PhysicsOpenLab Modern DIY Physics Laboratory for Science Enthusiasts
With the help of the spectrometer for alpha particles, described in the post DIY Alpha Spectrometer, we looked at some isotopes and radioactive substances. The main difficulty of the alpha spectroscopy is the preparation of the source: to obtain net lines and good resolutions it is necessary that the active layer is very thin (ideally only a few atoms) and homogeneous. Of course this is quite hard to get, so we will content ourselves of the sources that can be prepared in a “home” laboratory.
Spectrum α Americium (241Am)
Americium-241 was directly obtained from plutonium upon absorption of one neutron. It decays by emission of a α-particle to 237Np; the half-life of this decay was first determined as 510 ± 20 years but then corrected to 432.2 years. Americium is the only synthetic element to have found its way into the household, where one common type of smoke detector uses 241Am in the form of americium dioxide as its source of ionizing radiation. This isotope is preferred over 226Ra because it emits 5 times more alpha particles and relatively little harmful gamma radiation. The amount of americium in a typical new smoke detector is 1 microcurie (1 µCi, 37 kBq) or 0.28 microgram. This amount declines slowly as the americium decays into neptunium-237, a different transuranic element with a much longer half-life (about 2.14 million years). With its half-life of 432.2 years, the americium in a smoke detector includes about 3% neptunium after 19 years, and about 5% after 32 years.
Americium decays by means of the 4n + 1 chain of Np-237 is commonly called the “neptunium series” or “neptunium cascade”. The americium decay takes place by means of emission of alpha particles with energy of 5443 KeV and 5486 KeV.
For our test we used a americium capsule contained in a smoke detector. Keep in mind that the active element is deposited on a support protected by a very thin golden plate, as seen in the image above, this has the effect of lowering a bit the energy of alpha particles and enlarge the line emission decreasing the resolution.
In the diagram below you can see the alpha spectrum of americium emission : a line centered at about 4800 keV with a FWHM of about 4%.
The measurement of the americium spectrum displayed above was made in a vacuum, if no air is evacuated from the measuring chamber the result that is obtained is different because the air has the effect of slowing the alpha particles and dispersing their energy.
The chart below shows two spectra of americium alpha taken with the source at two different distances from the detector without evacuating the chamber. You can see that how the lowest peak, corresponding to a greater distance from the sensor, has lower energy and greater line width than the largest peak, taken at a shorter distance, this effect is known as energy straggling.
Spectrum α Radium (226Ra)
Radium is a chemical element with atomic number 88. Its symbol is Ra. The word radioactivity is derived from the name of this element (for historical reasons) even if it is not the element with the highest known radioactivity. White, black on exposure to air. It is an alkaline – earth metal present in trace amounts in uranium minerals. Its most stable isotope, Ra 226, has a half life of 1602 years and decays into radon.
Radium 226 belongs to the decay chain 4n+2 of U-238 called the “uranium series” or “uranium cascade”.
Beginning with naturally occurring uranium-238, this series includes the elements: astatine, bismuth, lead, polonium, protactinium, radium, radon, thallium, and thorium. All are present, at least transiently, in any natural uranium-containing sample, whether metal, compound, or mineral. The series terminates with lead-206.
The image below shows the radium section of the decay chain.
As you see there are many alpha decays that should leave their mark on the alpha spectrum of a radio source. In the following table the main radium decays are reported, highlighting the alpha decays and the corresponding energies.
For our test we used two watch hands with radium luminous paint, the level of radioactivity is very low, but the layer of paint is very thin and this facilitates the formation of emission peaks. In the image below we report the spectra obtained. The first spectrum without line broadening compensation, the second with this algorithm applied (it is for gamma spectroscopy but applicable also in this case) in order to make the energy peaks more visible.
In the spectrum we recognize the emission peaks of Radium 226 (4782 KeV), of Polonium 210 (5305 KeV), Radon 222 (5490 KeV), Polonium 218 (6002 KeV) and Polonium 214 (7687 KeV). There is also a peak at around 5000 keV, which corresponds to isotope Protactinium 231 and emission peaks which could be correspond to Uranium 235 emission.
