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| Version | User | Scope of changes |
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| Apr 3 2009, 2:58 PM EDT (current) | amyremer | 63 words added, 1 word deleted |
| Apr 3 2009, 2:24 PM EDT | amyremer | 10 words added |
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The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus is emitted. The way it works is that the Helium nuclei are in constant collision with the walls of the nucleus and because of its energy and mass, there exists a nonzero probability of transmission. That is, an alpha particle (Helium nucleus) will tunnel out of the nucleus. Here is an example of alpha emission with americium-241.(6)
| Relevant Historical Data: | First produced in 1944 Americium (isotope Am-241) was discovered by nuclear chemist Glenn Seaborg and his colleagues(13) during the Manhattan Project at the University of Chicago's Metallurgical Labratory by bombarding plutonium with neutrons. It was the fourth transuranic element to be discovered. (12)Created by bombarding Plutonium-239 with high energy neutrons to create Plutonium-240. Plutonium-240 is bombarded with high energy neutrons to create Plutonium-241 which then decays to Am-241 through beta decay. Am-241 did not receive a name until 1946 in honor of the continent where it was discovered. | ||||||||||||||||||||||||||||||||||||||||||||||||
| Chemical/Radioactive Composition: | Chemical Symbol : AM -241, Am241 Specific Activity : 3.5 Ci/g Atomic number : 95 Physical appearance: silver metal that tarnishes slowly in air and is soluble in acid. AmO2 = Americium oxide | ||||||||||||||||||||||||||||||||||||||||||||||||
| Energy Characteristics: |
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| Exposure Rate Constant: | The "Specific Gamma Ray Dose Constant", sometimes known as the "Gamma Factor", is the dose rate at a specific distance from a given amount of a photon-emitting radionuclide. These constants are used frequently for radiation protection purposes. The following is a listing of Specific Gamma Ray Dose Constants for a variety of radionuclides, in units of Rem per hour (Rem/hr) at a distance of one (1) meter from a one (1) curie point source of that radionuclide.(5) Americium
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| Half-life Properties: | Half-life 432.2 years(2) , Biological half-life of Am-241 following injestion or inhalation is 50 years in bone, 20 years in liver, and permanent in gonadal tissues. Am-241 emits both alpha and gamma radiation (alpha radiation is utilized in AmBe "chemical" neutron sources), but here we are interested in low energy or "soft" gamma rays. Am-241 produces 59.5 keV gammas with a 36% probability of decay, and 14 keV gammas with a 43% probability of decay.(7) Decays to Np-237, decaying in turn to Pa-233 and U-233. (8) | ||||||||||||||||||||||||||||||||||||||||||||||||
| Forms available for use: | In aqueous systems the most common oxidation state is +3. It is very much harder to oxidize Am(III) to Am(IV) than it is to oxidise Pu(III) to Pu(IV).(2) Currently the solvent extraction chemistry of americium is important as in several areas of the world scientists are working on reducing the medium term radiotoxicity of the waste from the reprocessing of used nuclear fuel. See liquid-liquid extraction for some examples of the solvent extraction of americium. Americium, unlike uranium, does not readily form a dioxide americyl core (AmO2).(2) This is because americium is very hard to oxidise above the +3 oxidation state when it is in an aqueous solution. In the environment, this americyl core could complex with carbonate as well as other oxygen moieties (OH-, NO2-, NO3-, and SO4-2) to form charged complexes which tend to be readily mobile with low affinities to soil.
