectroscopic factors obtained by the peak-fitting method at 3.1 MeV and those resulting from high-order Legendre coet~cients in stripping channels are in general agreement with previous determinations. 4. D i s c u s s i o n

The reaction which at low energies depends in a selective way on the distribution of the deuteron p-wave strength is deuteron radiative capture to the low-lying negative parity states of liB accompanied by E1 ?-emission. Del Bianeo e t al. 14) have compared the predictions of the direct one-step capture mechanism with the experimental data on the 9Be(d, ~)tlB(g.s.) transition. The experimental cross sections are found to exceed the calculated ones by more than one order of magnitude. Thus the direct capture hypothesis has been rejected and the enhancement has been ascribed to the T = ½ part of the giant dipole resonance. The energy range where the maximum of the capture intensity is observed overlaps the energy range covered in the present experiment. One can thus assume that the anomalies found here are also associated with the semidirect processes involving the giant dipole resonance. As far as we know,

9Be(d, Po. 1)t°Be, 9Be(d, to) sBe

103

TAnI~ 2 Narrow resonances in 11B (15.82 ~ Ex ~ 18.4 MeV) as possible candidates for analog states x1B(y, p)XOBe a) Ex (MeV)

9Be(d, p) zOBe F~ (MeV)

/ ' (keV)

9Be(d' 7,)11B b) Ez (MeV)

/1 (keV)

16.2 16.5 16.9 17.5 17.8 18.2

16.43 d)

17.46 a) 17.70 t)

9Be(t ' p) t 1Be e) Ex (MeV)

r ' (keV)

3.41 (15.96) 3.89 (16.44)

1454-30 --< 10

3.96 (16.51)

154-5

5.25 (17.80) 5.86 (18.41)

454-10 ,~, 300

~ 40 d)

,~, 200 ¢) ~ I00 t)

17.44

2254-50

18.37

3204-100

a) Ref. 19). b) Ref. 17). c) Ref. is). d) Ref. is). ") Present work, sect. 3. t) Ref. 4). methods have not yet been developed to treat in a unified way the direct transfer and the compoend transitie~ via a strong collective state. One may note, however, that the (d, particle) data may add important information about the phase relationships. This cannot be inferred from the (d, ?) data since an "isolated" level is involved and the interference terms are lacking. In the second and third columns of table 2 the relevant information is collected on narrow resonances in lIB excited in the 9Be(d, p)t°Be and9Be(d, ?)tlB reactions in the excitation energy range from the 9Be+d threshold to 18.4 MeV. Hitherto only the barrier penetration factors have been considered to be responsible for their narrow widths at these high excitations. In the sixth column of table 2 we present the energies of the llBe (T, T, = ½, ½) states established in ref. is) via the 9Be(t, p)tlBe reaction together with the expected positions of their analogs in l tB (in parentheses). The energy 12.55 MeV has been taken as the position of the ground state analog according to refs. t 6). In the seventh column the parent widths are given. The comparison of the positions and widths of the 9Be+d resonances with those from the 9Be(t, p)HBe spectra strongly indicates that the resonances at 16.43, 17.70 and 18.37 MeV are the isobaric analogs. The 16.43 MeV resonance may correspond to either one or both members of the narrow 3.89-3.96 MeV doublet in ~Be. It appears that the isospin selection rules determine their narrow widths. The T = states in l~B can decay by particle emission in an isospin-allowed way only to the T = 1 states of mass-10 nuclei. In the first column of table 2 resonances excited in ~~B(?, Po, ~)t°Be reaction (see ref. 19)) are listed. The level at 17.70 MeV excited in the 9Be(d, p) is observed +) to decay exclusively to the ~°Be(2 +) first excited state. The same is true for the 17.8 MeV resonance in the photoproton work. Thus a definite correspondence between these two sets of levels does exist in spite of the fact that the energy definition in the latter case is inferior. On the basis of the angular distributions of photoprotons, J~ = ~+

104

B. ZWIEGLII~ISKI et aL d

has been ascribed in ref. 19) to all listed levels except for the 16.9 MeV to which J~ ----~- has been assigned. The efficient excitation of the positive parity T = ½ states in an isospin-forbidden 9Be + d reaction may thus reflect the isospin mixing of the T = ½ giant resonance with the isobaric analogs. In the present case they represent a sort of fine structure of the giant resonance. Among the ones listed in table 2 the 17.5 MeV resonance is the strongest in the 11B(7, p) and thus a candidate for an isobaric analog according to the foregoing arguments. The fact that it has been probably excited also in 7Li(~t, n)l°B (ref. 20)) does not necessarily invalidate this suggestion. There the integrated yield of neutrons has been measured by the longcounter method and selective decay to the T = 1 states of : °B cannot be excluded. The reasons why its counterpart is not excited in the 9Be(t, p) reaction are not evident.

