Selectivity

 

The detection of the fission fragments in kinematic coincidence with the $\gamma$-rays and the geometry of the detector system provide additional selectivity in analysis of the $\gamma$-ray spectra. The resulting mass measurement can be used to create $\gamma$-ray spectra gated on a particular mass region. This is most useful for studying the weak mass production channels near the symmetric and very asymmetric regions, where mass gates can reduce the $\gamma$-ray ``background'' of the stronger mass channels.

The Doppler correction allows a $\gamma$-ray to be assigned unambiguously to either the light or the heavy fission fragment and, with $\gamma$-ray gating, provides spectra which include sharp $\gamma$-peaks from only one nucleus. In thick source experiments, since most of the $\gamma$-rays are emitted after both the fission partners come to rest in the source material, it is not possible to identify from which nucleus the $\gamma$-ray originates without relying on the systematics of the $\gamma$-ray coincidence relationships. Also, gating on $\gamma$-rays of one particular nucleus brings back not only coincident $\gamma$-rays from that nucleus, but also the $\gamma$-rays from the several possible partner nuclei (since the number of evaporated neutrons can vary). Using a thin source, the $\gamma$-rays emitted by the recoiling fission fragments are shifted from their true energies due to the Doppler effect. For a given transition, the ``raw'' or detected $\gamma$-ray peak is broad. When the Doppler correction is applied for the fragment of interest (i.e. the heavy or light fragment), the sharp peak shape is reconstructed for the $\gamma$-rays originating from that fragment, whereas the the $\gamma$-peaks are broadened further for $\gamma$-rays originating from the other fragment. This results in spectra which contain only sharp $\gamma$-peaks for the nucleus of interest (see Fig. 3). This selectivity is useful when building level diagrams for nuclei where little is known, such as nuclei with odd Z and/or N.

  

Figure 3: Comparison of spectra from experiments with thin/thick fission sources. In the thin source spectra the sharp $\gamma$-ray peaks are only from one nuclei, whereas in the thick source data the $\gamma$-rays from the partner nuclei are also present. New levels found with the thin source data are shown as thick lines in the level diagram.

If a $\gamma$-ray is seen in the Doppler corrected spectrum, then it was emitted during the flight of the fragment. Sharp $\gamma$-ray peaks which can be seen when no Doppler shift has been applied originate from transitions occurring after the fission fragments implant in the PPACs. This provides a means of distinguishing prompt from delayed $\gamma$-rays. Since the flight time before implantation is $\sim$15 ns, nuclei with isomeric transitions longer than this flight time can be studied. Coincidence relationships between the delayed $\gamma$-rays can be used to identify the levels lying below an isomeric state, whereas the delayed-prompt (with the appropriate Doppler shift) coincidences can be used to identify transitions feeding the isomeric state. The delayed-prompt (with the other Doppler shift) coincidences can be used to identify the fission partner, useful for identifying the isomeric nucleus if it is unknown.

The distinction between prompt and delayed $\gamma$-rays can also be used as a means of performing a recoil-distance type lifetime measurement for lifetimes which are on the order of the flight time (see Fig. 4). The full intensity of these lines can be measured with the thick source data. The Doppler corrected thin source data only recovers that fraction of the decays that occur in flight. The fraction that decay after implantation can be measured from the stopped (no Doppler correction) component, and, after correcting for the off-center efficiency of Gammasphere, the ratio can be used to determine the lifetime. The lifetimes can also be measured directly from the $\gamma$-ray time, since the fission time can be deduced from the time-of-flight measurement.

  

Figure 4: Lifetime measurement with CHICO. The E3 transition shown has a lifetime on the order of $\sim$20 ns. The transitions occurring in flight are recovered by performing the Doppler correction (as shown in the thin source spectrum). The loss of intensity of the E3 relative to the E1 in the thin source spectrum is due to transitions occurring after the recoiling ¹³⁵I implants in the PPAC. This allows a recoil-distance like lifetime measurement.

Because of the Doppler correction, the analysis of the thick source data and thin source data differs. For both data sets the primary method of building coincidence relationships among the $\gamma$-rays is to build 2-(matrices) and 3-(cubes) dimensional data structures in which the $\gamma$-ray energy is plotted versus the energy of the $\gamma$-rays in coincidence, allowing easy gating and projection. In the thick source experiments these structures are symmetric, since the $\gamma$-rays require no Doppler correction. The thin source data requires the use of asymmetric matrices/cubes as well as symmetric matrices/cubes. The symmetric structures, consisting of $\gamma$-rays with like Doppler corrections, are used for building decay schemes for a particular nucleus, where the asymmetric structures are used for establishing the partner fragment, or prompt/delayed coincidences.