The 13C(d,y,)15N reaction around E,,, = 17.7 M ~ V '

W. DEL BIANCO Lahorcrroire tle Plzysiqrre NrrclPuire, Utliversirk cle M o t ~ r ~ P o l , MorzrrPul (Qrrk.), Cnt~nda H 3 C 3 J 7

A N D

N. MARQUARDT, K . FARZINE, A N D H. V. BUTTLAR R~r l~r -Ut l i~~ers i l i i r Boclz~rtrc, Bochrrm, Wesr Crrt t~ony

Received August 12, 1977

The 95" differential cross section of the '3C(d,y,)'5N reaction has been measured at deuteron energies from 1.1 to 4.2 MeV. The y rays have been detected by a 25.4 cm long by 25.4 cm diameter NaI(T1) crystal spectrometer. The yield curve reveals a single resonance structure at Ed = 1.74 2 0.04 MeV (E,,, = 17.7 MeV).

La section efficace differentielle a 95" de la reaction 13C(d,y,)'5N a ete mesuree pour des energies des deuterons allant de 1 .1 84.2 MeV. Les rayons gamma ont ete detectks par un crystal de NaI(T1). La courbe d'excitation montre une resonance B Ed = 1.74 2 0.04 MeV (E,,, = 17.7 MeV).

Two measurements of the 90" differential cross section of the '3C(d,yo)'5N reaction have recently been performed. Weller and Blue (1) have studied the l3C(d,y0)l5N reaction in the energy range Ed - 1 to 3.7 MeV. The yield curve a t 8, = 90" shows a resonant behaviour near Ed = 1.7 MeV with a peak cross section of 1.6 pb/sr and a width of about 0.6 MeV. The measurements of Weller and Blue have been extended by Del Bianco et al. (2) to higher deuteron energies (Ed = 2.9-9.95 MeV). In the region of overlap (Ed = 2.9-3.7 MeV) the yield curves obtained in the two experiments show a similar energy dependence. However, the absolute values of the differential cross section are in dis- agreement by nearly a factor of three (Fig. 1).

Because of the large discrepancy, it was felt necessary to extend the measurements of Del Bianco et al. (2) to lower deuteron energies with the aim of determining the absolute value and energy dependence of the yield curve below Ed = 2.9 MeV. The experiment described in this article was per- formed with the deuteron beam produced by the Bochum University Dynamitron Tandem acceler- ator. Incident deuteron energies were varied from 1.2 to 4.2 MeV and currents ranging from 0.5 to 1.2 pA were used on target. The I3C target had a thickness of approximately 500 pg/cm2 and was pre- pared by cracking 90% enriched acetylene on a tantalum disc. The energy loss in the target amounted to 160 and 65 keV at the deuteron energies Ed = 1.2

NaI(T1) crystal spectrometer, its axis forming a 95" angle with the deuteron beam. The NaI crystal was collimated and surrounded by a 10 cm thick lead shield and a 40 cm paraffin layer to minimize back- ground radiation. It was not attempted to extract the absolute cross section directly from the data; instead the y ray yield was normalized to the differential cross section of Del Bianco et al. (2) in the deuteron energy range Ed = 2.9 to 4.2 MeV; the normaliza- tion factor was then used to determine the absolute yield below Ed = 2.9 MeV.'

The differential cross section at 8, = 95" of the '3C(d,y,)15N reaction measured as a function of deuteron energy is shown in Fig. 1 (solid symbols). The error shown represents the statistical error and the error resulting from the procedure used to unfold the y-ray spectrum by a least-squares-fit method. Additional errors are that due to the normalization procedure (f 18%) and the absolute error in the yield curve of Del Bianco et al. (2). The latter amounts to and is mainly to be attributed to the calibration of the y-ray spectrometer. In Fig. 1 for comparison purposes we have also drawn the 90" differential cross sections measured by Weller and Blue (1) and by Del Bianco et al. (2). Over the region of overlap (Ed = 2.9 to 4.2 MeV) the yield curve of our experiment and that of Del Bianco el al. show a similar behaviour. Below Ed = 2.9 MeV, the shape of the differential cross section measured in this

and 4.2 MeV, respectively. The capture y rays were should be noted that the yield curves of Weller and Blue detected by a 25.4 cm long by 25.4 cm diameter (1) and of Del Bianco et a/. (2) were measured at 8, = 90°,

whereas in this experiment the y-ray yield was monitored a t 'Research supported by grants from the National Research 8, = 95". However, the experimental angular distributions a t

