Apparent Diurnal Variation in Background Radiation

by John Walker
January 27 MIM


Aware Electronics RM-80 Radiation Monitor Since October 16th, 1998, I've run, pretty much continuously, an Aware Electronics RM-80 radiation monitor connected to a serial port on a 1992 vintage 486/50 MS-DOS PC. The RM-80 uses a 7313 pancake Geiger-Müller tube, which is halogen quenched and has a minimum dead time of 30 µs, and is equipped with a mica window to allow alpha particles to enter. The unit generates high voltage for the tube from the Data Terminal Ready and Request to Send pins of the serial port and toggles the state of the Ring Indicator line whenever a count is detected. The serial port can be programmed to interrupt on changes in the state of this signal, making it straightforward to implement radiation monitoring in software. Tube sensitivity, calibrated with Cesium-137 (137Cs), is 3.54 µR/hour per count per minute.

The second generation HotBits generator uses an RM-80 detector illuminated by a 5 microcurie 137Cs check source. I decided to attach the HotBits spare detector to a PC and let it run as a background radiation monitor, as much as anything to let the detector run for a while to guard against "infant mortality" in any of its components, should it have to take over for the in-service detector. Aware Electronics supplies the detector with a DOS driver program called AW-SRAD, which I used to log the number of counts per minute, logging the collected data to files in CSV format. After a few months of data collection, I decided to run some analyses of the data, and I'm not sure exactly what I found. Let's take a look.


Counts per Minute Histogram

Counts Times
22 1
23 0
24 0
25 0
26 4
27 6
28 9
29 6
30 22
31 35
32 38
33 83
34 126
35 169
36 234
37 319
38 453
39 553
40 761
41 1039
42 1270
43 1580
44 1794
45 2129
46 2579
47 3001
48 3436
49 3773
50 4257
51 4656
52 4873
53 5274
54 5372
55 5541
56 5740
57 5753
58 5679
59 5535
60 5467
61 5187
62 4847
63 4687
64 4358
65 3988
66 3599
67 3263
68 2910
69 2523
70 2184
71 1883
72 1670
73 1347
74 1193
75 924
76 801
77 638
78 528
79 421
80 339
81 264
82 210
83 171
84 131
85 88
86 66
87 52
88 53
89 25
90 26
91 12
92 16
93 4
94 4
95 6
96 0
97 3
98 0
99 0
100 1
101 0
102 0
103 1
First of all, I plotted a histogram showing the distribution of counts per minute; the vertical axis is the number of minutes in the database in which the number of counts on the horizontal axis were recorded. The histogram table is reproduced in the bar at the right and plotted below.
Per-minute Hourly Count Histogram
This could hardly be a prettier Gaussian normal distribution. Yet the outliers on the high end and the apparent bias toward that side of the curve are suggestive, though probably not significantly so, that we're seeing sporadic cosmic ray air showers generating large counts in the minutes in which they occurred. Unambiguous detection of an air shower requires coincident detection by two or more counters separated by distances of metres to dozens of metres (a typical air shower is a pancake about a hundred metres in diameter and two or three metres thick in which all the particles are moving at essentially the speed of light). I'm planning to get another RM-80 and try coincident detection in the future.

Radiation by Local Solar Time

The next thing I tried is binning the results hourly by local solar time. The following chart shows the results, plotted in terms of average background radiation flux in micro-Roentgens per hour. (The average background radiation of 16.2 µR/hr--142 mR per year--may seem high, but recall that Fourmilab is at an altitude of 800 metres above mean sea level in the Jura mountains. Both the soft and hard [primarily muon] components of secondary cosmic rays are absorbed by the atmosphere, so the greater the altitude the more intense the radiation [historically, this provided the first clue the source must be in the upper atmosphere or space]. At sea level, cosmic rays contribute about 30 mR/year, but at the 10 km altitude at which jet airliners fly, cosmic radiation accounts for about 2000 mR/year; more than 60 times as intense as at sea level.) When I plotted the hourly local time averages, I was surprised to obtain the following result.
Local Solar Time Hourly Flux
I've read about variations in cosmic ray flux with latitude, in Easterly and Westerly incidence, modulation by the solar cycle, and due to changes in the geomagnetic field, but I've never heard mention of a diurnal cycle. Yet this plot appears to show a sinusoidal variation, with a magnitude variation between the highest three-hour period and the lowest of almost 6% of the mean value, with the trough in the curve apparently just about 12 hours from the peak.

To explore whether this might be nothing but an artifact or statistical fluctuation, I then re-binned the same data minute by minute, resulting in the following plot, in which the blue curve is the raw minute-binned data and the red curve is the same data filtered by an exponentially smoothed moving average with a smoothing factor of 0.9.

Local Solar Time Flux by Minute

Randomly selected data subset.
Selected Solar Time Hourly Flux
Well, it still looks credibly sinusoidal, with the maximum and minimum at about the same point. As we all know, the human eye and brain are extraordinarily adept at seeing patterns in random data. So let's try another test frequently applied as a reality check when apparently significant results appear in a data set. The chart at the left was created by randomly selecting 25% of the points appearing in the complete data set and plotting them hour by hour. We find that the selection has little effect on the shape of the curve or the location of its maximum and minimum.

