Far Time-of-Flight calibrated, noise problem eliminated
[Report from LPP - “we” refers to LPP] The Far Time of Flight (FTF) has now been calibrated against background radiation.
We first were able to greatly reduce the noise pick-up from the electoral oscillation of FF-1 simply by removing the pre-amplifier from the FTF, which is not needed, as the instrument is sensitive enough without it. We then tested the FTF with ambient radiation, which mainly consists of the decays of radon nuclei in the atmosphere, emitting 6.4 MeV alpha particles. These individual decays were easily detected by the FTF and, since we knew how big they were, they could be used to calibrate the device. For those interested, read the calibration report below:
Calibration of the Far Time of Flight (“FTF”) Instrument
The FTF Instrument is designed to measure the energy of the neutrons generated by the fusion reactions in the DPF. It does this by measuring the time it takes the neutrons to get to the detector, which gives their velocity and therefore their energy.
From the neutron energies, we can calculate the energy of the ions in the plasma that produce the neutrons. In November, we set up the FTF, and discovered that it was picking up too much radio frequency noise from the dpf experimental device. This noise effectively masked any data we hoped to get from it. We had to reduce the noise, and then calibrate the device to determine how big a signal represented how many neutrons.
We reduced the noise down to an acceptable 20 mV simply by removing the pre-amp that amplified the signal from the FTF. According to its literature, the pre-amp had also been responsible for the saturation of the signal at 2.5 V. The unamplified signal is large enough for our purposes.
Measurement of the FTF signal with background radiation showed that the smallest consistently negative-going pulses were with a trigger level of -56 mV. This presumably is due to the radon decay alpha particle with 6.4 Mev. So for single alpha particles, the sensitivity is about 8.75 mV/Mev.
The maximum pulse is 15-20 V, presumably due to saturation of the photo-multiplier tube (PMT). As a check on the sensitivity, 10 V signals were observed every 22 sec on average. From the manufacturer’s literature, that corresponds to about 90 Mev for the muons that make up the bulk of cosmic rays at the surface. For that energy, the range is 50 cm, so the absorption fraction is 5x10-2. This gives 1.0x10-2 particle/cm2-sec. This is exactly what we expect.
A typical pulse is shown in figure 1. Taking the area under the curve, we find the typical pulse is 250 mV-ns. Since the manufacturer says a 6.4 mev alpha particle produce 5x103 photons, assuming 450 nm, one alpha per ns works out to 0.44 nw. With the PMT’s sensitivity (as given by the manufacturer) of 0.5 V/nW, we expect 220 mV for one alpha per ns, essentially what we got.
A 2.54 Mev neutron will produce a 1.25 Mev proton which has half as much light as the alpha, so 1 neutron/ns produces a 125 mV signal. So we need at least 80 neutrons over 100 ns for a 100 mV peak. With a geometric factor of 8x105, this is 6.4x107 neutrons. A 10 V peak is 6.4x109 neutrons.
