Q: At the moment of exposure during a recent X-ray I distinctly felt the
hairs on my legs move. The radiographer insisted it was not the discharge of
an electroscope by ionising radiation, even though my recently removed
trousers were a polyester/wool mix and the atmosphere hot and dry. This
explanation is out by several orders of magnitude when the amount of radiation
used is taken into consideration. Can anyone provide an alternative
explanation?
A: Removing polyester trousers in a dry atmosphere could produce a static
potential of 10 or more kilovolts. Discharging this from a smooth body would
require more X-ray flux than is wise or likely. However, hot hairy legs are
different. The hairs will have high field strengths around them due to their
small diameter. This could allow the X-rays to trigger localised 鈥渁valanche
ionisation鈥�, especially in the presence of easily ionised organic vapour from
sweat. Thus hairs on end would lose some charge and move.
Avalanche ionisation around hair-thin wires spaced a few millimetres apart
is used in multiwire detectors to count and image single X-ray photons and low
energy betas. These typically use between 10 and 30 kilovolts and use argon at
atmospheric pressure with a trace of methane to provide the organic component.
They were developed by Charpak et al, CERN 1968, and are now widely used in
high energy physics and nuclear medicine.
Advertisement
A: I beg to disagree with the radiographer. The capacitance of the human
body is around 0.1 nanofarad. Ionisation of the surrounding air by the X-ray
beam, for a typical lumbar spine radiograph, is sufficient to discharge about
100 nano-coulomb, so a patient鈥檚 potential can change by as much as 1000
volts, (potential equals charge divided by capacitance,) during an exposure,
if he is well charged, insulated and close to an earthed metal object such as
an X-ray film holder. Dry body hair is deflected by the (entirely harmless)
reduction in local electric field. A highly charged patient may experience a
repeated sensation during a series of exposures, but such reports are
rare.
Most X-ray equipment is bonded to earth, so patients usually discharge
themselves as they climb onto the couch, but some foam mattresses and
filmholders are insulators. The moral is, undress slowly in the presence of
strangers.