4.3 MINIBALL data calibration
4.3.3 Background correction
Figure 4.13 displays the timing correlations between fission fragments andγ rays. The dominant component stems from the prompt γ rays arising from the prompt fission events (region A), whilst the interesting events with promptγ rays in coincidence with isomeric fission fragments can be found in region B, the separation of the two components is obtained by gating on the walk - corrected energy - dependent time signal of the MINIBALL detectors and the coincident fission fragments. A third component (region C) is visible, comprised of delayed γ rays following the delayed fission of shape isomers. A fourth region D is indicated, this is used for the subtraction of random background. γ rays emitted from the decay of shape isomeric excited states of237fPu occur within the typical lifetime of rotational states (≈picoseconds after the excitation of the shape isomer), while the isomeric ground - state decays via delayed fission after the lifetime of the respective shape isomer. Therefore the time window accessible for the analysis of fission fragments belonging to the shape isomers from the short - lived 115 ns isomer in 237Pu covers
approximately two halflives, whilst for the long - lived isomer (t1/2 = 1120 ns) only a fraction of
D
B
C
A
Figure 4.13: Timing correlation between fission fragments and γ rays. Highlighted are four different regions. Region A corresponds to the promptγ- rays in coincidence with prompt fission fragments. Region B contains promptγ- rays in coincidence with delayed fission fragments, while region C corresponds to the fission γ rays from the delayed fission fragments and region D (see Fig. 4.8) represents a region of random background.
Having calibrated the MINIBALL and PPAC data, the promptγ- ray energy spectrum in coincidence with delayed fission events could be extracted. Gating only on those fission events, where both fission fragments are delayed by at least 70 to 90 ns (due to the energy loss in the thick 235U target) with respect to the promptαbeam pulse, and selecting promptγ rays within
±30 ns of the beam pulse, results in the final spectrum ofγ rays belonging to the shape isomers in237fPu (see Fig. 4.13 region B). Figure 4.14 a) shows theγ- ray energy spectrum, summed over
all 24 Ge detectors, obtained by gating on region B in Fig. 4.13 containing prompt γ rays in coincidence with delayed fission events. A background component from neutron - inducedγ rays is visible (indicated with dashed lines in the spectrum of Fig. 4.14 a), furthermore it is assumed that a random background of γ rays is also present. For this reason theγ ray spectrum shown in Fig. 4.8 is normalised to the neutron - induced γ rays present in Fig. 4.14 a) and added to a random background component ofγ rays obtained by gating on region D in Fig. 4.13 resulting in the background spectrum shown in Fig. 4.14 b). Subsequent subtraction of the γ ray spectrum shown in Fig. 4.14 b) from the spectrum in Fig. 4.14 a) results in the γ ray energy spectrum shown in Fig. 4.14 c). It was the task of the analysis described in chapters 5 and 7 to extract a level scheme for 237fPu from this rather fragmented complexγ- ray energy spectrum.
4.3. MINIBALL DATA CALIBRATION 71
b)
c)
a)
Figure 4.14: a)γ- ray spectrum following the235U(α, 2n) reaction, obtained after gating on re- gion B (promptγ- rays in coincidence with delayed fission events) of Fig. 4.13. Background from neutron - inducedγ rays is visible and indicated with dashed lines. Figure 4.14 b): background component obtained by taking random coincidentγ- rays (Fig. 4.13 region D) and adding these to the normalised prompt fission γ- rays seen in Fig. 4.14 a. Fig. 4.14 c) Background - corrected prompt γ- ray energy spectrum following the 235U(α, 2n) reaction, obtained after gating on
region B of Fig. 4.13 and subsequent normalised background correction (Region A and D of Fig. 4.13).
Chapter 5
Extraction of experimental results
Following the calibration, event building and subtraction of random background from the ex- perimental data, a detailed analysis of the experimental findings was performed. For the two known fission isomers in 237fPu half lives of t
1/2= 110 ns and t1/2= 1180 ns, respectively, were
reported [RUS71, VAN73, SPE74]. In order to verify the consistency of the present data with the published properties of the two fission isomers in237Pu, the isomeric decay curve of237fPu as displayed in Fig. 5.1 was analysed. The spectrum displays the time spectrum of fission fragments detected with respect to theα- beam pulse, exhibiting the tail of the prompt fission component extending up totf rag≈15 ns, followed by a delayed component originating from a superposition
of both fission isomers. An exponential fit applied to the decay curve in the delayed time region (beyond 15 ns) exhibits a half-life of 165 ns (solid line), originating from a superposition of con- tributions from the two fission isomers in237Pu with t
1/2= 110 ns and 1.1µs, respectively. Using
a two-component fit with fixed decay constants for the two isomeric contributions (dashed and dash - dotted lines in Fig. 5.1) results in a relative intensity of σlong
σshort ∼ 10 in agreement with
literature values, where a value of σlong
σshort= 9.7 has been reported [VAN73].
