Experimental Description

The experiment was carried out at Gammasphere with 92 Ge detectors. A thin ²⁵²Cf source was used allowing the fission partners to recoil out of the source and be detected by the Rochester parallel plate avalanche counter (PPAC) array (CHICO) as shown in Fig. 1. The source was 70 $\mu$ Ci of ²⁵²Cf electroplated onto a 500 $\mu$ g/cm² Ni foil with a 20 $\mu$ g/cm² carbon cover foil to prevent self-transport of the Cf. Events consisting of 2 particles and at least 3 $\gamma$ -rays were collected at $\sim$ 4000 Hz. The total data set contains $\sim$ 900 million events.

**Figure 1:** Schematic of the experimental setup. The PPAC array (CHICO) is shown with two representative Gammasphere Ge detectors.
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The particle detector (CHICO) consists of 2 conical arrays of PPACs covering a total solid angle of 68 $\%$ of $4\pi$ sr, although the usable solid angle for this experiment is $\sim$ 50 $\%$ due to energy loss of the fragments in the Ni backing foil. The detector measures time-of-flight differences of the fission fragments with 500 ps resolution and angular position of the fission axis to within $1^{\circ}$ in $\theta$ and $9^{\circ}$ in $\phi$ . Although PPACs do not measure the energy of the fission fragments, as silicon detectors do, they accept large count rates (up to $\sim$ 1 MHz for CHICO), can be tuned to be insensitive to the significant $\alpha$ -field (32 times the fission rate), and are resistant to radiation damage.

The total kinetic energy (TKE) variation of the nascent fission fragments as a function of mass division is well known (see for example Refs. 1,2). By using this TKE and simple 2-body kinematics, correcting for energy loss in the backing foil and geometric variation of the flight path, the time-of-flight difference gives the mass division and velocities of the fission partners. The minimum flight path from the source to the detector is 13 cm, giving a time-of-flight difference for the strongest channels of about 5 ns. The short flight path along with the time resolution of the detector limits the experimental mass resolution to $\sim$ 8 amu as shown in Fig 2.

**Figure 2:** Mass Resolution. The experimental mass spectrum is projected out by setting double gates on the $\gamma$ -rays of a particular nucleus. The relative intensities of the mass distributions are not to scale.
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Since the fission products are recoiling at large velocities, the $\gamma$ -rays emitted in flight need to be Doppler corrected. As mentioned above, the velocities of the fission partners are measured along with the angular position of the fission axis, allowing the Doppler correction to be performed on an event-by-event basis. The $\gamma$ -ray resolution after the Doppler correction is $\sim$ 7 keV for a 1 MeV $\gamma$ -ray, compared to $\sim$ 2 keV intrinsic detector resolution. The degradation of resolution after the Doppler correction is due mainly to the $7^{\circ}$ opening angle of the Gammasphere Ge detectors.