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Influence of Protons on the Capture of Electrons by 7Be in The

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Influence of Protons on the Capture of Electrons by 7Be in The PHYSICAL REVIEW C 75, 034608 (2007) Influence of protons on the capture of electrons by 7Be in the Sun V. B. Belyaev Bogolyubov Laboratory of Theoretical Physics, Joint Institute for Nuclear Research, Dubna RU-141980, Russia M. Tater and E. Truhl´ık Institute of Nuclear Physics ASCR, CZ-250 68 Reˇ z,ˇ Czech Republic (Received 21 November 2006; published 20 March 2007) The capture of electrons by the nucleus 7Be from the three-body initial state p + e− + 7Be in the continuum is studied. On the basis of the expansion of the three-body continuum wave function in the small parameter 1/2 ≈ (me/mp) [me (mp) is the electron (proton) mass], the role of the protons on the electron capture is considered. The results are compared with the traditional treatment of the electron capture by the nucleus 7Be. For stars with density and temperature like in the center of the Sun the studied mechanism can make a non-negligible contribution to the capture rate. DOI: 10.1103/PhysRevC.75.034608 PACS number(s): 23.40.−s, 25.10.+s, 97.10.Cv I. INTRODUCTION difference between the binary and ternary mechanisms1 of the The process of the electron capture by the nucleus 7Be is electron capture. Indeed, if one starts from the three-body important because it contributes to the low energy part of the initial state, then the processes (1.1) and (1.2) should be spectrum of neutrinos radiated by the Sun. In addition, it is interdependent because the wave function of three charged obvious that the balance of the disappearance channels of 7Be particles cannot be presented as a product of pair wave 8 functions, as is required by the binary processes (1.1) and in the Sun regulates the amount of the nucleus B that is the 2 source of the high energy solar neutrinos. This is the main (1.2). Here we apply the method and results obtained in reason why this process has attracted considerable attention Ref. [9] where an alternative to the Born-Oppenheimer over many years [1–7]. approach has been suggested for the calculations of wave Practically all the discussion so far of the electron capture in functions in the continuum for three charged particles. 7Be is concentrated on considering the electron wave function in the vicinity of the nucleus and on the screening effects on it; II. CALCULATIONS the study of this capture in the plasma is described in Ref. [7]. In what follows we estimate the role of the process that is In analogy with the approach developed in Ref. [9], the usually not included in the standard theory of the pp cycle in continuum wave function of three charged particles can be the Sun. Let us first note that in the standard theory of this expanded in a small parameter, , cycle the destruction of the nucleus 7Be takes place in the Mm 1/2 m 1/2 binary reactions = e ≈ e , (2.1) (M + mp)(mp + me) mp p + 7Be → 8B + γ, (1.1) where, in addition to the electron and proton mass, also M,the 7 e− + 7Be → 7Li + ν. (1.2) mass of the nucleus Be, enters. The expansion of the wave function of the studied three-body system is then 7 Because the nucleus Be participates in both processes, instead (r, R ) ≈ (r, R ) + (r, R ) +···. (2.2) of the binary reactions (1.1) and (1.2) we consider the 0 1 contribution to the electron capture rate from the three-particle Because ≈ 0.0233, one expects the effects of the second initial state p + e− + 7Be. In this case the following reactions term on the right-hand side of Eq. (2.2)tobeatthelevelof can take place: 2% in comparison with the first term. It was found in Ref. [9] that in the limit → 0 the Jacobi 7 coordinates r and R (see Fig. 1) separate. This means that the Li + ν + p, (1.3) structure of the wave function 0(r, R)is p + e− + 7Be → 8B + γ + e−, (1.4) 8 − C C B + e . (1.5) 0(r, R) = (R) (r,Z = Z1 + Z2), (2.3) As was shown in Ref. [2], the screening corrections for the 1We use the terms binary reaction and ternary reaction as synonyms electrons in the continuum are rather small. Therefore, we for the reactions in the two- and three-component systems. consider in the initial state the bare Coulomb interaction in all 2There is only one exception corresponding to the case when all − − two-body subsystems e + p, p + 7Be, and e + 7Be. In three particles are at very large distances between themselves [8], this case one can immediately realize that there is a qualitative which is not applicable to the electron capture. 