Apparent Diurnal Variation in Background Radiation
by John Walker
January 27 MIM
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.
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.
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.
| Randomly selected data subset.
|
|
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.
|
|
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.
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.
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
