Adaptation of a superconducting-solenoid-transporter Si(Li)-Si(Li) spectrometer for in-beam studies of internal-pair transitions

The Debrecen superconducting magnetic spectrometer (SMS) has been adapted for in-beam internal-pair studies. Test measurements have been carried out using a 207Bi radioactive source and the 19F(p, αγ)16O, 23Na(p, αγ)20Ne, 27Al(p, p'γ)27Al, and 42Ca(p, p'γ)42Ca reactions (at bombarding energies Esub(p)=3.5-4.0 MeV). Convenient spectrometer parameters, backscattering of electrons and positrons from one detector to the other, and methods for background reduction have been investigated. Experimental values of (14 +- 3)%, (12 +- 3)%, and (14 +- 2)% for the one-detector pair-line efficiencies were determined for the 20Ne(E2; 1634 keV), 42Ca(E0; 1837 keV) and 27Al(M1+E2; 2211 keV) transition, respectively. The observed pair-line detection efficiencies for two detectors operated in sum-coincidence mode were (35 +- 7)% and (34 +- 6)% for the 42Ca(E0; 1837 keV) and 27Al(M1+E2; 2211 keV) transitions, respectively. The energy resolution of the spectrometer was approx.= 0.5% in singles and approx.= 0.6% in sum-coincidence measurements for the 2211 keV M1+E2 pair line of 27Al. Effective pair-formation coefficients for one-detector and opposite two-detector geometries have been calculated theoretically for various multipoles. Different methods for the determination of the multipolarity of internal-pair transitions have been investigated. Optimum multipole discrimination effects have been discussed. The experiments show that a good multipole discrimination power can be achieved with the SMS. (orig.)