FAQ - Radiation Shielding - Making a shield

[Richard Hull]

Shielding from fusor radiation is not a real issue for most of us until we are actually fusing with a fusor operating in the range of 20KV applied or more.

First danger - X-rays

X-rays will arise and stream out of any fusor device at about 15kv and above, but only if the device is made entirely of glass or has glass viewports or rubber hose ports. These X-rays will be very soft and not particularly dangerous. The fusor designer should wisely position the device so that the ports do not face towards the operator or observers.

At about 20kv we are all virtually operating stainless steel vessels and doing fusion. No x-rays will penetrate the steel shell. However, those X-rays that exit the ports will be a bit more nasty and some minor lead shielding is a must. A small port cover or shield can be readily fashioned from lead sheeting like that used in gutters of about 1/16th inch thickness. Most view ports can have their window access remain unshielded by the use of mirrors and a small viewing telescope or a small modern video camera and monitor setup. Again, all of these issues are taken care of without any shielding, provided you designed your fusor correctly from the beginning such that all ports face away from you.

ABOVE 30KILOVOLTS..........

You are now starting to penetrate the shell in many fusors. A much more robust X-ray shielding effort is needed now. This means LEAD! ..........a decent amount of it! You can shield yourself against x-rays up to 70kv with a lead thickness of less than 1/4" (7mm).

Since we are going to also see neutrons as well, I will cover the construction of a complete combination shield near the end of this post.

********************************************************************

Neutrons- Fast and dangerous!

The RBE (relative biological effectiveness) of fast neutrons is the highest of all radiation. Thus, we will need to consider special protection in this area.

Fortunately, most operational fusors produce very minor neutron fluxes. Most fusors hum along in the 20-30 kilovolt supply ranges where, based on the rariifed opeartional period lengths and operational frequencies, both x-ray and neutron numbers and intensities are rather low. Still, shielding ought to be afforded for nearby operators.

At voltage over 30 kilovolts, neutrons can become a serious issue and above 40 or 50 kilovolts, serious shielding efforts against both powerful X-radiation and increased neutron output fluxes are demanded.

****************************************************************

The best shielding is Distance

Nothing is so cheap or effective as placing distance between you and any source of radiation. The inverse square law is a mighty protector indeed. If you have a serious setup, you might place it in a far away corner and remotely operate it from another room or even place the device in an outside shed.

Barring your ability to highly remote a serious fusor system, you should place it, ideally, at an outside corner (best) or against an outside wall of your home. This will shield about 180 degrees of the device, mostly due to the inverse square law on the outside of the dwelling. Hopefully, you are on the ground floor as well.

*********************************************************

A serious, all inclusive, shadow cone shield .............................

We attack this issue of construction with two types of shielding in a layered design. Assuming a rather small device and an operator placed at 6 feet from the device, a small 14 inch by 14 inch movable "shadow cone shield" can be fabricated.

I propose a hollow 1/2" plywood box 3 " thick and 14 X14 inches on the face. The box should be lined with polyethylene of .010 thinkess like a big sealed baggie! The box should be filled with molten paraffin mixed with borax. Once cooled and solidified, a top or lid with an eye hook should should be firmly affixed.

Next, a 1/16 to 1/8th inch thick, 14X14 sheet of lead should be attached on the faces of both sides of the box. Your combo shield is now done.

Ideally, this shield is placed as close to, and infront of, the fusor shell or body in such a way that a relatively radiation free shadow cone is cast at the operator and his control panel. This shield should prove effective up to about 60-70 kv applied.

This shield will be heavy, probably between 40-80 lbs. Some thought migh be directed at a firm ceiling mounted support system on a rail that will allow the shield to be readily slid into and out of place.

The great shadow zone (central part of th shadow) should protect, completely, the head and torso of the operator. This, as noted earlier, is based on critical positioning of the shield prior to operation.

It is hoped that this single simple "combo" shield will act as a model for all fusor shields were it is deemed necessary. [b]See image attached below[/b]

See also viewtopic.php?f=13&t=5270#p33714

Richard Hull