Most people’s fusor or neutron source is placed relatively close to their moderator. This FAQ is more directed at the fusor as a neutron source. In the volumetric reduction, lines can be drawn from a composite top and side view of the system to define the volume of possible straight line fast-neutron radiation within the predefined volume. Remember, this pre-determined volume is based solely on your moderator in which your slow or thermal-neutron detector is located.

All of this assumes your detection consists of a homemade moderator with a thermal neutron detector buried within it, and that you know the relative efficiency of your thermal neutron detector within a suitably sized moderator. This method will also work for manufactured thermal neutron counters that have their own moderator, (REM balls, polyethylene attached or detached moderator’s, etc.

In the diagram below I take my system as an example. Its volume is totally determined, as noted above, by my 46cm x 15cm x 15cm moderator. The longest dimension of the moderator determines the size of the required cubic volume. In my case it is a 46cm cubic volume. Fortunately, this includes the entirety of the fusor as well. Lines are drawn in the 3D representation in the two planes to the moderator within this volume. in both views the moderator is drawn in the center of, and touching one wall. You will note the dark gray area represents the uncaptured or missed straight line neutron emissions from the fusor. The white volumes are the source and volume of the neutron straight line field volume. This neutron field volume from the fusor can and do enter the moderator which has defined the cubic volume. Study this drawing and its two views.

You will note that the gray area where lost neutrons are extant are broken up into carefully measured rectangles and right triangles. It is important to first figure out the volume of your entire cubic volume. In my case it is 97,366 based on the cube of 46cm which is the moderators longest intercept side. Figure out the cubic volume of your moderator. This is very important. Find the areas of all the triangles and rectangle in both drawing making sure not to figure the same areas twice. It is important to multiply all these two view areas one-by-one by the long dimension of the cube to get their volume in cubic centimeters.

Hopefully when you add them all up, you will have the lost volume to neutron detection. Dividing the lost neutron volume by the moderator volume. (This volume intercepts the neutrons to be moderated and counted). In this case the multiplier ratio is 7.2. However, the 3He detector in its moderator is only 70% efficient. Thus only 70% of the intercepted Neutrons ware counted to correct for this the volume multiplier

7.2 must be divided by 0.7.....7.2/0.7 = ~10.3. Rounding this to 10 makes this factor the new multiplier for Fusor V's digital 3He cpm rate to yield the true TIER. (As seen in the diagram)

**Now, flies in the ointment**

We now know that the average fusor is not totally isotropic in emission. Beaming is a suspected culprit in this anisotropy. While fusion does take place in velocity space within the fusor which is isotropic, the beaming is shown to perform beam-on-target fusion to such an extent that beam-shell contact points tend to show a preference for forward neutron emission. This forward beam emission, it is believed, accounts for a significant amount of the emitted neutrons. Rare is the perfect spherical fusor produced now. With a simple 3 ring grid in a spherical fusor there are six beams. My straight-line emission from these beams only intercepts the moderator with 2 of the 6 beams. As such, assuming equal beam currents, I am missing up to ½ to 2/3 of the neutron emission counts! This anisotropy leads to either a much higher or much lower TIER than I think I am getting! I feel I just cannot assume to justify a multiplication factor of 15 or 18X to my count for TIER. I feel there must be a beam, shell impacts where some or even many neutrons are, indeed, issued at back angles. If not a 12X multiplier would make my 92500 X12 be a 1,110,000 reading and I just refuse to believe this, as the calibrated electronic measurements did not show this to be the case!

Regardless of anisotropy, only a pure beaming machine without a wire grid system can be assumed to give a more decent count using this volumetric system. Then the back half of a wire gridded machine, be it a sphere, a cylinder, a cross or a cube will be lost counts.

Expect me to use this factor of 10X the 3He cpm rates to report my tier rates.

Richard Hull