Answer to Question #13667 Submitted to "Ask the Experts"

I am a radiation protection technician still new to this world. With respect to ATE question 10421 already answered, I observed static interference with an DP2R/4A probe, a ZnS(Ag) scintillator probe with photo-multiplier tube. As described, I assume the physics are similar, yet was wondering if, due to the construction of the probe if the static influence would be more?

Also due to the nature of ionizing radiation, is it correct to assume that even an instrument that is resistant to static influence would eventually become influenced by static as the detector equipment becomes electron deficient due to prolonged exposure over years?

Electrostatic fields are often characterized by high electric fields that may develop between the insulating material carrying the static charge and other materials in the vicinity, sometimes resulting in the release of charge from either material, depending on the sign of the charge on the insulator. This can affect a variety of types of radiation detectors. The previous question you cited referred to possible effects on GM detectors. Photomultiplier (PM)-based detectors may also be affected. An electrostatic field in the vicinity of the PM tube could influence the propagation of electrons through any of the stages of acceleration as they travel from the photocathode through multiple dynodes of increasing voltage as they proceed to the collecting anode. Conceivably, such electrons could either be enhanced or depleted by the interactions of spurious electric fields associated with nearby materials/generators. Another effect possible is the production of light scintillations in the glass tube envelope in the presence of a high electric field, which become more likely when the tube is coated with a conductive coating and the electric field results in an induced increased voltage between the conductive coating and the cathode, with the release of energetic electrons in the glass wall; the glass scintillations produced can result in additional photoelectrons being released from the photocathode. This results in a general increase in the dark current from the tube.

Additionally, electrostatic fields may also induce possible noise in the other electronics associated with the PM instrument, leading to possibly increased background readings. The degree of interference from electrostatic fields with the PM tube or associated electronics depends quite strongly on how well shielded the PM tube and electronics are against electrostatic fields. Such shielding varies with types of materials used, the degree to which they shield vulnerable parts, and the integrity of the grounding system used to conduct induced currents to ground. For a given electrostatic field it may not be possible to say whether the GM detector or the PM-scintillator detector would exhibit the greater relative effect from electrostatic fields since the details of the detectors’ construction and their orientations in the fields would be critical considerations.

Regarding the last part of your question, any depletion or enhancement of charge that might result in an instrument because of exposure to transient electrostatic fields is not likely to persist for long after the electric field is removed. Any such charge excess or depletion that would persist at all would be on insulating surfaces that are incapable of rapidly dissipating such charge. If the electrostatic influence is removed, any charge imbalance that might have been induced on insulating surfaces would likely dissipate through natural processes over time. The buildup of charge on insulating surfaces in detectors while in use can produce internal fields that may enhance or depress collection of radiation-induced charge, and may yield low level noise associated with charge leakage.

Proper design of radiation measuring instruments must consider possible effects of both extraneous electrostatic fields and magnetic fields that can affect detector behavior, and it is common to include design elements, such as appropriate shielding to minimize such effects. It is also common practice, especially with instruments such as ionization chambers, that measure very small radiation-induced currents, to use specialized techniques. One common technique is the use of guard electrodes to negate effects of charge leakage associated with the buildup of charge on insulating surfaces. Detailed discussion of this is beyond the scope of this discussion.

I hope this has been helpful to you.

George Chabot, CHP, PhD