5. Conclusions

The conclusion is made that the 1.85 MeV resonance manifests itself in all three reaction channels, giving rapidly changing interference patterns when the energy is varied from below to above its location. The excitation by deuteron p-waves and positive parity is suggested. The T = ½ part of the giant dipole resonance in ' 'B may be responsible for the p-wave effects. In 9Be(d, Po, 1) t°Be reactions in the energy range from 0.9 to 3.1 MeV, satisfactory agreement has been found between the high-order experimental and the DWBA Legendre coefficients calculated with fixed spectroscopic factors. These have values comparable to those theoretically predicted 7) and previously obtained at Ed = 12 MeV (ref. s)). The validity of Bowcock's method stems from the fact that matrix elements associated with the deuteron p-wave weakly contribute to these coefficients. As a result of the deuteron p-wave contribution enhancement due to the resonant process the isospin-forbidden excitation of the positive parity T = ½ states in ttB is expected to occur. A comparison of the level positions of the parent ttBe with the narrow resonances in 11B has been performed. This indicates that the 16.43, 17.70 and 18.73 MeV resonances are the isobaric analogs. Positive parity has been previously assigned to them. The authors thank the staff of the Van de Graaff Laboratory and particularly Dr. Jask6la for their cooperation in running the accelerator. We would also like to thank Dr. Turkiewicz for many helpful discussions.

References 1) T. Lauritsen and F. Ajzenberg-Selove, Nucl. Phys. 78 (1966) 1; F. Ajzenberg-Selove and T. Lauritsen, Nucl. Phys. A114 (1968) 1 2) M. L. Roush, F. C. Young, P. D. Forsyth and W. F. Hornyak, Nucl. Phys. A128 (1969) 401

9Be(d, Po. 1) x°Be, 9Be(d, to) sBe

105

3) A. Saganek, I. Sledzi6ska, Z. Wilbelmi and B. Zwi~gli6ski, Prec. Int. Conf. on nuclear physics, eel. 1, ed. J. do Boer and H. J. Mang, August-September 1973 (North-Holland, Amsterdam, 1973) p. 436 4) E. Friedland, H. W. Alberts and J. C. van Staden, Z. Phys. 267 (1974) 97 5) J. P. Schiffer, G. C. Morrison, R. H. Siemssen and B. Zeidman, Phys. Rev. 164 (1967) 1274 6) J. E. Bowcock, Prec. Phys. Soc. A68 (1955) 512 7) S. Cohen and D. Kurath, NucL Phys. A|01 (1967) 1 8) D. L. Powell, G. M. Crawley, B. V. N. Rao and B. A. Robson, Nucl. Phys. A147 (1970) 65 9) A. Saganek, I. Sledziliska, A. Turos, Z. Wilbelmi and B. Zwi~gliltski, Acta Phys. Pol. B2 (1971) 473 10) F. Machali, Z. A. Saleh, A. T. Baranik, F. Asfour, L. Boundouk and V. E. Storizhko, Atomkernenergie 13 (1968) 29 11) A. Saganek, I. Sledzi6ska, Z. Wilbelmi and B. Zwiggll~ski, Report INR, to be published 12) B. Zwi~gli/iski, J. Piotrowski, A. Saganek and I. Sledziflska, Nucl. Phys. AI09 (1973) 348 13) F. Bertrand, G. Grenier and J. Pornet, Report CEA-R-3504 (1968) 14) W. Del Bianco, S. Kundu and B. Reuben, Nucl. Phys. A232 (1974) 333 15) F. Ajzenberg-Selove, R. F. Casten, O. Hansen and T. J. Mulligan, Phys. Lctt. 40B (1972) 205 16) D. R. Goosman, E. G. Adelberger and K. A. Shover, Phys. Rev. C1 (1970) 123; W. Bencnson, E. Kashy, D. H. Kong-A-Siou, A. Moalem and H. Nann, Phys. Rev. C9 (1974) 2130 17) K. Battleson and D. K. McDaniels, Phys. Rev. C4 (1971) 1601 18) H. J. Annegarn, D. W. Mingay and J. P. E. Sellschop, Phys. Rev. C9 (1974) 419 19) Yu. I. Sorokin, A. Kh. Shardanov, V. G. Shevchenko and B. A. Yuriev, Yad. Fiz. 11 (1970) 8 20) M. K. Mehta, W. E. Hunt, H. S. Plendl and R. H. Davis, Nucl. Phys. 48 (1963) 90 21) R. H. Bassel, R. M. Drisko and G. R. Satchler, Report ORNL-3240 (1962) 22) W. Fitz, R. Jahr and R. Santo, Nucl. Phys. AI01 (1967) 449