Comparison between the ground state transitions of the sub-Coulomb deuteron stripping reactions on 6Li

'~ E~E~ER

Nuclear Physics A621 (1997) 119c-122c

NUCLEAR PHYSICSA

Comparison between the ground state transitions of the sub-Coulomb deuteron stripping reactions on 6Li K.Czerski, G.Ruprecht, H.Bucka and P.Heide Institut fiir Strahlungs- und Kernphysik, Technische Universit£t Berlin, D-10623 Berlin By means of recoil nuclei spectroscopy the 6Li(d,p0)7Li and ~Li(d,n0)TBe mirror reactions have been measured for deuteron energies between 65 and 135 keV. The results have been compared with theoretical calculations including effects of the isospin mixed subthreshold resonance state in 8Be. 1. I n t r o d u c t i o n The inhomogeneous models of primordial nucleosynthesis [1-3] predict in low-density neutron-rich regions a relatively high 2H abundance [4], so the deuteron induced reactions may play an important role in creation and destruction of chemical elements in the early universe. Furthermore, the mirror stripping reactions (d,n) and (d,p) may contribute significantly to the neutron-proton density ratio which is one of the most important parameters for the inhomogeneous models and consequently for the nucleosynthesis of heavier elements. According to charge symmetry considerations in the network calculations of primordial nucleosynthesis the reaction rate ratio between the (d,n) and (d,p) reactions on 6Li has so far been assumed to be equal to one [5]. In our previous study of the 6Li(d,nl)7Be and 6Li(d,pl)TLi reactions leading to the first excited states of the recoil nuclei [6] it has been shown that this simple assumption is not quite correct. Whereas for deuteron energies above 200 keV the neutron reaction channel is more probable than the proton channel, for lower energies a decrease of the branching ratio (d,nl)/(d,pl) by about 20% is observed. We have explained the drop of this ratio by the influence of a broad 2 + isospin mixed subthreshold resonance state in the compound nucleus 8Be (E~ = 22.2 MeV) with a mixing parameter e = 0.20 corresponding to a partial resonance width ratio F n l / F p l = 0.3. Theoretical calculations [6] carried out for the ground state transitions %i(d,n0)TBe and Li (d,p0)7Li with the same isospin mixing parameter predict a branching ratio (d,no)/(d,p0) that drops to about 0,5 at zero deuteron energy. It is the aim of this work to experimentally check this behavior and to present S-factors for the astrophysically relevant energy region below 200 keV. 2. E x p e r i m e n t a l m e t h o d The 6Li(d,p0)TLi and %i(d,n0)TBe reactions have been investigated for deuteron energies between 65 and 135 keV by means of recoil nuclei spectroscopy. The magnetically analyzed deuteron beam provided by our cascade accelerator was focused to a spot of 2 0375-9474/97/$17.00 © 1997 - Elsevier Science B.V. All rights reserved. PII: S0375-9474(97)00221-2

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K. Czersla et al./Nuclear Physics A621 (1997) 119c-122c

mm in diameter on a thin 6LiF target (10 #g/cm 2 on a 10 #g/cm 2 carbon backing). The target thickness corresponded to an energy loss of 2-3 keV for deuterons and to 10-20 keV for the measured 7Li and 7Be recoil nuclei. Charged particles were detected by a 100 mm 2 PIPS-detector being placed at an angel of 150 degrees with respect to the beam in 10 cm distance from the target. A spectrum obtained in a single run for an incident energy of 105 keV is presented in Fig.1. Four different recoil nuclei lines corresponding

, 100

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,

2000

3000

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Figure 1. Charged particle spectrum measured at Ed,tab = 105 keV. The low energy part of the spectrum has additionally been expanded to present the recoil nuclei lines in more detail; the grey line is a Gaussian fit.