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| HVL in lead: | 0.01cm (1) | ||||||||||||||||||||||||||||||||||||||||||||||||
| Measurement/Calibrations/QA: | Prototype tests for calibration or reference sources containing americium-241 An applicant for a license pursuant to Sec. 32.57 shall, for any type of source which is designed to contain more than 0.005 microcurie of americium-241, conduct prototype tests, in the order listed, on each of five prototypes of such source, which contains more than 0.005 microcurie of americium-241, as follows: (a) Initial measurement. The quantity of radioactive material deposited on the source shall be measured by direct counting of the source. (b) Dry wipe test. The entire radioactive surface of the source shall be wiped with filter paper with the application of moderate finger pressure. Removal of radioactive material from the source shall be determined by measuring the radioactivity on the filter paper or by direct measurement of the radioactivity on the source following the dry wipe. (c) Wet wipe test. The entire radioactive surface of the source shall be wiped with filter paper, moistened with water, with the application of moderate finger pressure. Removal of radioactive material from the source shall be determined by measuring the radioactivity on the filter paper after it has dried or by direct measurement of the radioactivity on the source following the wet wipe. (d) Water soak test. The source shall be immersed in water at room temperature for a period of 24 consecutive hours. The source shall then be removed from the water. Removal of radioactive material from the source shall be determined by direct measurement of the radioactivity on the source after it has dried or by measuring the radioactivity in the residue obtained by evaporation of the water in which the source was immersed. (e) Dry wipe test. On completion of the preceding test in this section, the dry wipe test described in paragraph (b) of this section shall be repeated. (f) Observations. Removal of more than 0.005 microcurie of radioactivity in any test prescribed by this section shall be cause for rejection of the source design. Results of prototype tests submitted to the Commission shall be given in terms of radioactivity in microcuries and percent of removal from the total amount of radioactive material deposited on the source. (9) Methods of detection (in order of preference) (Radiation detectors will detect americium-241. Most detection will be based on detection of the 59.5 keV gamma). 1. A radiation survey meter equipped with an energy-compensated Geiger Mueller detector. 2. Ion chamber survey meter – tends to be less sensitive than a Geiger Mueller survey meter but is able to respond more precisely in higher radiation fields. 3. Gamma scintillation detector – very sensitive but is also energy dependent. Must be calibrated for Am-241 before it can be used for dose assessment surveys. (1) QA:
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| Used in formula/calculation: | Radioactive decay problem: Americium-241 is used in some smoke detectors. It is an alpha emitter with a half-life of 432 years. How long will it take for 43.0 % of an Am-241 sample to decay? Use the formula A=Pe^(-rt) and r=ln2/halflife r=ln2/432=0.0016 Let P(the starting amount) be equal to 100 If 43% will be decayed the remainig will be 57 Substituting it in the expression 57=100e^(-0.0016t) ln (57/100)=ln e^(-0.0016t) ln (57/100)=-0.0016t t=ln (57/100)/-0.0016 t=350.34 years Reference (10) Calculation: Question: I need the exposure rate in mR/hr for a 500 mCi 241Am source at a distance of 30 feet. I'm using the formula D=6*C*E/d2. What is E (energy of gamma in MeV) for 241Am. Where can I find this information or other ways to make this calculation? Answer: The energy and intensity of the gamma or x rays of 241Am can be found in tables in the NUDAT retrieval program made available on the Internet by Brookhaven National Laboratory. A form will open asking various parameters. Fill in the mass number, nuclide, and radiation. (The radiation is G.) On the pull down for sort order, select the option beginning with intensity and energy of radiation. The sort will be ascending by intensity, so the most important contributions to exposure, most intense, will be at the bottom of the list. You will find these three photons:
All other photons are negligible relative to these three, so substituting the energy values in your equation will give the exposure from 241Am. The formula you propose will give a larger number of photons at 30 feet the correct value because attenuation in air was not considered. Nevertheless, because incorrect energy absorption coefficients are used, the formula predicts less energy will be deposited per unit volume and the resulting exposure rate will be too low. The formula you propose should not be used for the case of 241Am. The formula will give the answer of 0.093 mR/h. The actual result is higher, as will be shown below. The distance of 30 feet (9.144 m) in air attenuates these photons significantly. The mass energy absorption coefficients (mE) to estimate the attenuation can be obtained from the National Institute of Science and Tachnology. It will be necessary to interpolate the coefficients for the 241Am energies. The exposure rate by the simple formula should be reduced for each photon energy by the attenuation of 30 feet of air. The result must then be increased using the appropriate attenuation factor for the energy deposited in a unit volume. Rather than attempting to correct a simple approximation formula, it is better to perform the full analysis. The analysis below is intended for use in a spreadsheet.