Outliers removed.
Outlier-Filtered Solar Time Hourly Flux
Next, I decided to explore whether the apparent sinusoidal variation might disappear if I discarded outlying values, which might conceivably vary differently in time than those which make up the bulk of the database. I pruned the bell curve at about one standard deviation, then used the remaining data to prepare the plot at the left. As you can see, the case for a sinusoidal variation is eroded somewhat, but the general shape, magnitude, and location of extrema is conserved.

In the Stars?

Finally, I decided to plot the average radiation flux against local sidereal time. Sidereal time tracks the position of the distant stars as viewed from a given point on the Earth. At the same sidereal time, the same celestial objects (external to the solar system) will cross the meridian in the sky above a given place on the Earth. Since the viewpoint of the Earth shifts as it orbits the Sun, the sidereal day (the time between successive meridian crossings of a given star) is about 4 minutes shorter than the solar day (between solar meridian crossings). Correlation with the sidereal period is powerful evidence for a distant source for a given effect. For example, it was such correlation which provided early radio astronomers evidence the centre of the galaxy and Crab Nebula were celestial sources of the noise they were monitoring. Here's a plot of average background radiation flux by sidereal time.
Sidereal Time Hourly Histogram
Any inference based on sidereal time is highly suspect if based on a database covering less than a year, and these data cover only three months, so the apparent bimodal curve may simply be the result of limited data collection time. It will be fun to see how the peaks in the local time and sidereal time evolve as the database grows over the next year. The flux of primary cosmic ray particles is isotropic to better than 0.1%, so one would not expect to see any correlation with the sidereal period in data covering one or more complete years.

What's Going On Here?

Darned if I know! The floor is open to wild conjecture and unbridled speculation.

First of all, I think it's reasonable to assume that any diurnal variation in background must be due to cosmic rays. The balance of background radiation is primarily due to thorium, radon, and daughter nuclides in the local environment. In the vicinity of Fourmilab, the terrain is almost entirely thin, rocky topsoil over a thick layer of limestone. Limestone has little or no direct radioactivity (as opposed to, for example, granite, which is rich in thorium), nor does it contain radon precursors. In such an environment, it's hard to imagine a background radiation component other than cosmic rays which would vary on a daily basis. (This would not be the case, for example, in a house with a radon problem, where you would expect to see a decrease when doors and windows were opened during the day.)

If the effect is genuine, and the cause is cosmic ray flux, what are possible causes? The two which pop to mind are atmospheric density and the geomagnetic field. During the day, as the Sun heats the atmosphere, it expands. If you're at sea level, the total absorption cross section remains the same, but the altitude at which primary cosmic rays interact with atoms of the atmosphere may increase. Further, an increase in atmospheric temperature may change the scale height of of the atmosphere, which would perturb values measured at various altitudes above sea level. But if this were so, I'd expect the variation curve to be more or less in phase with the solar day, while what we seem to be seeing is skewed by about six hours.

Let's move on to the geomagnetic field. It's well documented that the Earth's magnetic field and its interaction with the Sun's create measurable changes in cosmic ray incidence, since the proton and heavy ion component of primary particles is charged and follows magnetic field lines. As any radio amateur or listener to AM radio knows, the ionosphere changes dramatically at night, allowing "skip propagation" of medium- and high-frequency signals far beyond the horizon. Perhaps this effect also modifies the geomagnetic field, affecting the number of charged cosmic rays incident at a given location.

If there is a diurnal effect, why should it peak around 07:00 local time? Beats me. Nor if the apparent (though I believe illusory, due simply to the database only covering a few months) correlation with sidereal time is genuine, why that should have a peak at 12 hours and a trough 8 hours later at 20 hours local sidereal time?

References

Clay, Roger, and Bruce Dawson. Cosmic Bullets. Reading, MA: Addison-Wesley, 1998. ISBN 0-201-36083-7.

Gregory, A. and R.W. Clay. "Cosmic Radiation." In CRC Handbook of Chemistry and Physics (73rd ed.), edited by David R. Lide, 11-133 -- 11-137. Boca Raton, FL: CRC Press, 1992. ISBN 0-8493-0473-3.

Wheeler, John Archibald, and Kenneth Ford. Geons, Black Holes, and Quantum Foam: A Life in Physics. New York: W.W. Norton, 1998. ISBN 0-393-04642-7.

Download Data and Analysis Programs (449 Kb Zipped archive)

If you'd like to massage these data yourself, you can download cosmic.zip, a Zipped archive containing the composite background radiation database in the file rad.csv and a variety of Perl programs which read the database and create the data sets for the various reports, also in CSV format, from which any spreadsheet or GNUPLOT can be used to prepare charts.

Other Radioactive Fun at Fourmilab


by John Walker