Council of Canada and the Bundes Ministeriurn fiir Forschung deuteron energies below 4.2 MeV indicate a variation of the und Technologie of W. Germany. y-ray yield from 8 = 90" to 8 = 95' of less than 1% (2).

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C A N . J. PHYS. VOL. 56, 1978

2 .O 3.0 4.0

DEUTERON ENERGY (MeV) '

13c( d , y o ) 1 5 ~ REACTION - THIS EXPERIMENT

@ DELBIANCO et 01. (2) 0 WELLER ond BLUE ( I )

-

-

- - - -

o G O - - 0 -

@ P - 0

- P O O O o O o

0 0 0 0 - 0 0 o 0 O 0 0

FIG. 1. Differential cross section of the 13C(d,yo)'5N reaction as a function of deuteron energy at 0, = 90': 0, the results of this experiment; 0, the data of Weller and Blue (1); 0, the results of Del Bianco st a / . (2). The error bars of Weller and Blue represent the statistical errors, whereas the errors of the other two experiments contain also the uncertain- ties resulting from unfolding the y-ray spectra.

experiment agrees with that of Weller and Blue. Thus the resonant behaviour at Ed = 1.7 MeV is con- firmed; however, the peak cross sections in the two cases differ by about a factor of 2.5. The parameters of the resonance are Ed = 1.74 f 0.04 MeV (E,,, = 17.7 MeV in 15N) and I-,,, - 0.70 MeV, in fair agreement with those of Weller and Blue (1).

The differential cross section of the inverse l5N(y,d0)l3C process, obtained by means of the principle of detailed balance, is shown in Fig. 2. The solid symbols are the results of this experiment; the solid line was drawn through the experimental points of Del Bianco et al. (2) and is intended to be only a guide to the eye. In the same figure, for comparison purposes, the experimental cross section of the '5N(y,po)'4~ reaction has also been plotted (3).

Radiative capture of composite particles is a suitable tool to investigate multi-particle multi-hole excitations in the GDR of light nuclei. This has first been pointed out by Gillet et al. (4) and nicely exemplified by the experimental work of Bromley and co-workers in 160 (5). Recently Weller and Blue (I) and Del Bianco et al. (2) have investigated 2p-3h states in the GDR of mass-15 nuclei by means of the '3C(d,y,)'5N reaction. Most of the discussion in the latter article is relevant to this work; thus we shall restrict ourselves here to a few points of par- ticular interest.

1 1 1

The angular distribution of the l3C(d,y0) resonance at Ecx, = 17.7 MeV has been measured by Weller and Blue (1) and found to be isotropic. Hence, assuming dominant El transitions, the state at Ecxc = 17.7 MeV is formed by p-waves deuterons and has quantum numbers J n = 112'. In addition, since the deuteron has isospin zero, in the hypothesis of charge independence of nuclear forces, the isospin of the state is T = 112.

If we now consider the inverse l5N(y,d0)l3C process (Fig. 2) and use the principle of detailed balance, we obtain an integrated l5N(y,d0)l3C total cross section over the resonance approximately a factor 1.6 larger than the corresponding quantity for the 15N(y,po)'4C reaction. The experiment of Weller et al. (6) shows that mainly s- and d-wave protons are responsible for the 15N(y,po)14C reaction in this energy region. Hence deuteron decay is hindered with respect to proton emission approximately by a factor 9 for s-wave and by a factor 4 for d-wave protons (7). Taking into account this result, the ratio of the relative deuteron and proton strengths in the 17.7 MeV region is unexpectedly large. It is reason- able to assume that deuteron emission is directly related to the 2p-3h component of the continuum wave function. Therefore the large ratio is indicative of the considerable 2p-3h strength of the resonant state.