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| Uses in Radiation Oncology: | Small amount is used in an ionization chamber inside the detector | ||||||||||||||||||||||||||||||||||||||||||||||||
| Treatment Planning: | There is no treatment planning currently being done because it is still being studied and is not commercially available to this date. (11) | ||||||||||||||||||||||||||||||||||||||||||||||||
| One other interesting fact: | Americium-241 does not occur in nature; however, some americium may be found in the environment as the result of atmospheric testing of nuclear weapons and improper disposal of wastes. Americium-241 is commonly found in ionizing smoke alarms. Even though it is radioactive, it is not very dangerous. A piece of paper, a few centimeters of air, or even the layer of dead cells on your skin are adequate shielding. About the only way this could hurt is as if a person ingest it. It is one of the nine reactor produced isotopes that is at risk of being used in bombs for terrorist attacks. (12) Americium-241 is the only isotope of americium to have widespread commercial use. It is the radiation source for a number of applications:
There are tests that reliably measure the amount of americium in a urine sample, even at very low levels. Using these measurements, scientists can estimate the total amount of Am-241 present in the body. Other tests can measure Am-241 in soft tissues (such as body organs) and in feces, bone, and milk. None of these tests are routinely available in a doctor's office because they require special laboratory equipment.(13) It is the only synthetic element that has found its way into households. There is one type of common smoke detector that contains a small amount of the substance, only .2 micrograms. (12) | ||||||||||||||||||||||||||||||||||||||||||||||||
Links:
Decay Radiation Search
X-Ray Mass Attenuation Coefficients
http://en.wikipedia.org/wiki/Americium
References:
1. Radioiactive Material Safety Data Sheet. Available at: http://www.safety.uncc.edu/Forms/Nuclide%20Safety%20Data%20Sheets%20NIH/Americium%20241.pdf. Accessed March 15, 2009.
2. Americum. Available at:http://en.wikipedia.org/wiki/Americium. Accessed March 16, 2009.
3. Health Physics Society. Dose and dose calculations. Available at: http://www.hps.org/publicinformation/ate/q1488.html. Accessed March 16, 2009
4. Radioactive material safety data sheet : http://www.stuarthunt.com/Downloads/RMSDS/Am241.pdf
5. Americum. Available at http://www.iem-inc.com/toolgam.html. Accessed March 17, 2009.
6. Americum Images. Available at:www.impcas.ac.cn/.../prc/radiation_types.html.Accessed March 17, 2009.
7. Analog. Gamma Ray Detector Calibration. Available at: http://www.logwell.com/tech/nuclear/GR_cal.html. Accessed on March 21, 2009.
8. Princeton University. Contained Source Radiation. Available at:http://web.princeton.edu/sites/ehs/ContainedSources/AmBeSource/ambesource.htm. Accessed on March 21,2009.
9. Title 10: Energy. Nuclear Regulatory Commission. Available at: http://edocket.access.gpo.gov/cfr_2005/janqtr/10cfr32.102.htm. Accessed March 21 2009.
10. Yahoo Answers. Ask.Answer.Discover. Available at: http://answers.yahoo.com/question/index?qid=20070217085004AAgZR9y. Accessed March 21,2009
11.Perez, Carlos A., Halperin, Edward C., Brady, Luther W., Schmidt-Ullrich, Rupert K.. Principles and Practice of Radiation Therapy, Fourth Edition. Lippincott Williams & Wilkins. 2004. pg 479-480.
12.Wikipedia. Available at www.wikipedia.org/Dirty_bomb. Accessed on March 24, 2009.
13. Radiation Protection. Americum. Available at: http://www.epa.gov/radiation/radionuclides/americium.html. Accessed March 30,2009.
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Mat C.. . .