to the ground and first excited states of 7Be and rLi could be identified in the low energy part of the spectrum. Due to the very low kinetic energy (~ 200 keV in the case of Be*) of the recoil nuclei no protective foil against elastically scattered deuterons could be used in front of the detector. Since the deuteron to recoil nucleus ratio is of the order of 106 an external pile-up rejector (with a pulse pair resolution of better than 50 ns) working together with an ultra high rate spectroscopic amplifier (Ortec 973U) has been applied to reduce the number of deuteron pile-ups. Details of the experimental set-up will be published elsewhere. Two opposite effects limited the investigated deuteron energy region. A lower limit of 65 keV was given due to the strong decrease of the reaction cross section with decreasing incident energy. The upper limit of 135 keV was determined by the necessity to separate the Be-lines from the elastically scattered deuterons. 3. R e s u l t s a n d

discussion

Astrophysical S-factors have been determined assuming an isotropic angular distribution for the 6Li(d,p0)7Li and 6Li(d,n0)TBe reactions at deuteron energies below 200 keV [7]. The results for the (d,p0)-reaction are presented in Fig.2 together with data from Elwyn et al. [7] up to 1 MeV deuteron energy. In the overlapping energy region the values are in a good agreement. An increase of the astrophysical S-factor with decreasing

K. Czerski et al./Nuclear Physics A621 (1997) 119c-122c

25

i

i

i

i

b

i

I

*

*

*

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i

O Elwyn et al. • present work

2O ;> lO d~ 5 0

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I

200

i

i

I

500

1000

Ed,lab (keV) Figure 2. Astrophysical S-factors for the 6Li(d,p0)TLi reaction. The dashed line represents the direct reaction component only (DWBA calculation) and the solid line is the sum of the direct and resonance components.

incident energy arises from the 2 + subthreshold resonance contribution [6]. This resonance has a rather large width of about 800 keV and lies 80 keV below the reaction threshold [8]. Because of its s-wave character the resonance contribution can be fitted by a Lorentz-curve (see Fig.2). The direct reaction component, also shown in Fig.2, has been calculated in the frame of DWBA [9]. The theoretical angular distributions were compared with the experimental data from [7] for deuteron energies between 145 keV and 875 keV to determine the zero range strength factor (for details see [6]). In difference to the stripping reactions leading to the first excited states of the mirror nuclei 7Li and 7Be the transitions to the ground states are dominated in the low energy region by the subthreshold resonance. This effect results from the large ratio of the partial resonance widths Fpo/Fpl = 9.7 ± 1.0 [6]. The ratio of the 6Li(d,n0)TBe to 6Li(d,p0)TLi recoil nuclei reaction yields corrected for the different center-of-mass solid angles is presented in Fig.3. The theoretical branching ratio resulting from the direct reaction contribution only (dashed line) clearly overestimates the experimental data. The solid line represents the calculations including the resonance contribution, where the only free fitting parameter is the ratio between the neutron and proton partial resonance widths Fno/Fpo. The value such determined is Fno/Fpo = 0.95 ~= 0.03; it can be compared with the theoretical ratio obtained according to the relation (see [6])

rpo -

kpoP~o [~/1 - ~ +

(1)

where k and P are the wave number and the penetration coefficient in the neutron and proton channel, respectively, and e stays for the isospin mixing parameter. In the case of e = 0, i.e. without any isospin mixing, the resonance width ratio reaches its maximum value of 0.95 which is in agreement with the experimental value. This result seems to be in contradiction to that obtained for the transitions to the first excited states [6] for which

122c

K Czersla et al /Nuclear Physics A621 (1997) 119c-122c

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Figure 3. Experimental and theoretical ratio of the 6Li(d,n0)TBe to 6Li(d,p0)TLi reaction yields. The curves are explained in the text.

c = 0.20, corresponding to Fno/Ppo = 0.41, was determined; the resulting curve is the dot-dashed line in Fig.3. However, this curve can be treated only as the maximum value of the neutron-proton asymmetry because no structure details of the wave function of the subthreshold resonance have been taken into account. The decay widths of this resonance state depend on the probability of finding the final state configuration in the resonance wave function. The ground states of 7Li and 7Be can be described as a mixture of the pl/2 nucleon states coupled with the 6Li-core, whereas the first excited states contain only the P3/2 component [10]. If we assume that the isospin mixing with e = 0.20 takes place only for that part of the resonance wave function which includes the P3/2 component and take into account that Ppo/Ppl ~ 10 we obtain a value of Pn0/Fpo ~ 0.92 which agrees with our experimental result.

and P3/2

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

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