From the fission fragment lifetimes we conclude that indeed states in the second potential minimum of 237Pu were populated. Inspecting the γ- ray spectrum displayed in Fig. 4.14, we
see a highly fragmented distribution ofγ rays, as expected for an odd nucleus. In the following sections it will be described how the level structure of237fPu was extracted from these data (see
chapters 5.2 and 6). The first step presented in the following section will be to disentangle the inclusiveγ- ray energy spectrum of Fig. 4.14 into its disjunct contributions from the two fission isomers, thus enabling to build level schemes separately for each isomer and then to merge these two independent level - schemes into a common level - scheme describing the nuclear structure of
237fPu.
5.1
Disentanglement of the
γ
spectrum
Having shown that states in the superdeformed minimum of 237Pu were populated, the next
task of the analysis was to separate the underlying contributions from the long - lived (high - spin) isomer from the short - lived (low - spin) isomer. In chapter 4.3 the MINIBALL Ge detector
tfrag[ns]
Counts/ns
Figure 5.1: Time spectrum of fission fragments measured with respect to the prompt beam pulse. An isomeric decay curve is visible in the delayed time regime, exhibiting an integral life - time of 165 ns (solid line), consisting of a superposition of contributions from the two fission isomers in 237Pu (t
1/2=110 ns and 1.1µs, dashed and dashed - dotted lines, respectively). The
relative intensities of the two isomeric contributions are consistent with the published isomeric population ratio σlong :σshort [VAN73].
resolution was found to be 2.9 keV, only those transitions in Fig. 4.14 exhibiting a line width of about this value were accepted as a ”real” γ transition. In this way 149 γ transitions were identified, only theseγrays were considered in the following discussion. Using the same technique as applied to measuring the overall lifetime of all fission fragments in Fig 5.1, we can gate on each of these individualγ transitions and inspect the corresponding coincident fission fragment decay curve. This results in the individual isomeric half - life connected to this specificγ transition. In this way for each γ transition the decay time for the corresponding fission fragments measured with respect to the beam pulse were analysed and tabulated. As a typical example Fig. 5.2 shows the decay curve for the 204 keV γ transition. It was found to belong to the long - lived fission isomer, since the exponential fit to fission fragment times establishes a half life of 730 (30) ns for this transition.
Table A1 (see Appendix A) lists allγ- ray transitions identified in the spectrum shown in Fig. 5.4 with a line width corresponding to the resolution of the germanium detectors. Furthermore the half - lives for the coincident fission fragments of each of the γ transitions and the number of counts in the respective peak are shown.
Following the assignment of the decay time to each of the 149 γ transitions, the resulting distribution (see Tab. A) of the half - lives has been plotted in Fig. 5.3. Two components can be distinguished, first a well - localised peak around 115 ns, which corresponds to the half - life of the short - lived fission isomer (see Fig. 5.3). 97 γ transitions contribute to this component. A second component (52 transitions) contains the remaining long - lived transitions with half - lives
5.1. DISENTANGLEMENT OF THEγ SPECTRUM 75 Eγ= 204 keV = 730(30) ns t1/2 0 100 150 200 250 10 25 45 40 35 30 15 20 5 50 counts/25ns t ff[ns]
Figure 5.2: An example of an individual γ- transition half - life. Shown is the measured time difference between the beam pulse and the detected fission fragments gated on the 204 keV
γ- transition. An exponential fit to the decay curve results in a half - life of 730 (30) ns, thus qualifying this transition as originating from the long - lived fission isomer.
up to ∼1.2µs. However, due to the limitation of the available delayed fission time window to ≈270 ns (seen in Figure 5.1) and thus to only a fraction of the half - life of the long - lived fission isomer, an unambiguous identification of the long - lived fission isomeric component in the half - life distribution was not possible. Nevertheless, the correct population ratio between the two isomers inferred from Fig. 5.1 justifies to identify the long - livedγ transitions with the decay of the 1.1µs isomer.
Fig. 5.4 shows again the complete energy spectrum of prompt γ rays in coincidence with delayed fission fragments as already displayed in Fig. 4.14. Marked by red arrows are those γ
transitions identified as belonging to the short - lived isomer, while the green arrows indicate transitions corresponding to decays of the long - lived isomer.