1 1 1 1 1 1 1 1 1 I I I I I I I I I 1 1 1 1 1 1 1 1 1 1 1 1

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DEL. BIANCO ET A L .

of the present results that the neglect of 2p-3h con- FORD. Phys Rev. C, 13,922 (1976). figurations in the region around E,,, = 18 MeV is 7. 1. BLOCK etnl. Rev. Mod. Phys. 23, 147 (1951).

not justified. 8. W. DEL BIANCO, S. KUNDU, and J . KIM. Can. J. Phys. 55, 302 (1977).

In conclusion the results of this experiment confirm 9, R, F,,,,,, R , G ~ ~ ~ ~ , ~ ~ , ~ ~ , ,,d B, spIcER. ~ ~ ~ 1 . the resonant behaviour of the 13C(d,yo)15N cross ~ h y s . A, 156,489 (1970).

6 0 0

- I 5 ~ ( y , do 1 I3c REACTION -

--- ' 5 ~ ( y , p o I4c REACTION

500 -

- 3 300 - D \

b D

\

100 -

16 18 2 0 22 2 4

EXCITATION ENERGY (MeV)

FIG. 2. Comparison of the l5N(y,pO)l4C and 15N(y,do)13C differential cross sections at OL = 90' obtained from the cross sections for the inverse processes by means of the principle of detailed balance. The dotted line (---) was calculated from the data of Harakeh et nl. (3) for the l4C(p,y,)l5N reaction. The l5N(y,d0)l3C cross section is a composite of the results of this experiment (0) and the measurement of Del Bianco et a/. (2) (-).

It should also be noted that the resonance in the section at E,,, = 17.7 MeV and indicate that the l5N(y,d0) cross section is located close to a dip corresponding state in 15N is characterized by a large (E,,, = 17.2 MeV) in the '4C(y,po) yield curve (Fig. 2p-3h admixture. 2). The correlation might be accidental, but it could Acknowledgements also be the type predicted by Gillet et 01. (4) in the case of single particle states with a large multi- The experiment was performed when One the

particle multi-hole component. It is interesting to authors (W.D.B.1 was visiting the University of

note that the same correlation has been found when Bochum. He wishes to thank the members of the comparing the structure in the yield curves of the nuclear physics group for their warm and

"B(a,y0)"N and l4C(p,y0)l5N reactions (8). the Quebec-Germany exchange program for financial

Several calculations of GDR states in mass-15 nuclei have been published (9, 3, 6) . They all yield an 1. H. R. WELLER and R. A. BLUE. NUCI . ~ h y s . A, 211,221 El-strength concentrated in the 15-25 excitation (1973). energy region; however, there are differences in the 2. W. DEL BIANCO, S. K U N D U , and J. KIM. N u c ~ . Phys. A,

270,45 (1976). location, quantum numbers> and strength of 3. M, H A R A K ~ ~ , P. PAUL, H, K"AN, and E, W A ~ B ~ ~ ~ O N . the single levels. No theoretical level is present at phys, R ~ ~ , C, 12, 140(1975). 17.7 MeV, although the calculation of Weller et 01. 4. V. GILLET, M . A . MELKONOFF, and J. RAYNAL. Nucl. (1) yields two levels with the correct quantum Phys. A, 97,63 I (1967). numbers at E,,, - 17 and 18.2 M ~ V . should be 5. H. D. SHAY, R. PESCHEL, J. M. LONG. and D. A. BROM-

LEY. Phys. Rev. C, 9 ,76 (1974). noted that in all the calculations only lp-2h excita- 6, H. R, WELLER, R. A. BLUE, N. R, ROBERSON, D, G. tions were considered. It would appear in the light RICKEL, S. MARIPUU, C. P. CAMERON, and R. D. LED-

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