Instrument calibration

In late November, Van Allen and I had a series of discussions dealing with the calibration of the GM counters. One of our key experimental goals was to obtain an accurate measurement of the absolute intensity of cosmic rays as a function of orbital position. Good calibrations were crucial to achieving that goal.

Calibration of GM counters was not a new subject. The procedures were well established, not only at Iowa, but also in all laboratories making cosmic ray ob­servations. This topic was discussed in Chapter 1 in connection with Les Mered­ith’s work in helping to establish the cosmic ray balloon program at the Univer­sity of Iowa. His master’s thesis contains an especially detailed discussion of this subject.23

Although the California Institute of Technology enjoyed a rich history of cosmic ray research, the engineers in the satellite program had not shared in that experience. For that reason, and because Van Allen and I were especially concerned about the calibration of the GM counters, I set up the equipment and procedures personally, made many of the runs, and supervised the entire process.

During a 29 November conversation, Van and I discussed the procedures in consid­erable detail. He agreed to send several items that I needed for the purpose, including a laboratory nuclear event counter, the coincidence circuit schematic diagram that I

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had developed there, two actual coincidence circuit assemblies, and several test jigs. He underscored our discussion several days later with a handwritten note that read as follows24:

State University of Iowa Department of Physics Iowa City 12/2/57

Dear George,

1) Rc’d your wire on [cost of] data reduction machinery, etc. I will add to it some for labor, publication, etc. and submit it in the near future.

2) Principal purpose of this note is to remind you of the essential importance of:

(a) Good effective length measurements on Geiger tubes

(b) Absolute efficiency of Geiger tubes for cosmic rays

(c) Counting rate vs. voltage curves (temp. fixed) and counting rate vs. temperature curves (voltage fixed) (preferably for cosmic rays, but since data comes in so slowly that way, at least for radioactive source!)

These tests must be made!

3) If you can send one or two payloads which realistically simulate the Geiger tube’s physical environment we will fly them here for the orientational test (i. e. effect of orientation on Geiger counter rate at balloon altitude). We will supply telemetering transmitter.

Regards,

JAVA

It should be noted that item 3 in that note was never accomplished. Although I carried a spare Explorer I payload back to Iowa City in April, events had overtaken us by that time. We were completely immersed in analyzing the unexpected trapped radiation data, and GM counter orientation information was by then of relatively little importance.

In early December, Van informed me that the laboratory instrument that I had requested was in use elsewhere and not available. That turned out to create a major problem in the light of calibration difficulties that developed later.25

The GM counter calibrations ultimately applied to the Deal I and Deal II instru­ments were extensive. A short list of them includes the following:

• GM counter rate as a function of its applied voltage, typically referred to as the counter’s plateau

• Counter rate variation as a function of counter temperature

• High-voltage regulator tube voltage as a function of temperature (those three calibrations were needed so that we could correct the counting rate for changes in the instrument temperature)

• Counter absolute efficiency for cosmic rays, i. e., the fraction of cosmic rays entering the counter that produce output pulses

• The effective counter length, used for defining its cross-sectional area (those latter two calibrations were needed to compute the absolute cosmic ray intensity)

CHAPTER 8 • GO! JUPITER C, JUNO, AND DEAL I 237

By 11 December, the technicians had assembled the coincidence circuits needed for the later two calibrations, and I made the first absolute efficiency run. Everything appeared to be working properly, so I prepared a set of detailed instructions and turned that operation over to John Collins.26 27

During the following week, I set up and checked the equipment and procedures for determining the counter effective length and turned that operation over to John as well.28 Making those measurements on a number of GM counters continued for several weeks. Reasonable-appearing calibrations were obtained between 28 December and 2 January 1958 for the counters destined for the Deal I payloads.

However, during the second week in January, I observed that the plateau measure­ments that we had begun to make for the Deal II counters were not fully reproducible. It became obvious that spurious counts were being registered during at least some of the runs. That called into question the validity of all the measurements that had been made up to that time, including those for the Deal I counters.

Analysis of the problem required a frustratingly long and tedious effort, in­cluding countless overnight runs. At various times, I attributed the problem to (1) improperly operating laboratory pulse counters, (2) interference from relay contact closures in a nearby crystal oven, (3) contact arcing in a nearby tempera­ture test chamber, (4) possible unstable discrimination of pulse height in the cir­cuit coupling the GM counter to the scalers, (5) unknown background radiation sources in the laboratory, and (6) damage to the GM counters in previous tests. Even after eliminating all of those possibilities, reproducible readings could not be achieved.

The pulse counters being used were standard laboratory instruments, made by Berkeley Instruments. They were designed to determine the frequencies of sinusoidal, square-wave, and other periodic signals, that is, waveforms that repeated in a regular pattern, as opposed to the randomly occurring pulses from our instrument.

On a hunch, on 21 January, I tried the simple expedient of connecting two identical frequency counters in parallel, to see if they would produce identical results. Even though the dial settings were identical on the two counters, their readings were different, and neither one was repeatable. I concluded that the Berkeley counters were too waveform-sensitive and therefore unsuitable for those measurements.

The need for one of the special nuclear pulse counters that I had earlier requested from Van Allen became urgent. I called him with a frantic appeal for one, and he arranged to send it. But it did not arrive for another week.

Meanwhile, I continued with more tests. I improved the coupling between the GM counter and the Berkeley frequency counters. That, too, produced unrepeatable results. I had planned to leave for Cape Canaveral on 26 January for the Deal I launch. But when I finally left the laboratory bench at 2:00 AM that morning in a very discouraged mood, I was compelled to cancel my plane reservations and continue with the calibrations. After a short sleep, I returned to the lab, packed the entire test

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setup in my car, and took it home, where I would be away from all interference sources at JPL. That was no better!

I was getting desperate, as Deal II payload manager Milton (Milt) Brockman was pressing me for the GM counters for the payloads. The following day, the Nuclear Radiation Instruments Model 161 laboratory pulse counter finally arrived from Iowa City. After substituting that instrument, the problem seemed to be solved, and I asked the technicians to proceed with calibration runs for the Deal II counters.

“Seemed to,” because I still had some lingering misgivings. Of course, by then it was far too late to make any further tests or changes on the Deal I instruments, as the payloads were already at Cape Canaveral and the launch was planned for two days hence.

The calibration of the Deal II counters was, from that point on, reasonably straight­forward, and the success with them helped to allay my concerns about the Deal I counters. As it turned out, the calibration of the Deal I flight instrument was shown to be fully valid.