Investigation of the 6Li(p,γ)7Be reaction at low energies

I N V E S T I G A T I O N O F T H E 6L i( p ,y ) 7B e R E A C T I O N A T L O W E N E R G I E S

A. Baykal1, İ. A. Reyhancan1, A. Elmalı1, Y. Özbir1, N. Erduran2

te k m e c e Nuclear Research and Training Centre, Department of Research and Development, Altinsehir Yolu 5. Km. Halkalı, 34303 Istanbul-Turkey

departm ent o f Physics, Faculty o f Sciences, University o f Istanbul, 34459 Vezneciler-Istanbul, Turkey e-mail: adnan.baykal@taek.gov.tr

ABSTRACT

A 6LiF target o f 26jug/cm2 thickness was bombarded with 250 keV atomic protons with 50 juA beam current intensity for the experiment. The gamma spectrum emerged from the reaction is measured by HpGe detector using a multichannel analyzer in the 0 - 7 MeV energy range at 3.3 cm away from the target in the proton beam direction. The ground and the first excited states o f 7Be and the energy levels o f 1 9F(p,ay)160 reaction can separately be detected in the resulted spectrum. The cross section o f 6Li(p,y)7Be reaction at 250 keV energy range has been measured.

1.INTRODUCTION

The basic problem o f nuclear astrophysics is to explain how the elements have come into existence and why the amounts o f them are as it is now. Nucleus synthesis theory assumes that after the big bang first light elements like H, He, Li, Be, B, come into existence and these elements became the source for the elements which have greater masses then these [1, 2, 3].

Radiative capture o f protons by light nuclei reactions at low energies is important in nuclear astrophysics [4]. At low energies reaction cross sections are very small so they can’t be measured directly. 6Li(p,y) 7Be radiative proton capture reaction is important for the production o f 7Li, when 7Be captures an electron 7Li comes to existence.

There are contraries in S factors and measured cross sections o f the above reaction when Ep > 150 keV [5, 6, 7 ]. There are two studies, which were carried out with polarized protons at Ep < 150 keV, [4,8]. In the present experiment 6Li(p,y) 7Be reaction is investigated with atomic protons.

2. EXPERIMENTAL PROCEDURE

In the present experiment SAMES T-400 low energies ion accelerator at Research and Development Department o f Çekmece Nuclear Research and Training Center were used.

The protons coming from ion source were accelerated under 50 - 400 kV. High voltage variance o f the accelerator is less than 5x1 O' 3 % and at maximum stable voltage, the maximum current is 2 mA and it is 3 mA when the working voltage is pulsed.

The atomic proton beam was obtained by passing the mixed beam (atomic and molecular ions) through a magnetic field. The proton beam diameter bombarding the target is 0.8 cm. Target was constructed by coating 6LiF to Copper disc of 49 mm diameter and 1 mm thickness. The used 6LiF consists o f 95.6 % 6Li and 4.4 % 7Li. The thickness o f 6LiF on the target is 26 jug/cm2. During the experiment proton current kept stable at 50juA. Target was cooled by natural air convection. In this experiment ORTEC GMX-10180 HpGe detector was used and its resolution was 2 keV for 6 0Co, 1332.5 keV gamma energies. Detector crystal’s diameter is 4.5 cm and its thickness is 4.2 cm. The thickness o f the Be window in front o f the HpGe crystal is 0.5 mm. The distance between the window and HpGe detectors crystal is 5 mm. In the experimental setup 6LiF target and detector were in the same direction with the incoming proton beam and the distance between the crystal and 6LiF target set to 3.3 cm. Proton current was measured from target directly. Proton mark on the target for 250 keV has been form a 0.8 cm circle. Center o f the proton marks circle and detector crystal’s center were on the same line with the proton beam (Figure 1). There was an electron-suppressing ring near the target. Calibration o f detector for gamma energies has been done using standard gamma ray sources. Energy level diagram o f 7Be can be shown in Figure 2 [4].

For a rough detection o f 6Li(p,y) 7Be reaction for mixed beam Nal (3x3 inch) and GMX HpGe detectors were used.

