Characterizing Radioactivity

Detection of radioactivity depends on the type of radiation being targeted.

Geiger Counters

Gieger counters are short tubes filled with a low-pressure mixture of gas with electrodes maintained at a large potential difference. When radiation entered through the window, the radiation ionized the gas, producing an electrical discharge between the electrodes. The current is used to drive a speaker (the iconic clicks) or increment a counter. However, the long recharge period after each count, and sensitivity mainly to geologically uninteresting alpha particles, has rendered this instrument mostly obsolete.

Scintillometers

Sodium iodide crystals photoelectrically absorb gamma rays, producing a flash of light during the process. This flash of light is detected by photomultiplier tubes, converting the energy into an electric current which drives a speaker or increments a counter. The sensitivity of the instrument is dependent upon crystal size, with larger crystals detecting more events. The dead time after each reading is on the order of tens of milliseconds, and thus only requires corrections when working with very highly radioactive materials.

If a pulse-height analyzer is integrated into the photomultiplier tubes, the energy of a gamma ray can be estimated and a spectrum produced. The overlap of gamma ray energies produced by different decay processes is corrected by stripping the measurements, a calculation which is highly dependent on the crystal size of the scintillometer. The equipment will also drift with temperature. Core Laboratories produces several integrated scintillometer systems that will automatically correct the measured spectra, from the laboratory-based Spectral Core Gamma System to the more portable Well-Site Gamma Logger.

Alpha Particle Monitors

Alpha particles are measured by leaving a small detector on site for at least 12 hours. The detectors can be metallic membranes that collect daughter products, photographic emulsions that are etched by ionizing radiation, or sealed units of detector and data storage in a single apparatus.

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