ANTIOQUIA Y LOS PLANES DE DESARROLLO.

It is firstly important to discuss a misassignment effect observed when tagging nuclei in an isomeric state from their conversion electron emissions as proposed by Jones [17].

Figure 5.6 shows the time of the conversion electron decay for all R-ce tagged events after the tagged recoil implantation with both linear and logarithmic scales. Over the short time scale we see the desired isomeric decays and also a low level random misassignment rate due to background in the DSSSD. From the rate observed

10 15 20 25 30 35 40 45 50

b (%)

Leppanen.. Belozerov Oganessian Bemis Andreyev

Figure 5.5: Value obtained for the spontaneous fission branching fraction in 252_No

bSF (red line) compared with those found in [54], [39], [53], [46] and [50] respectively.

in the Figure, this would give around 31 random misassignments within the isomeric time gate, which compares well with the result from the approximated model given in Table 5.3 which gave 24. This value, along with Nrandom = 6 for R-ce-DT events

are satisfactorily low. There is also however, a clear contribution from the decaying ground state of252_{No, indicated by the half-life, when gating for conversion electrons}

between 37 and 500 keV. These events are thought to be predominantly due to252_No

recoils implanted in the ground state where by _∼50% of their emitted α particles are away from the surface of the DSSSD. They then deposit only a small fraction of their fullα energy as shown in Figure 5.4. This can cause a tagged recoil, implanted in the DSSSD in its ground state, to be misassigned as an isomeric event. From the decay curves in Figure 5.6 it was found that 7.2(10)% of isomeric R-ce tagged events were actually implanted in the ground state. This proportion is enhanced by the fact that the half-lives of the isomeric and ground state decays are not several orders of magnitude apart. It can be seen that this misassignment effect is much more prominent than that caused by random background in the DSSSD in this case, which accounts for only 1.0(1)% of R-ce-T events. It should also be noted that this effect is

0 10 20 30 40 50 Counts / 92 ms Background missassignments No decays, T = 2.25(26) s 8 isomer decays Total decays 0 2000 4000 6000 8000 10000 12000 Time (ms) 10 100 1000 Counts / 92 ms 252 - 1/2

Figure 5.6: Time after implantation of R-ce tagged isomeric decay events, with fit- ted functions for the constituent decays are shown. Given with both linear (a) and logarithmic (b) scales.

not predicted by the model represented in Equation 5.3, due to the assumption taken that the background was randomly occurring.

This effect is also present in R-ce-DT isomer events if the ground state decay tag used is that of248_Fm

α or244Cfα. Figures 5.7 and 5.8 show the isomer conversion

electron and decay time spectra respectively of R-ce-DT events, split between the four possible ground state decay mode tags used. In both cases the spectra are clean for a subsequent 252_{No ground state decay, but significant background appears >500 keV}

forEce and>700 ms for decay time when using the248_Fm

α or244Cfα tags. This again

indicates252_{Noαdecays present in the spectra and shows once more the dominance of}

this as a cause for misassignments, over those due to random background which will be present using all ground state decay tags. The proportion of these misassignments when using 248_Fm

α or 244Cfα tags will be around twice that for R-ce tagged events

at 14.4(20)% as preceeding 252_{No α decays will be emitted away from the DSSSD}

∼100% of the time as opposed to_∼50%. These misassignments explain the increased proportion of252_{No recoils produced in the isomeric state when using}248_Fm

αor244Cfα

as opposed to 252_{No ground state tags, as shown in Table 5.2. This is also the reason}

that these events are omitted when finding properties such as the isomeric half-life. The total misassignment proportion due to this effect for R-ce-DT isomeric events, including also those using 252_{No ground state tags, is then found to be 4.0(6)%. In}

light of this effect it is important to take care when analysing isomer tagged spectra. Reference to the R-ce-DT spectra using only ground state 252_{No decays as tags is}

important, as these are not susceptible to this misassignments effect.

Focal plane γ-ray spectroscopy was performed using the PLANAR and CLOVER detectors and Figure 5.9 shows spectra in coincidence with the conversion electron decay tag for R-ce-DT events. The transitions observed are from the decay of the 8−

isomeric state and transitions from both within the 2− _{side band and those to and}

within the ground state band, can be seen. This is consistent with the observations by Sulignano [24] as given in the level scheme in Figure 5.1.

Figure 5.7: R-ce-DT isomer decay conversion electron energy spectra in the DSSSD-X strips using ground state decay tags of252_No

α, 252NoSF, 248Fmα and 244Cfα.