Geant4 Simulations for the Radon Electric Dipole Moment Search at

4.2.1 Multipolarity Effects
The shape of the γ ray angular distribution has a significant impact on the observed
signal inthe GRIFFINdetectors. Figure4.6 gives examples of some common γ
ray angular distributions in the 223 Rn decay. Figure 4.7 illustrates the resulting signal
and fits for various multipolarities. The simulations presented in Figure 4.7 consisted
of two million 223 Rn decay events from an initial two billion nuclei. The decay scheme
was modified to enhance the 592 keV transition, such that the 592 keV γ ray was
emitted in every decay. The multipolarity of the 592 keV transition was modified to
reflect that of the multipolarity of interest. In these simulations the EDM cell, oven,
magnet and µMetal were absent. The initial polarization of the 223 Rn nuclei was
100% with a spin-decoherence time (T 2 ) of 10 seconds. The precession frequency was
simulated to be 1 Hz and the ring of eight GRIFFIN detectors were in their highest
efficiency mode in the forward position.
The resulting fits were good, with reduced χ 2 values (see Figure 4.7) of approximately
one. The observed precession frequencies are twice the input value due to
the symmetric nature of the γ ray angular distributions about 180 ◦ . The exception
is the J i = 7 2 to J f = 5 2
E2 transition which is four times the input frequency due
to its four lobed angular distribution. The second term in the fitting Equation 4.2
(A 3 e −x
A 4 ) is shown to be necessary as the average intensity varies on the timescale of
the depolarization as the angular distribution becomes isotropic. A positive polarization
intensity (A 3 ) gives an overall decreasing intensity on the time scale of the
depolarization time, whereas a negative polarizationintensity gives anoverall increasing
intensity on the time scale of the depolarization time. The polarization intensity
(A 3 ) is positive for “dumbbell” shaped γ-ray angular distributions and negative for
“donut” shaped γ-ray angular distributions.
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