Reading the drum spectrogram
The striped panel in the middle of the RadBeeper window, and how to read it. It answers one question: is anything reaching the counter on a schedule?
What it is for
Radioactive decay has no rhythm. Counts arrive at random, and a healthy counter in an ordinary room shows that: no pattern at all.
If something does arrive on a schedule, it is not decay. It is a fan carrying something past the tube every few seconds, electrical noise on the counter's supply, or the counter itself batching its reports. In the big number on the screen all of those look exactly like more radiation. Here they look like a stripe.
What you are looking at
It is a paper chart, like the one a seismograph draws. Every eight seconds a new line is drawn at the bottom, just above the counts, and all the older lines move up to make room. Each line is made from the last eight and a half minutes of counts, and the line at the top is about twelve minutes older than the one at the bottom.
The very bottom line is the same measurement averaged over hours. It is drawn in the same colours as the lines above it, in the same places, so a bump in it sits directly under the same bump in the lines above. It is steadier than they are, and slower to change.
Left to right is how often. Slow rhythms are on the left and fast ones on the right. The labels under the panel say where you are: in the picture, the left edge is once an hour, 8m is once every eight and a half minutes, and the right edge is once every six seconds.
Up is more. Where a line rises, more of the counts arrived at that rhythm.
The colour and the thickness say how much. The pen gets hotter and wider as the signal gets stronger:
| You see | It means |
|---|---|
| Thin green lines | The ordinary background. Nothing there. |
| Lime to yellow | Stronger, but still within what chance does on its own. |
| Orange and red, thick and glowing | More than chance usually manages. |
| White | Much more. |
| A dot on a line | The top of a peak. |
| A white ring | The single strongest peak on the whole chart. |
How bright a whole line is says how busy the counter was. A line drawn while the counter was counting twice its usual rate is at full brightness. One drawn while it was counting half is faint.
How to read it
Mostly green: all is well
This is a counter in an ordinary room. The chart is green from edge to edge, with a few warm patches that come and go. This is the good answer.
A warm patch that comes and goes: chance
Random counts make short-lived patterns by luck, and the chart shows them. A patch like this one, ten lines tall, is what luck looks like. It will drift up the chart and off the top, and another will turn up somewhere else.
The rule of thumb: a patch shorter than about a third of the chart is chance. Measured on simulated background, chance patches are usually five lines tall, almost never more than thirteen, and were never seen to reach half the chart. A patch that stays hot for longer than that is worth a look.
A stripe from top to bottom: something real
A hot stripe that runs straight up the chart, in the same place, and stays there as new lines arrive is a rhythm that is really present. Read where it is against the labels under the panel to see how often it repeats.
Then look for the cause near the counter: something that turns, switches or pulses at that interval.
A whole line lit up, side to side: the counts came in a rush
If an entire line is hot from the left edge to the right, nothing is arriving on a schedule. For those minutes the counts came less evenly than usual: in a rush, or in clumps. That is worth a look at the counter itself as much as at the room.
Three things to keep in mind
The left side moves slowly. Slow rhythms need a long time to measure, so the lines on the left hardly change from one to the next and run down the chart as parallel streaks. A streak there is not proof of anything. For slow rhythms, trust the bottom line, which averages over hours.
It only sees strong rhythms. A faint one is invisible here. The bottom line is what finds those, by averaging for a long time.
It never goes off the scale. Whatever is strongest on the chart sets the top, and everything else is drawn against it. The colours do not rescale, so yellow always means the same thing.
Where it is
Run radbeeper-gui with a counter attached. The chart is drawn when the window is at least 640 pixels tall; in a smaller window the counts and the bars keep the space.
The details
The drum spectrogram is the lab report: how a line is computed, how much time the chart holds, the measurements behind the rule of thumb above, and why it is drawn the way it is.