0556-2813/2007/75(3)/034608(4) 034608-1 ©2007 The American Physical Society V. B. BELYAEV, M. TATER, AND E. TRUHLIK´ PHYSICAL REVIEW C 75, 034608 (2007) the state with zero angular momentum F (η, ρ) C (ρ) = 0 , (2.6) E ρ where the function F0(η, ρ) satisfies the equation d2F 2η 0 + 1 − F = 0, (2.7) dρ2 ρ 0 FIG. 1. The Jacobi coordinates for the p + e− + 7Be system. √ √ with ρ = pβR, and η =−Zα me/E =−Zα 2/v is where C (R ) is the Coulomb wave function describing the the Sommerfeld parameter. The function F0(η, ρ) can be relative motion of the proton and the nucleus 7Be, and expressed in terms of the Kummer function M (see Ref. [11], C Chap. 14) as (r,Z = Z1 + Z2) is the Coulomb wave function that describes the motion of the electron in the field of the effective −iρ F0(η, ρ) = C0(η)ρe M(1 − iη,2, 2iρ), (2.8) Coulomb potential of the charge Z = Z1 + Z2. The crucial C point is that the wave function (r,Z = Z1 + Z2) depends 2 = 2πη − where C0 (η) 2πη/(e 1). on the distance between the electron and the center of mass Instead of the quantity ς(R,T ), one can consider of the subsystem of heavy particles. It means that even if the distance between the electron and the nucleus 7Be is zero, as ∞ C = 2 −E/kT 0 E (βR,Z 5) e dE is required by the Hamiltonian of the weak interaction, the ςC (R,T ) = . (2.9) ∞ 2 − C C ,Z = e E/kT dE wave function (r,Z = Z1 + Z2), defining the probability 0 E (0 4) 7 of the electron capture by the nucleus Be, should be taken at Using Eq. (2.7), one obtains ||= | | = a non-zero distance r β R , where β 1/7 is the ratio of the proton and 7Be masses. C 2 = 2πη − E (0,Z) 2πη/(e 1), (2.10) It is clear that this phenomenon appears to be due to the electron movement in the Coulomb field of two charged which√ is of the same form as the Fermi function (2.5); however, = particles with positive charges. Following the arguments η 2ν. presented above, we now consider two effects acting in One can find the following integral representation (see opposite directions. On one hand, increasing the effective Ref. [11], Chap. 13), positive charge of the heavy particles system by one unit will shπη 1 1 − t it enlarge the electron capture rate. On the other hand, using M(1 − iη,2, 2iρ) = e2iρt dt, (2.11) the Coulomb wave function at finite distances instead of the πη 0 t function taken at zero distance should damp the capture rate. for this function. In the numerical calculations of the integral Taking into account that the nuclear matrix elements of the over energy in Eqs. (2.4) and (2.9), we used this representation reactions (1.2) and (1.3) are the same, as a measure of influence of the Kummer function for the energies E>0.1keV.In of the third particle (the proton in this case) on the capture rate the interval E<0.1 keV, the function under the integral in 7 of electrons by the nuclei Be we introduce the ratio ς(R,T ), Eq. (2.11) strongly oscillates which makes the calculations which is the function of the distance R between the particles difficult. Instead, we applied the program PFQ developed in and the temperature T , Ref. [12]. Let us note that for the energies E>0.1keVthe ∞ program PFQ and Eq. (2.11) provide the same results to a high C (βR,Z = 5)2 e−E/kT dE = 0 E degree of accuracy. ς(R,T ) ∞ − . (2.4) = E/kT We also introduce the mean value ς(R0,T) of the function 0 F (Z 4,ν)e dE ς(R,T ) Here the denominator on the right-hand side of the equation − 2 − (R R0) contains the quantity that enters the electron capture rate from 2 = 2R0 the continuum for the reaction (1.2)[1]. The Fermi function ς(R0,T) N e ς(R,T )dR F (Z,ν) is given by the equation ∞ − (x−1)2 = 3 2 2 4πR0N e x ς(R0x,T )dx, (2.12) F (Z,ν) = 2πν/(e2πν − 1), (2.5) 0 where where the parameter ν is given by the equation ν = − 2 − =− − (R R0) Zαm√e/p Zα/v,α is the fine structure constant, and −1 R2 = 2 0 p = 2m E is the electron momentum. It is obtained by N e dR e using the solution of the Dirac equation with the Coulomb 4πR3 √ 1 potential [10].
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