The GM Probe has a simple construction. Just under the surface, you can see a gas filled chamber connected to a simple circuit. Radiation interacts in the chamber and is changed into an electronic pulse

Inside the chamber is an electrode called anode which collects the charge created by ionization from incident radiation. The electric circuit is completed by using the chamber wall as the cathode. The chamber is filled with a special gas that amplifies the signal after the ionization.

Incident radiation, shown as a beta particle, passes through the thin window of the probe and into the gas filled chamber.

The ionization creates an electric current in the circuit. The chamber's quench gas stops the amplification from an interaction (approx. 80 microseconds) and prepares the chamber for another interaction.

The electric current is processed in the scaler and sorts out the radiation interaction current pulses from the system current from the batteries.

Inside the scaler, the signals are reported visually by a meter (needle) and audibly by clicks.

A scintillation probe is based on the light emission by substances (i.e. a crystalline sodium-iodide salt) that emits light or "scintillates" when struck by ionizing radiation. These light flashes are collected by a photomultiplier tube, which also amplifies the signal. These sensitive components are all encased in a magnetically shielded, light tight aluminum shell. This probe is used to primarily detect low energy photons (125I) and low energy x-rays.

The probe contains a scintillation material (a NaI crystal), an optical window, a photocathode, a photomultiplier, and an amplifier.

An incident x-ray or gamma ray passes through the probe window depositing energy in the scintillation crystal. The scintillation material converts the energy into light (scintillates).

The emitted light spreads out and strikes the optical window and is transferred to the photomultiplier (PMT).

At the front of the (PMT) is a window of special material called the photocathode (shown in red). When light strikes the photocathode, it produces electrons. One electron is shown ejected. The electron is pulled towards the PMT.

The electron ejected from the photocathode passes into the photomultiplier. 


 

The ejected electron hits the PMT's dynodes and multiplies the number of electrons. This essentially causes an "avalanche" of electrons that result in an amplification of the original interaction.

The avalanche ends at the last dynode, where an electron pulse that is millions of times larger than it was at the beginning. The electrons are then collected by an anode at the end of the tube.

The anode collects the electrons and forms an electronic pulse.

The pulse is directed along the circuit to the scaler as a series of pulses.

Inside the scaler, the signals are reported visually by a meter (needle) and audibly by clicks