Neutron Time of Flight Detectors

Another update from Eric Lerner.  “We have installed, but not fully tested, two important instruments, the neutron time-of-flight detectors.” 

See also accompanying Time of Flight Detector photos courtesy of yours truly.  Now, back to Eric:

Neutron Time-of-Flight detectors installed

We have installed the two time-of-flight detectors that will give us a good deal of information about the plasmoid’s density and temperature as well as another way of measuring total neutrons. 

How they work

The detectors work as follows: a neutron hits a proton in plastic scintillators, causing the proton to move and give up its energy to an electron.  This in turn emits visible photons of light.  The light goes to a photomultiplier tube, where the photons knock electrons off thin electrodes charged to 2 kV.  The electrons accelerated by the field smash into other electrodes, creating more electrons. 

This happens several times, leading to a great amplification of the initial signal.  The output is then electronically amplified further, so that individual neutrons can be observed.  Photons displace electrons directly as well, but the neutrons can be distinguished because the neutrons take hundreds of ns to reach the detectors, traveling far slower than the photons. 

By measuring when the neutron burst passes each detector, we can find out the velocity and thus the energy of the neutrons and determine the average energy of the ions that fused to produce them.

Detector sensitivity

An initial test of the far time-of-flight detector (located at 17.15 meters from the axis of FF-1) showed that the detector saturated (recorded more than its maximum) for both the X-ray and neutron signal from a relatively small pinch.  So this shows that it is more than sensitive enough for our purposes. To put the signal within the region where the PMT responds linearly, we shuttered off all but 6.25 square cm of the scintillator.  We will be testing further with it and the near time of flight (located at 11.05 meters).

Instruments in use

We are now using six instruments:  the two time-of-flight detectors, two neutron detectors, the main Rogowski coil and the lower Rogowski coil to detect ion beams.  Except for the main Rogowski coil, we are still testing the instruments.