RadAl™-1: Radiation Monitor and Alarm

cost-effective protection against radiological risk

RadAl™-1


Sensitivity of the RadAl™-1/1 radiation alarm instrument

The maximum workplace exposure to external radiation allowed by law is presently in general set at 30 mSv/y (ancient units: 3 REM/year), approximately equivalent to 20µGy/h (2 mR/hour), assuming 1 Gy = 1 Sv (1 RAD = 1 REM) and 200 workdays per year of 8 hour each. Radiation levels at or above this value are considered dangerous to human health.

The RadAl™-1/4 will signal alarm at radiation levels 1000 times lower than this, namely, at 20 nGy/h (typical).

Discussion

The RadAl™-1 uses a proprietary algorithm to provide fast response to high radiation fields, while maintaining high sensitivity (with slower response time) at low radiation fields.

The alarm is programmed to trigger when the detected radiation field exceeds significantly (in a strictly mathematical sense) the (instrument + natural) background.

The instrument background is (in worst case) of 1 cps (count per second) equivalent to 0.08 µGy/h. Natural background is by definition variable but can be assumed to be in the range of 0.04 to 0.20 µGy/h at sea level.

Sensitivity varies with local background, nature of source, and measurement time, namely, how long the field is presented to the instrument.

The following table represents sensitivities calculated assuming normal background, Co60 source, and worst case electronics. Calculations are based on a RadAl™-1/1 model with a (worst case) GM tube sensitivity of 12 cps·µGy-1·h-1.

Time elapsed

Trigger Field

6 seconds

0.4 µGy/h

1 minute

0.12 µGy/h

10 minutes

0.04 µGy/h

In other words, in the presence of a 0.4 µGy/h (or stronger) gamma field, the RadAl™-1/1 alarm will trigger within 6 seconds; if the source produces less then 0.4 µGy/h but more than 0.12 µGy/h, the alarm will trigger within 1 minute, and if the source is between 0.12 and 0.04 µGy/h, the alarm will trigger within 10 minutes. These values should be halved for the RadAl™-1/4 model.

In the energy range of gamma radiation produced by Plutonium, Uranium and their decay products, the GM tube used in the RadAl™-1 is up to seven times more sensitive than it is to Co60   radiation, hence the actual sensitivity in the presence of Special Nuclear Materials is even better than indicated above.

It is very complex to translate sensitivity data from µGy/h units (radiation field strength) to µCi (radiation source strength) and from here to weight figures, since the nature of the source, its distance from the detector, and of course any intervening shielding must be taken into account.

Typically, the RadAl™-1 is tested using a 1 µCi Cs137   radiation source. This source triggers the alarm within 6 seconds when at a distance of less than ten centimeter.

It can be calculated that 1 µCi Cs137 produces approximately the same field as 30 µCi Am241 , but since the instrument has higher sensitivity, only 5  µCi Am241 are necessary to produce the same signal. This amount of Americium is approximately equivalent to 1 µg  Am241.   Am241 is present as a contaminant in all Pu samples, in concentrations between 0.5% and 4%. Hence 0.2 mg of high purity (military grade) Pu, if unshielded, will trigger the RadAl™-1 alarm from a distance of less than ten centimeters. In reality, Pu samples emit larger amounts of gamma radiation, which increases sensitivity, but on the other hand in reality they are probably going to be heavily shielded, which will increase the minimum detectable amount in real word situations.

One fact should be clear from the above discussion:

The RadAl™-1 radiation alarm guarantees that the workers in its vicinity will not be exposed to dangerous levels of radiation

Finally, we are often asked to compare the sensitivity of the RadAl™-1, based on Geiger - Müller detectors, with that of competitor units using NaI(Tl) crystal detectors.

NaI detectors are more expensive and fragile; they also have a complex response curve vs. gamma energy, whereas GM tubes are essentially flat on a Gy scale, with a single pronounced peak at 60 keV, which is the range of maximum interest in radiation protection.
NaI detectors also have have higher sensitivity per unit volume, which allows faster response. Such faster response is necessary when trying to detect, for example, radiation in cars driving by at 60 mph. Over longer time scales (seconds), sensitivity becomes somewhat less relevant since what matters is being able to detect a significant change over natural background: that is where the algorithm used becomes critical.
Other (NaI crystal based) instruments specify fixed alarm levels as low as 1 µGy/h. As indicated, the RadAl™-1 automatically trains itself to produce an alarm at levels as low as 0.2 µGy/h in 6 seconds, and 0.02 µGy/h in ten minutes.

Can we do better? Can anybody do better?

That 's hard. Let us look at a practical example.

We observe that the RadAl™-1/4 measures between 2000 and 4000 background counts every ten minutes depending on geography, in absence on shielding. Let us take 3000 counts/10 minutes (10 cps) as a reasonable value. When not in tracking mode, we then compute that the internal algorithm sets the threshold level to 11 % more than the background. We have observed in very few cases "false" alarms, clearly due to natural background fluctuations (related to wind direction, etc.). These "false" alarms were eliminated by use of shielding or by setting the background tracking feature. We come nevertheless to the conclusion that natural background can fluctuate up to 15 % in the medium / long term, hence higher sensitivity becomes meaningless because it is impossible to distinguish real radioactive sources from variations in background.

Do we fulfill the FEMA criteria?

This is a question we often get from industrial users.
The US Federal Emergency Management Agency (FEMA) has produced these documents:

( Copies of the documents may be obtained by contacting Ralph A. Myers, Federal Emergency Management Agency, 500 C Street SW., Washington, DC 20472, (202)646-3084, (facsimile) (202)646-3486.)

These documents pertain to the sensitivity required by portal monitors employed in the event of radiological accidents (civilian and power plant scenarios), hence they are not specifically relevant to the applications of the RadAl™-1. Nevertheless, we believe the RadAl™-1 to comply with FEMA specifications.

The standard (on the Federal Register, Vol. 60, Num. 56, page 15290, of March 23, 1995, which we shall not paste here for copyright considerations)  specifically calls for the capability to detect 1 µCi Cs137 on any position on a vertical line between 0.5 and 5.5 ft from the ground.

These are very tough specifications, and few if any reputed portal manufacturers claim compliance.
Nevertheless:

We experimentally observe (less then) 2000 counts/10 minutes as background using a shielded RadAl™-1/4.
Independent calibration data assure us of a minimum signal count of 288 counts/10 minutes with 1 µCi Co60 at 1 m.
Note that 1 µCi Cs137 will give a lower count rate: response is approximately flat when expressed in Gy (or Sv); equal source strength in Cs137 will give a signal proportional to the ration of total gamma energies: 0.66/2.50 = 0.26.

The threshold for alarm will be set by the internal algorithm at 2268 counts.
In presence of 1 µCi Co60 at 1 m the alarm will be triggered (count rate 2288 counts/10 minutes), with > 50% probability.
Since 1 µCi Cs137 will give 0.26 times that signal, the threshold distance will be 50 cm.

Consequently, two rows of RadAl™-1/4 at 1 m distance will comply with the FEMA requirements.

Frankly, asking a detector to respond to signal levels less than 15% of normal background, is asking a lot. But we can do it.

 
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