We finish this "Long & Short (of it)" Image du Jour with this image.

Is it another astronomical photo?

It could be.

A stellar burst of energy in an active and hostile space environment?

It could be.

This image is not telescopic.

And it isn't microscopic.

It could be considered not of our material world because the image source never existed in nature as it appears.

Yet, it is nothing but pure nature -- matter.

In the mid-60s at NASA in Houston, I was to calibrate ionizing radiation detectors which would be attached to lunar-traveling astronauts' suits. At the time, lunar travel seemed like a long time into the future. We had confidence in ourselves for the Gemini program because we had developed our technologies during Mercury, but Apollo was going to be the first step for man traveling in deep space.

We needed to know how much energy was lost by the particles going through the thin wall of the detector, and we were using sensor technology designed for high-radiation environment.

We needed to measure the same particles, but those with much less energy and in a smaller population.

I decided that I needed access to a Van de Graaff generator.

This is an atomic particle linear accelerator and I needed a simple one. The more advanced Van de Graaff that would eventually be installed at NASA-Houston was being held up by Kennedy family politics. They wanted NASA to do that kind of work in Boston.

A local oil refinery had a Van de Graaff and I made arrangements to use it as a controlled source of particles. It could eject a few electrons at a time.

I designed what is known as a Faraday Cup. This one was 24" long, 15" in diameter, and in the front was a 4" diameter hole, 14" deep. It was made from one piece of aluminum.

The outside was electrically grounded and an insulated target inside could be electrically connected to an electrometer (electrostatic voltmeter). These instruments had virtually no input load, consequently, they had little effect on what was being measured -- a prime requirement for this application.

As electrons left the generator and went into the target, the target’s electrical charge would change and this voltage would be read. (We are not talking about a bolt of lightning here but just a relatively few very high-speed electrons spit from the generator toward the cup.)

The detector chamber on the astronauts' suits would be about 3/4" diameter so I wanted to measure the radiation in just that area in the cup. To make this possible, I devised a collimator from an aluminum cylinder 4" in diameter and 10" long. It was filled with aluminum and lead wafers, each with a 3/4" hole in the center.

The sensor was placed at the back of the cup and this collimator slipped into the cup between the generator and the sensor. Now the beam from the Van de Graaff would be masked down to 3/4" diameter for the detector and other energy masked out.

I had one problem (that I was aware of). It is called S-band scattering.

We know that light going past the edge of an object is scattered by diffraction. This is why the shadows of something like a telephone pole are sharp at the bottom on the ground but as the pole shadow progresses across the ground, that shadow edge gets more fuzzy. That is evidence of light diffraction.

Some of the energy from the Van de Graaff, while going through the 3/4" holes in the collimator, would be "diffracted" away from the sensor by the inside edges of the holes. What came out of the Van de Graaff would not necessarily impinge on the sensor I was trying to calibrate.

To determine if this would be the case, and if so, how I could adjust for it, I developed a little test.  I got a bunch of 4" x 5" film sheets, cut them to 3" x 3", wrapped each sheet in light-proof paper, and then sealed the edges of the paper with black electrical tape.

Why use that sort of tape?

Why not?

It is what I had handy when I walked into my dark room to wrap the film. (We have learned that much good science can come from non-decisions.)

I transported everything back across Houston to the Van de Graaff facility. I aligned the cup with the Van de Graaff exit gate, put a piece of film in the back of the cup, slid in the collimator, left the generator room, and in the control room had the Van de Graaff opened to expose the film to the particle beam for about one minute (some small amount of time -- I don't recall now).

I repeated this procedure with different exposure times for as many sheets as I had -- maybe ten.

I went back to my lab and processed the sheet film in an open tray.

This was the same lab where I had processed thick-film nuclear particle tracks, and later would process emulsions on 4" x 10" glass spectrographic plates to adjust solar filters. (And it was where I did the work on the WWII German aircraft film seen in an earlier Image du Jour series.)

The image shown today is a print from one of the test films from the Faraday Cup. It was exposed entirely by nuclear particles. What is white in the image was exposed black by the radiation on the negative film.

When I first saw the film in the darkroom I thought, 'What kind of crap is this?'

I was expecting a center spot with a little fuzzy edge caused by the scattering. I had been planning on just running a scanning densitometer over the spot, integrating the resulting pattern, and then I would know how much of the total energy coming in was being diffracted away from the sensor rather than being measured by it.

This should have been easy.

But what is all that other stuff that exposed the film around the center disc? ('Well,' I thought 'at least the disc looked like I expected.')

The other "stuff" wasn't from a light-leak because it was not uniform, yet it appeared on each sheet of film. It had to come from the Van de Graaff generator - but what energy coming from that could penetrate all the lead that I had in the collimator?

None.

The energy had to come from the Faraday Cup itself.

Static electrical discharges?

I didn't think so. I had seen that on a lot of film before and that looked different.

Could the Van de Graaff exit be scattering so much as to come through the back of the cup?

Through 10" of solid aluminum?

That thought was the clue I needed.

The particles that had exposed the disc had continued through the film with plenty of energy and they went into the back of the cup and were absorbed but many came back as back-scatter and some of the exposures could be from another type of energy.

It happens.

This affect is called "bremsstrahlung" -- German for "braking energy" -- and it can cause short-lived, secondary emission. And when human tissue is involved, bremsstrahlung can cause more damage than the energy causing the bremsstrahlung.

This cup was not going to work because of back-scatter and bremsstrahlung.

But I noticed that if I squinted my eyes I could see an "X" in the image. It went from corner-to-corner. That "X" was a little lighter on the negative film. (You can see it as darker areas in the image -- if you squint.)

What caused THAT?

Remember that if it was dark on the print then it was light on the negative and this meant that that area of the "X" didn't get as much radiation as the rest of the film background.

What made the "X"?

The electrical tape.

Which was made of vinyl.

And there was my solution. I would not defy nuclear physics (as if I could) but use plastic to convert this incoming energy to something that wouldn't be a problem. By the time the energy reached the back wall of the cup, it would have already been through the space suit detector. I didn't want the back scatter to come back through the back of detector and be "detected" again. That would create some bad calibration data.

I would just moderate the back-scatter -- acrylic is good for this.

I cut a 4" diameter disc of 1" acrylic, shoved it to the back of the cup, and repeated the experiment. That time I had very little secondary emission and back scatter.

Then I scanned the exposed film with a densitometer, and determined the percent of area that was outside the geometric limits of the detector. The energy that the generator beam calibration said went into the cup was reduced just by the percent of S-band scattered energy that did not reach the space suit sensor.

This would be as accurate as we could get. (And better than we needed.)

But remember that this whole Long & Short Image du Jour series started with nuclear measurements of +/- 50%.

It takes the sort of measurements, engineering, and guess-work as used with the Faraday Cup to get all the accuracy the discipline will allow.

So "close enough for government work" has lots of dimensions -- and I seem to have worked them all -- from counting individual electrons going into human tissue to measuring energy emitted from targets 92,000,000 miles away.

And this realization contributed to my feelings of "getting through."

It might be of interest that back then, none of this work utilized a PC. Indeed, we didn't have any electronic hand calculators, either. They hadn't been invented.

Our hand calculators were slide rules, paper, and pencils. The pencils had red lead, blue lead, green lead -- and our slide rules were clipped to our belts and our pencils were in pocket protectors.

All we nerds had them.

But back then, "nerd" had not been invented either.