Answer to Question #10234 Submitted to "Ask the Experts"

During my working day with x rays, I wear a body thermoluminescent dosimeter (TLD). Inside the TLD are two crystals similar to a cylinder in shape, with short height (around 0.5 mm) and radius (around 1 mm). My question is, Does my equivalent dose depend on the angle of incident radiation on the crystals?

If radiation is incident at 0 degrees (between normal line of circle and radiation beam), the active element is seen as a circular surface, but at 90 degrees the profile appears as a rectangular shape. This leads me to conclude that perhaps the same radiation in the same distance and time can yield different dose values (for 0 and 90 degrees). Am I right?

We should point out that for a given fluence of photons at the location of the active element (assuming a condition of secondary charged-particle equilibrium), the dose to the active element is independent of the orientation of the element in the photon field as long as the photon attenuation in the element is not significant. Generally, the dimensions of the active elements are sufficiently small that the photon attenuation effects in them are acceptably small, and the dose to the active element is not significantly degraded because of variations in its geometry. Most personal dosimeters, however, do show some angular dependence in their response. Usually the difference in response from what is observed at 0 degrees incidence (i.e., radiation incident normally on the badge) increases with increasing angle and with decreasing photon energy. The angular dependence effect is not generally associated so much with changes in perspective associated with the active element, such as the cylindrical elements that you describe, but more with the changes in attenuation of the photons as they traverse the other badge materials and, to some extent, with scattering effects in the badge and in the human body.

For example, most personal dosimeters use an active element positioned below a tissue-equivalent thickness of 1 gram per square centimeter (equivalent to 10 mm thickness of soft tissue) for assessing the so-called deep dose or 10 mm personal dose, which is typically used as a surrogate for effective dose. When 60 keV photons pass at zero degree incidence through this material, they are attenuated by about 20 percent so that roughly 80 percent of the photons reach the dosimeter. If the same energy photons are incident at an angle on the face of the badge such that the angle between the direction of the photons and a normal to the badge surface is 60 degrees, the effective path length through the covering material is increased by a factor of two compared to normal incidence, and only about 67 percent of the primary photons will reach the active element. If the photon energies are higher, the attenuation effects are less severe, and for lower energies the effects are more pronounced.

This angular dependence phenomenon is well known, and it is recommended that established dosimetry programs/providers perform angular response testing to demonstrate that the deviations of response from what is expected at normal incidence are acceptable. In the United States, the standard commonly used is ANSI/HPS Standard N13.11 (2009), in which there are some acceptance criteria given that allow deviations of + 30 percent for angles (between the normal and line of incidence) of up to 60 degrees. If you are a member of the Health Physics Society you may download this document free of charge here. You can also find criteria for angular dependence in the International Organization for Standardization standard ISO 4037-3:2019.

I hope this addresses your concerns.

George Chabot, PhD, CHP