After atomic and molecular proton beams were passed through analyzer magnet’s field, atomic proton beam obtained perpendicular to the initial beam direction and with this beam the target has been bombarded. Using two different proton beams, three-gamma spectrum was obtained (Figure 3, Figure 4, Figure 5). Figure 5 shows the acquired spectrum obtained with atomic protons on 6LiF target with irradiation time 653 sec. using CANBERRA 85 multi channel analyzer. The spectrum shows the 6.130 MeV gamma ray coming from 1 9F(p,ay)160 reaction and 5.61 MeV gamma ray coming from 6Fi(p,yQ)7Be reaction (ground states) and 5.18 MeV gamma ray coming from 6Li(p,y!)7Be reaction (first excited states). The gamma spectrum from atomic

Figure 1. The schematic view o f experimental set up (target and the geometry o f detector).

Figure 2. Energy level diagram o f 7Be [4]

Figure 3. The gamma spectrum o f 6Fi(p,y) 7Be reaction obtained with mixed (molecular and atomic) proton beam with Nal detector at 250 keV.

ı o o o o ı

_fc

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₁

₁

2000 4000 6000 8000

Channel

Figure 4. The gamma ray spectrum o f 6Li(p,y) 7Be reaction obtained with mixed (molecular and atomic) proton beam with HpGe detector at 250 keV.

Figure 5. The gamma spectrum o f 6Li(p,y) 7Be reaction obtained with atomic proton beam with HpGe detector at 250 keV.

3. CROSS SECTION MEASUREMENTS

The reaction cross section for 6Li(p,y) 7Be can be written in terms o f the acquired gamma ray photo peak yield Y(0,Ep), (where 0 is the laboratory angle o f the detector and Ep represents the beam energy) as follows [10],

c(0,E p)=Y(0,Ep) / (D(Ep) s Q (1)

D(Ep) is the lithium areal density of the target, 8 is the photo peak efficiency, is the solid angle o f the detector and P is the number o f protons reaching to target during the bombardment. The lithium areal density D(Ep) has been calculated as follow [10],

d D(E) - dE / STP(E) (2)

dD is the differential Lithium areal density, and dE is the differential energy with o f the incident beam, STP(E) is the stopping cross section for protons on 6LiF target [10]. STP(Ep) has been obtained using code SRIM 2003.

6Li(p,yj) 7Be reactions cross section has been obtained experimentally using expression,

In the present experiment photo peak counting number for the first excited states o f 6Li(p,y!) 7Be reaction was studied. The gammas from 6Li(p,y!)7Be reaction is isotropic [5]. The comparison o f our result with other researcher results is given in Figure 6.

Figure 6. The change o f the cross section o f 6Li(p,y)7Be reaction with respect to energy [5].

4. ASTROPHYSICAL S FACTOR

At the low energies o f astrophysical interest, charged particle cross sections are greatly inhibited by the Coulomb barrier, which causes them to fall exponentially with decreasing beam energy. This yields low experimental count rates, which often make it necessary to measure the cross section at somewhat higher energies, and then to extrapolate the result downward. This extrapolation procedure involves parameterizing the cross section at low energies in term of known energy dependencies, namely Coulomb barrier penetration and geometrical cross section effect. The remaining energy dependence, the so-called Astrophysical S Factor, varies only slowly with the energy and is, one hopes, easier to extrapolate then the cross section [2,10]. The cross section is defined in terms o f the S factor, S(Ec m) as follows

g (Ecm) = S(Ec m ) e'2mi / Ec m (3) Where r| is the Sommerfeld parameter and it is [13],

n = 0.1575 Z ,Z2 ( A / E , m.)1/2. (4) Zj and Z2 is protons and targets charge numbers, A is the reduced mass, A1 and A2 are protons and targets mass number

A = Aj A 2 / (A! + A 2)

And Ec m is equal to

Ec.m. — (A2/ Ai+A2)Eiab.

For 6Li(p,y!)7Be reaction for 250 keV protons S factor is found S(Ecm) = 60.1 ± 5.3 eV b.

(5)

(6)

Figure 7.

5. RESULT AND DISCUSSION

In the experiment, even though the cross section o f 6Li(p,y)7Be reaction is in nanobam level, we have measured it successively. We couldn’t obtain the gamma ray spectrum with 100 pA atomic proton currents, at proton energy 150 keV and 200 keV. Since we don’t have current integrator so we couldn’t take counts for a long time. Later we will repeat the experiment in the range at 150 - 350 keV with current integrator. In the proton energy range 150 to 200 keV it is necessary either to increase the current or to increase the reaction time.

6. ACKNOW LEDGM ENTS

We are grateful to Caner Yalçın for his help in calculations.

7. REFERENCES

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