Double Gamma Decay inCa40andZr

Abstract

The rare double gamma decay of the first excited 0 + state in 40 Ca and 90 Zr has been measured with a segmented 4TT Nal detector system, which allows suppression of the perturbing background due to positron annihilation in flight. In both cases the directional correlation of the two photons is found to be asymmetric around 90°, which is explained by an interference of 2EI and 2M1 transitions. The deduced Ml quenching factors agree with those from (e,e') and (p,p') measurements. PACS numbers: 23.20.En, 27.40, + z, 27.60. +j Many attempts have been made to observe the double gamma decay in nuclei such as 16 0, 40 Ca, and 90 Zr: As these nuclei have a ground and first excited state with spin and parity 0 + , the first excited state can only decay by internal conversion and pair production or by the simultaneous emission of two y rays, each with a continuous energy spectrum but summing up to the O^ -> 0+ transition energy. As a result of the small 2y branching ratio ( = 10~4), these early experiments suffered from insufficient statistics and background problems. In this Letter we present the first unambiguous observation and detailed investigation of the 2y decay mode in nuclei, which was performed by use of the Heidelberg-Darmstadt "crystal ball," 2 a 4TT y-ray detector system comprising 162 individual Nal(Tl) modules. The surprising result of our measurements on 40 Ca and 90 Zr is that the nuclear 2y decay proceeds not only via 2EI but also via equally strong 2M1 transitions. The resultant 2M1 strengths supply an independent measurement of the Ml quenching factors deduced from recent (e,e f ) and (p,p f ) experiments. 3 The 0+ first excited states 4 of 40 Ca (E 0 = 335 MeV, r 1/2 = 2.1 ns) and 90 Zr (#0=1.76 MeV, Ty 2 = 62 ns) were populated via known (p,p') resonances at ^ = 5.08 MeV and ^ = 7.08 MeV, respectively, with use of a pulsed beam (0.5 ns width) supplied by the Emperor accelerator of the Max-Planck-Institut. Inelastic protons were detected at backward angles in four surface-barrier detectors, each subtending a solid angle of 0.17 sr; they were mounted together with the metallic targets ( 40 Ca: 0.3 mg/cm 2 , 99.97%; 90 Zr: 0.7 mg/cm 2 , 97%) in the center of the "crystal ball" (CB) detector. Special care has been taken to suppress the perturbing y background stemming from the decay of the O2" state via internal pair conversion and the subsequent positron annihilation in flight (PAF). For these events the y-ray energies E yX ,E yl and their relative angle 0 12 are kinematically related bywhereas for the two-quantum decay all relative angles are allowed. Since the angle resolution of the CB is ±9°, the disturbing PAF events with E y i + E y2 = E e + + 2m e c 2 =£0 can De identified by means of Eq.(1) as long as they occur in the CB center. We therefore surrounded the target and the silicon detectors by a small Lucite box of 5-mm wall thickness to ensure that the positrons were either stopped within 5 cm from the CB center or at least sufficiently degraded in energy. In the data analysis we accepted onl...

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Frequency dependence of coherently amplified two-photon emission from hydrogen molecules

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