| General | |
|---|---|
| Symbol | 85Kr |
| Names | krypton-85, 85Kr, Kr-85 |
| Protons(Z) | 36 |
| Neutrons(N) | 49 |
| Nuclide data | |
| Half-life(t1/2) | 10.756 years |
| Isotope mass | 84.9125273(21)Da |
| Spin | 9/2+ |
| Excess energy | −81480.267keV |
| Binding energy | 8698.562 keV |
| Decay products | 85Rb |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| Beta decay | 0.687 |
| Beta decay | 0.173 |
| Isotopes of krypton Complete table of nuclides | |
Krypton-85 (85Kr) is aradioisotope ofkrypton.
Krypton-85 has ahalf-life of 10.756 years and a maximumdecay energy of 687keV.[1] It decays into stablerubidium-85. Its most common decay (99.57%) is bybeta particle emission with a maximum energy of 687keV and an average energy of 251 keV. The second most common decay (0.43%) is by beta particle emission (maximum energy of 173 keV) followed bygamma ray emission (energy of 514 keV).[2] Other decay modes have very small probabilities and emit less energetic gamma rays.[1][3] Krypton-85 is mostlysynthetic, though it is produced naturally intrace quantities bycosmic ray spallation.
In terms ofradiotoxicity, 440 Bq of85Kr is equivalent to 1 Bq ofradon-222, without considering the rest of theradon decay chain.
| t½ (year) | Yield (%) | Q (keV) | βγ | |
|---|---|---|---|---|
| 155Eu | 4.76 | 0.0803 | 252 | βγ |
| 85Kr | 10.76 | 0.2180 | 687 | βγ |
| 113mCd | 14.1 | 0.0008 | 316 | β |
| 90Sr | 28.9 | 4.505 | 2826 | β |
| 137Cs | 30.23 | 6.337 | 1176 | βγ |
| 121mSn | 43.9 | 0.00005 | 390 | βγ |
| 151Sm | 94.6 | 0.5314 | 77 | β |
Krypton-85 is produced in small quantities by the interaction ofcosmic rays with stablekrypton-84 in the atmosphere. Natural sources maintain an equilibrium inventory of about 0.09 PBq in the atmosphere.[4]
As of 2009, the total amount in the atmosphere is estimated at 5500 PBq due to anthropogenic sources.[5] At the end of the year 2000, it was estimated to be 4800 PBq,[4] and in 1973, an estimated 1961 PBq (53 megacuries).[6] The most important of these human sources isnuclear fuel reprocessing, as krypton-85 is one of the seven commonmedium-lived fission products.[4][5][6] Nuclear fission produces about three atoms of krypton-85 for every 1000 fissions (i.e., it has a fission yield of 0.3%).[7] Most or all of this krypton-85 is retained in thespent nuclear fuel rods; spent fuel on discharge from a reactor contains between 0.13–1.8 PBq/Mg of krypton-85.[4] Some of this spent fuel isreprocessed. Current nuclear reprocessing releases the gaseous85Kr into the atmosphere when the spent fuel is dissolved. It would be possible in principle to capture and store this krypton gas asnuclear waste or for use. The cumulative global amount of krypton-85 released from reprocessing activity has been estimated as 10,600 PBq as of 2000.[4] The global inventory noted above is smaller than this amount due to radioactive decay; a smaller fraction is dissolved into the deep oceans.[4]
Other man-made sources are small contributors to the total. Atmosphericnuclear weapons tests released an estimated 111–185 PBq.[4] The 1979 accident at theThree Mile Island nuclear power plant released about 1.6 PBq (43 kCi).[8] TheChernobyl accident released about 35 PBq,[4][5] and theFukushima Daiichi accident released an estimated 44–84 PBq.[9]
The average atmospheric concentration of krypton-85 was approximately 0.6 Bq/m3 in 1976, and has increased to approximately 1.3 Bq/m3 as of 2005.[4][10] These are approximate global average values; concentrations are higher locally around nuclear reprocessing facilities, and are generally higher in the northern hemisphere than in the southern hemisphere.
For wide-area atmospheric monitoring, krypton-85 is the best indicator for clandestine plutonium separations.[11]
Krypton-85 releases increase the electrical conductivity of atmospheric air. Meteorological effects are expected to be stronger closer to the source of the emissions.[12]
Krypton-85 is used in arc discharge lamps commonly used in the entertainment industry for large HMI film lights as well ashigh-intensity discharge lamps.[13][14][15][16][17] The presence of krypton-85 in discharge tube of the lamps can make the lamps easy to ignite.[14] Early experimental krypton-85 lighting developments included a railroad signal light designed in 1957[18] and an illuminated highway sign erected inArizona in 1969.[19] A 60 μCi (2.22 MBq) capsule of krypton-85 was used by therandom number server HotBits (an allusion to theradioactive element being aquantum mechanical source of entropy), but was replaced with a 5 μCi (185 kBq)Cs-137 source in 1998.[20][21]
Krypton-85 is also used to inspect aircraft components for small defects. Krypton-85 is allowed to penetrate small cracks, and then its presence is detected byautoradiography. The method is called "krypton gas penetrant imaging".[22] The gas penetrates smaller openings than the liquids used indye penetrant inspection andfluorescent penetrant inspection.[23]
Krypton-85 was used in cold-cathode voltage regulator electron tubes, such as the type 5651.[24]
Krypton-85 is also used for Industrial Process Control mainly for thickness and density measurements as an alternative toSr-90 orCs-137.[25][26]
Krypton-85 is also used as a charge neutralizer in aerosol sampling systems.[27]