Answer to Question #5388 Submitted to "Ask the Experts"

I routinely use ionization-chamber survey instruments that are vented to the atmosphere and thus contain ambient air ("Cutie Pie" type instruments). On the lowest ranges, I notice that occasionally the meter needle jumps abruptly and then settles back down to zero. These events seem to be sporadic. They occur in all the Cutie Pies I've ever owned regardless of age or condition or brand. Do other people observe these little sporadic pulses? Are they indicative of leakage in the circuits? Radon decaying in the chamber? I am just tossing out some of my guesses here. I try to keep these kinds of equipment in good order. Sometimes I've gone as far as to wash out the electrometer circuit with dry methanol, and I compulsively dry out any desiccant if present in the device. Those occasional little pulses won't go away.

Your experience of observing what appear to be transient upscale readings on a variety of ionization chambers is interesting, and I am not sure I can provide a definitive answer but will relay to you my experience and thoughts in this regard.

I have used a number of portable dose or exposure measuring air ionization chambers in many situations over quite a few years. Those that I have used have at times shown upscale readings that are sometimes but not always associated with an identifiable ionizing radiation field.

The reasons that I have ascribed to such readings have included:

1. Upscale drift probably associated with small changes in electrical components and the use of DC coupling in electrometer circuits, especially common in some of the early Cutie Pie designs; this, however is not likely the kind of spurious transient that you describe but rather a slow increase that requires rezeroing of the instrument.
    
2. Use of the ionization chamber close to a source of radio-frequency (RF) radiation; in this case the ionization chamber reading increases rapidly and decreases again once the detector is removed from the RF source or the source is removed—many electrical and electronic devices can generate electromagnetic radiation that may produce these effects, and the responses may be short-term transients, depending on the nature of the source and the ion-chamber instrument.
    
3. The infiltration of radioactive gas into the chamber volume; two situations come to mind—the first when an Eberline RO-2 chamber was brought into a reactor environment in which ¹³³Xe concentrations were elevated and the second when a chamber was purposefully exposed to a tritium gas environment.
    
4. The breakdown of plastic insulators in a small-volume Victoreen ionization chamber after the chamber had been exposed to high integral doses (somewhere between 107 and 108 rad); in this case the detector showed elevated, erratic, and unstable readings due to extreme charge leakage and did not decrease in a transient fashion.
    
5. An increase in charge leakage associated with elevated humidity that likely enhanced charge leakage across the insulators; this occurred in a high humidity area in which there was a slight overpressure of air, and the response slowly returned to normal after the chamber was placed in a normal, drier atmosphere.

Many portable air ionization chambers used in health physics dose/exposure measurements use a desiccant to remove water vapor as air enters the chamber; it is important to keep the desiccant fresh (as you have indicated you do) to avoid humidity-induced leakage problems.

It is certainly true that if you bring an open-to-air ionization chamber into a radon-laden atmosphere the reading may increase, as you implied, although I would not expect it to go down until the instrument was removed from the radon environment and the chamber allowed to equilibrate in clean air. If we consider a fairly typical 300 cm³ ionization chamber, we can easily calculate the expected current that might be generated by the significant alpha radiation from radon decay for typical ambient radon levels. Assuming a radon concentration of 1 pCi/liter in the chamber, we can estimate the short-term radon-induced current I_Rn (we shall neglect ion recombination and assume the alpha particles from ²²²Rn and ²¹⁸Po both, with a combined alpha energy of 11.5 MeV, lose all their energy in the air in the chamber and contribute to the current and that the average energy required to produce an ion pair in air is 34 eV):

I_Rn = (1 pCi l^-1)(2.22 d min^-1 pCi^-1)(1 min/60 s)(12.6 MeV d^-1)(10⁶ ev MeV^-1)(1 ip/34 eV)(1.6 x 10^-19 C ip^-1) = 2.0 x 10^-15 C s^-1 or 2.0 x 10^-15 amperes.

Many of the ionization chambers used for dose/exposure measurements operate as mean-level-type devices, providing an average response (current) to a large number of ionizing events considered together; many such detectors have a lower limit of measurement sensitivity of about 1 mR h^-1. The current that might be expected from the same chamber as above (300 cm³) at room temperature at an exposure rate of 1 mR h^-1 would be:

I_source = (10^-3 R h^-1)(2.58 x 10^-4 C kg^-1/R h^-1)(300 cm³)(1.2 x 10^-6 kg cm^-3)(1 h/3600 s) = 2.6 x 10^-14 amperes.

Thus, at the lowest level of interpretable response (1 mR h^-1), the radon at 1 pCi l^-1 would produce a negligible current, likely indistinguishable from the general background noise. Naturally, readings would be expected to increase approximately linearly with increasing radon concentration.

Some modern ionization chambers have the ability to measure exposure rates at least 10 times lower than the 1 mR/h noted above, and such chambers, if open to air, may show elevated responses to even modest radon levels—e.g., for a chamber capable of measuring 0.1 mR/h the alpha radiation from radon at 1 pCi l^-1 in the chamber would yield an average current equal to about 80% of that from a 0.1 mR h^-1 field. Some modern chambers also are equipped with thin Mylar^TM windows, often covered by a movable shield, that allow external alpha radiation to penetrate the chamber when the shield is slid away from the window. Radon and other alpha emitters external to the chamber could then cause a measurable signal.

It is also possible to obtain some transient elevations in response from unusual charge leakage events. Such might occur especially if high-voltage insulators are inadequate or damaged or if a guard electrode fails or is improperly designed. Free charge can accumulate on an insulator and, under some circumstances, may be released to the collecting electrode causing a short-term increase in reading. To my knowledge this is not a regular occurrence in properly designed and functional ionization chambers in common use. If all of your observations of transient responses had been confined to one particular design by a single manufacturer, I might suspect that a design flaw might be the cause, but since you indicate that you have used a large number of ion chambers from different manufacturers, this seems unlikely.

One might also conjecture that sudden changes in background radiation dose rates could result in the kinds of effects you have described. Such changes could occur from sources in the workplace but also might be associated with unusual cosmic bursts or enhanced sunspot activity. In general, however, such natural events large enough to produce a measurable reading on a typical mean-level ionization chamber are not frequent enough that there would be much likelihood of your observing them. Nearby lightning strikes might generate sufficient electromagnetic radiation to produce transient effects.

There have been some ionization chambers designed as pulse measuring systems rather than mean-level devices. These pulse measuring systems, by design, look at individual events and would have the capability to discern small increases in pulse rate associated with many of the causes of enhanced background radiation, especially the high specific ionization components of such background. Such event-type ionization chambers are not common and would not, I expect, have been used by you in your health physics work.

There are other external influences, such as electrostatic fields, that might also produce short-term transient responses. Other electrical/electronic effects may also give false short-term elevations. Transient currents induced through range-switching is a fairly common effect that you have likely observed.

In summary, there are a few possible explanations for the short-term transient effects that you describe, but I cannot provide a very confident endorsement of a single particular cause. Perhaps some of the thoughts outlined above might assist you in deducing the likely cause. Good luck.

George Chabot, PhD, CHP