Ken Kasper

Optically stimulated luminescence dosimeters.

Landauer is now offering a new flavor of NVLAPapproved dosimeters called OSL dosimeters. OSL, which stands for optically stimulated luminescence, uses laserproduced light to detrap charges and produce a measurable glow from an exposed material. The OSL dosimeters are touted as being more accurate, more sensitive, and more stable than traditional thermo-luminescent dosimeters (Fig. 1 and Fig 2). Like popular thermo-luminescent dosimetry, OSL dosimetry works by measuring photons from the visible light spectrum that are released when radiation-excited electrons drop to a lower energy state. Standard thermoluminescent processes help electrons get to the lowerenergy states using heat. The OSL process uses light energy. Optically stimulated luminescence has been used by archeologists and geologists to date natural materials such as quartz or feldspar (Akselrod and McKeever 1999). Luminescence emitted during exposure to a steady source of light will reveal the dose of radiation absorbed by the mineral since its last exposure to sunlight (Akselrod 1998). Through application of this method, the age of the mineral can be determined. The OSL dosimeter, which is used to measure radiation dose to people instead of rocks, uses an aniondeficient aluminum oxide doped with carbon (Al 2O3:C). Al 2O3:C makes a good OSL dosimeter since it is very chemically inert, very hard, has an abundance of dosimetric traps, and has excellent luminescent efficiency when optically stimulated (Akselrod et al. 1999). One may ask, how can you measure the soft glow of an Al2O3:C chip recalling its radiation exposure history through the pupil-scorching light of a laser? To resolve this problem, a technique known as pulsed optically stimulated luminescence was developed. This process uses an electronic gate system that only measures the light in between very short (nanosecond) laser pulses. Some 4,000 carefully timed laser flashes are used to stimulate the exposed chip in a single second. A single reading of an OSL dosimeter chip only uses a small portion of the “trapped” latent signal. Because of the remaining signal and a well-characterized laserinduced depletion rate of the Al 2O3:C, OSL dosimetry can easily be reanalyzed several times. The OSL dosimetry is good at keeping the latent signal from an exposure as long as it is not too hot and the time period is not too long. For temperatures less than 22°C (72°F), the OSL dosimeter can hold nearly the entire exposure signal for over 1 y. Between this temperature and 45°C (113°F), the dosimeter can maintain over 90% of the latent signal for 3 mo. Until recently, only thermo-luminescent dosimeters and film badges could be used for radioactive material licensees in certain categories under Title 10 of the Code of Federal Regulations. A change to the law was published in October of 2000 under “New Dosimetry Technology” to allow the use of other NVLAP accredited dosimeters. According to Betty Ann Torres with the U.S. Nuclear Regulatory Commission, the rule was adopted and became effective 8 January 2001 (66 FR 1573). The OSL dosimeter that is currently available through Landauer, Inc., is called the Luxel Dosimeter. For gamma exposure, the Luxel dosimeter has a minimum dose reporting level of 1 mrem. This dosimeter is

In-situ Object Counting System.

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