Dosimetry using silver salts

Details Dosimetry using silver salts Dosimetry using silver salts

1999-05-13

2003-06-24

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiation responsive nonelectric signalling

Reexamination Certificate

active

06583425

ABSTRACT:

FIELD OF THE INVENTION
Background of the Invention
It is important that workers who may be exposed to hazardous radiation be able determine whether they have been exposed, and their level of exposure, i.e. the radiation dosage. Situations may arise where workers must cease work at a particular worksite if they have been exposed to a radiation dosage that exceeds a predetermined value. Dosimetry relates to measuring a dosage of ionizing radiation, and a “dosimeter” is a portable radiation sensor that is used to determine whether a worker has been exposed to a particular dosage of radiation. Radiation impinging on the dosimeter produces physical and/or chemical changes in the dosimeter, and an analysis of the changes may provide the type and dosages of the radiation.
Dosimeters may be grouped into “non-real time” and “real-time” dosimeters. “Nonreal time” dosimeters do not provide the worker with an immediate determination of their particular radiation dosage. By contrast, “real time dosimeters” do provide the worker with an immediate determination of their radiation dosage. By determining immediately the radiation dosage, the worker may avoid additional unnecessary and prolonged exposure to radiation.
In general, dosimeters have a radiation-sensing element surrounded by a protective housing. The housing offers mechanical protection to the sensing element. Also, the housing may include materials that absorb various types of radiation to prevent these types of radiation from reaching the radiation-sensing element. For example, a thin housing made of paper can stop alpha radiation. A housing a few mm thick and made of plastic, aluminum, or cardboard can stop alpha and beta radiation, while a thinner plastic housing can stop alpha radiation but pass beta radiation. A 1-mm thick lead housing will attenuate a 150 KeV photon to 0.03 of its initial dose. A 2-mm thick lead housing will attenuate a 150 Kev photon to about 0.0001 of its initial dose. A 5-mm thick lead housing will attenuate the dose from a 250 KeV gamma or x-ray photon to 0.001 of the initial dose.
“Colorimetric dosimeters” are a general class of real time dosimeters that provide the user with a visible color change upon exposure to radiation. The appearance of a particular color may indicate that the radiation threshold for the dosimeter has been reached. Similarly, a range of colors, or of intensities of a particular color, may correspond to a range of dosages. A color chart may be provided with the dosimeter for color comparison to determine dosage. Alternatively, the dosimeter may be equipped with a mechanical meter that provides a numerical value of the dosage.
U.S. Pat. No. 3,691,380 entitled “Threshold Value Dosage Meter” by K. Hübner et al., which issued Sep. 12, 1972, describes a threshold value dosage meter for detecting 0.5-10 megarad dosages of ionizing radiation. An aqueous solution containing polyvinyl alcohol, methyl orange, chloral hydrate, and sodium tetraborate (borax) buffer is poured onto a support and dried to produce the threshold dosage meter. Irradiation of the dosage meter with ionizing radiation results in the ionization of the chloral hydrate and in the production of hydrogen chloride, which reacts with the methyl orange to produce a color change from yellow to red. The added buffer attenuates the threshold of the dosage meter by reacting with hydrogen chloride until the buffer is exhausted.
U.S. Pat. No. 3,899,677 entitled “Plastic for Indicating a Radiation Dose” by Y. Hori et al., which issued Aug. 12, 1975, describes a plastic film which undergoes a color change in response to a radiation dose. A solution of a chlorinated polymer, a plasticizer, and at least one acid sensitive coloring agent is flow coated onto a support and dried to produce the film. Irradiation of the film produces hydrogen chloride, which reacts with the coloring agent to produce a visible color change.
The G-value of a dosimeter is a measure of its sensitivity to radiation. The G-value is defined as the number of reactions produced per 100 eV of radiation deposited on the dosimeter. Useful colorimetric sensors often have a G-value of about 3, and are used to detect and measure relatively high, often megarad, dosages of radiation.
Dosimeters that employ silver as a radiation-sensing element are inherently very sensitive because silver has a high cross section for many types of radiation, including neutron and gamma radiation. Neutron cross sections for silver can be found in “Neutron Cross Sections,” vol. 2, V. McLane, C. L. Dunford, and P. F. Rose, National Nuclear Data Center, Brookhaven National Laboratory, Academic Press, Inc., Harcourt Brace Jovanovich, San Diego, 1988. Gamma radiation absorption coefficients for silver can be found in “Gamma-Ray Absorption Coefficients,” C. M. Davisson and R. D. Evans,
Reviews of Modern Physics
, vol. 24, p 79-107, 1952.
Both solid state and emulsion type dosimeters having silver as a radiation-sensing element are known. For example, a solid state, silver containing dosimeter is described in “Radiation Dosimetry”, edited by F. H. Attix et al., Academic Press, (1966), vol. II, chapter 13, page 258. A block of silver-activated phosphate glass was placed into a plastic locket to form the “DT-60” personal dosimeter. A dosimeter employing a silver-based emulsion is described in “Radiation Dosimetry”, vol. II, chapter 15, and vol. III, chapter 28. The emulsion contains microscopic silver halide crystals that are dispersed in gelatin and coated on a support to produce a film. Exposure of the film to radiation produces an image on the film. The image may be in the form of particle tracks that can be analyzed to yield detailed information regarding the identity and energy of the particles that produced the tracks. The following equations, which generally describe the chemistry of the well-known photographic process, summarize the chemistry involved when the film is exposed to radiation,:
(AgX)
n
+radiation→(AgX)
n-m
Ag
0
m
+(X
2
)
m/2
  (equation 1)
(AgX)
n-m
Ag
0
m
+(RH)
n-m
Ag
n
0
+(HX)
n-m
+R
n-m
  (equation 2)
 HX+B→HBX  (equation 3)
According to equation 1, radiation interacts with a grain of silver salt (AgX)
n
, where X is typically bromide, chloride, iodide, or a mixture thereof, to produce an aggregate grain ((AgX)
n-m
Ag
0
m
) having both silver salt and silver metal (Ag
0
). The radiation of equation 1 may include non-ionizing radiation such as visible and ultraviolet radiation, and ionizing radiation such as alpha-, beta-, gamma-, x-ray-, neutron-, electron beam-, and proton beam radiation. According to equation 2, a photographic developer (RH), i.e. a reducing agent, rapidly reduces the aggregate grain to silver metal (Ag
0
); acid (HX) and an oxidation product (R) are also produced. According to equation 3, acid (HX) reacts with a base (B) to form an acid-base complex (HBX).
The pH of the silver containing emulsion changes during the developing process as HX is produced. The activity of some reducing agents may change as the pH changes, which may result in the production of underdeveloped or overdeveloped images. To avoid this, a buffer is added to the emulsion to maintain the activity of the developer. This facilitates the production of high quality images.
Dosimeters employing silver emulsions are extremely sensitive to radiation. They provide images from which the identity and energy of the radiation can be determined. However, dosimeters employing silver emulsions do not allow workers to immediately determine their radiation dosage.
Therefore, an object of the present invention is to provide a method for detecting ionizing radiation.
Another object of the present invention is to provide a silver-based calorimetric radiation dosimeter.
Yet another object of the present invention is to provide a threshold dosimeter which blocks non-ionizing radiation and detects ionizing radiation.
Still another object of the present invention is to provide a dosimeter that achieves the foregoing objects and also ena

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