Neutron Ball

        The neutron ball was first installed at the Cyclotron Institute in 199?. It is a 4π neutron detector used to measure the free neutron multiplicity. Originally, the neutron ball consisted of 2 half-circle segments is a "hamburger" configuration. When NIMROD was added, the half-circles were rotated 90° to make the upstream and downstream components. A middle section consisting of 4 segments was added to accommodate the charged particle portion. The pseudocumene was replaced in 2006 (before S. Wuenschel's thesis experiment). The scintallator and the PMTs were replaced in 2016 by the NStars collaboration from LLNL. The data sheet for the R1250 can be found here. In 2018, the cave was reconfigurated, which included a new beam dump and removal of the half-wall and beam dump shielding. GEANT simulations were performed and can be found here.)

        The neutron ball consists of 6 segments (1 upstream, 4 middle and 1 downstream sections). The upstream ("top") and downstream ("bottom") section have 4 PMTs and the middle sections each have 3 PMTs for a total of 20 PMTs (reference to HAMAMATSU PMT?). The tanks are filled with EJ-355 (pseudocumene and mineral oil) doped with 0.25% wt. natural Gd.

        Historically, only the neutron multiplicity was determined by taking the difference between the 1st 100μs after an event was triggered and the 2nd 100μs. In order to measure an accurate free neutron multiplicity, the K500 beam pulser was used due to its short turn-off time (~30μs). The phase shifter turns off within 100μs. Results are seen in Fig. 1.

Fig. 1. Beam pulser (left) vs. phase shifter (right). The beam pulser turned off in 50μs with most within 40μs. There was a 20μs dead time from the VME crate and the controller only reads out every 7μs.

Experiments were done to measure the performance of the neutron ball after the new Cave 4 configuration. Results are seen the pptx below.
070318 Results
090418 Results
121718 Results

References for the neutron ball Nuclear Instrumentation and Methods A articles are found here:
1995 NIM A