Step 7 (Cont. *Final*)
If all goes well, you should see on your camera the image below, and you should be detecting neutrons at this point.

Operation is quite a balancing act, since the voltage is controlled by both the power supply, but also by Paschen's Curve and Ohm's Law relating to the pressure in the chamber. Great patience is required to "Get the hang of it", but after doing so it becomes quite simple to run. Operation can be aided by an ion-gun which will not be discussed in this article.

STEP 7: Fire It Up (and Cross Your Fingers)

Time to turn it on (don't forget to cover any viewports/cameras with lead! Also x-rays can pour out of ceramic feedthroughs so point them away from people. It is a good idea to be monitoring for x-rays where any people are present). The basic procedure is:

-Turn on the roughing pump and wait for sufficient backing pressure, turn on the diffusion or turbo pump and wait for it to fully warm up or achieve running speed
-Throttle the chamber back (with the valve between the diffusion/turbo pump and the chamber)
-Ever so slightly open the needle valve to the deuterium tank
-Turn up the high voltage until either plasma establishes on the camera, or you've reached 40kv and nothing has happened (don't forget, you only get one chance in your life to screw up with voltages of this degree)
-If nothing has happened, keep admitting more gas and the pressure should keep going up. Plasma should form around 40kv at about 10-15 microns of deuterium.

Step 6 (Cont.)
NIM Configuration: If you chose to make a NIM setup as I have, the typical layout is a charge sensitive pre-amplifier at the head of the proportional tube, which is plugged into both a high voltage power supply generating positive polarity voltage appropriate for the tube (in the range of 800V-2kV generally). The amplifier also hooks into a shaping amplifier, which is followed by a Single Channel Analyzer (for setting the detection discrimination level), followed by a pulse counter and/or rate meter.


Shown in the first picture is my NIM setup, the second picture is the pre-amplifier attached to a moderated helium-3 tube, the third picture is a bubble detector after being exposed to neutrons.

Step 6 (Cont.)
-A Proportional Tube
Tubes can be purchased which are filled with either BF3 or Helium-3 (some very old tubes are Boron-10 lined inert gas tubes). These tubes, similar to a geiger counter, can be used with a counting device to detect electrical pulses when neutrons pass through the tube. Either an all-in-one counter can be used, often made by a company called Ludlum, or a modular counting system can be made using NIM modules. The tube is surrounded by about 2 inches of moderating material such as wax or water. This is by far the most accurate and useful form of neutron detection, however the cost of a new tube is prohibitive to most people, and they are extremely rare on the surplus market. Also, counting equipment can become quite costly.

Step 6 (Cont.)
-Silver Activation (Cont.)
In my best runs, I have gotten a piece of silver to about 250CPM over background on a CDV-700 geiger-counter. The disadvantages of this are that it requires a decent neutron flux (at least about 100,000 neutrons/s) which is above the average "beginner's first run" neutron rate. Also, it is somewhat difficult to calibrate, and the counts can't be taken until after the fusor has been shut off.

Step 6 (Cont.)
-A Neutron Bubble Dosimeter (Cont.)
Additionally, they tend to be fairly worn out after a year of shelf life (although I've kept mine in a refrigerated storage container at 50*F and it seems to be like new after I think more than a year). The advantage is that calibration data is provided with purchase and of course it is easy.
-Silver Activation
When silver is placed near the reactor (with a moderator [paraffin wax, water, HDPE, etc] between it and the neutron source, since only thermal neutrons will activate the material) it becomes slightly radioactive with decent neutron fluxes. It has a short half life of only a few minutes, but if you quickly put a geiger counter next to the silver, counts can be detected.