Investigation of Positron Line Emission in Heavy Ion Collisions with the EPOS-Spectrometer

Abstract

Positron production in nuclear collisions is well known from internal pair conversion processes of nuclear transitions with energies exceeding 2m_Oc². The pair production is related to the competing γ-decay of the nuclear transitions through conversion coefficients and the resulting positron yield is essentially proportional to the $\gamma$-ray intensity. An additional strong source of positrons, however, has been detected in very heavy ion collisions already in the first year of the UNI LAC operation. ^1,2) Compared to the positron yield expected from nuclear processes as inferred from the γ-ray spectral intensity (E_γ > 1 MeV), the new source dominates the positron production for the high-Z collision systems with the combined charge Z_ua = Z₁+Z₂ > 160 (Figure 1 on page 196). From its steep increase with either Z _ua (see c)or with the decreasing internuclear distance R_min (see b), the new contribution was identified as quasiatomic positron production^3,4). Indeed, the deep (V_c > 20 MeV) Coulomb potential temporarily formed during the collision for some 10^-20sec changes fast. Mostly when the logarithmic derivative R/R of the internuclear distance R(t) (part (a) of Figure 2 on page 197) is maximized, this time variation gives rise to induced Coulomb ionization type⁶) transitions from the negative energy continuum states to bound quasiatomic or continuum electronic states (Figure 2 on page 197).