Spectrum α Radon-222 daughters
Adopting the setup shown in the images above you can make the spectrometry of some isotopes of Radon progeny. A uraninite sample is placed inside a sealed pouch with a piece of paper. Radon emitted from the ore with his daughters (in particular Po218, Pb214, Bi214 and Po214) settles in part on the surface of the paper and can be detected by the alpha spectrometer.
In the graph below you can see after few minutes the peaks of Po218 at 6000 KeV and Po214 at 7687 KeV.
In the chart below you can see how after several minutes remained only the peak of Po214 due to the fact the Po218 has a decay time of 3 minutes. Note the presence of an emission peak in correspondence isotope Bi211, product of the decay of uranium 235 through the Radon 223.
Using the electrostatic trap that will be described in the following paragraph it has been acquired an additional spectrum of radon progeny. The electrostatic trap allows for selectivity and concentration of the positive ions produced by beta decay of the radon daughters, in particular Pb214 and Bi214 which in turn produce the alpha emitter Po214.
We note the presence of the isotope Po211 product of the decay of uranium 235 through 223 Radon.
Spectrum α Radon-220 (Thoron) daughters
“Thoron” is the name that identifies the radon isotope with atomic weight 220. This radioactive isotope is produced in the decay chain of thorium and its decay time is about 55 seconds. In the thoron decay chain highlights the Pb-212 isotope, with a half-life of approximately 10 hours, with the 239 keV main gamma peak. In the field of alpha radiation important are the Bi-212 and Po-212 isotopes. The image on the left shows a part of the decay chain from radon 220 onwards. As thoron source were used the classical thorium mantles.
To capture the progeny of thoron an “electrostatic ion trap” has been used to capture and concentrate the isotopes on a metal plate, the positive ions produced by the beta decay of Pb-212 : Bi-212 and Po-212, both of them alpha emitters.
The ion trap is shown schematically in the drawing shown below : it is a metal container that is positively charged with respect to a metal plate placed inside at a center position. Inside is introduced (or generated directly inside) Radon gas, which in turn decays and produces positively ionized isotopes that are rejected by the external walls and attracted toward the plate by the electrostatic field. The images below show the device.
With this method it was possible to concentrate the two isotopes and obtain the alpha spectrum shown below, in which are seen, the emission peaks of the Bi212 and the Po212 with the corresponding beta emission.
Spectrum α Polonium (210Po)
This isotope of polonium is an alpha emitter with a half-life of 138.39 days. One milligram of this metalloid emits the same number of alpha particles of 5 grams of the radio. The decay of this element also releases a large amount of energy: half a gram of polonium-210, if it is thermally isolated from the environment, it can quickly reach temperatures of about 500 ° C, and develop about 140 W / g into thermal energy. A few curies (GBq) of polonium-210 emits a blue luminescence due to excitation of the surrounding air for Compton effect. Infinitesimal quantity of this isotope can be used as alpha source sample for energy calibration in the alpha spectrometer. It should be handled with great care since the source is open and this isotope is highly toxic.
In the images below shows the source and its location inside the instrument.
The graph below shows the spectrum of the source, as it is a specially prepared source (thin and uniform), it achieves a good resolution: FWHM = 2%
Spectrum α Uranium from Fiestaware
Fiesta ware was the largest selling dish line in American history – 200 million dishes were shipped since 1936. The red/orange color glaze contains uranium. The government seized the company’s uranium supply in 1943 out of fear it could be used to make a bomb. A single plate contains about 4.5 grams of uranium, mostly U-238. Production resumed in 1959 with depleted uranium (depleted of U-235) and continued until 1972 when it was discontinued out of concerns about uranium and lead leaching out of the glaze. Because it had not time to be produced, the glaze does not contain radium, it contains only uranium and some of its daughter isotopes :
The radioactivity comes mainly from U238 and U234 for alpha particles and from Th234 and Pa234 for beta particles. We tested a piece of this glaze with the our alpha spectrometer. The chart below shows the result :
The glaze does not permit to have clear alpha peaks but instead we have a large “blob” that starts from 1MeV and goes till the energy of U238 and U2354 emissions. Below 1MeV it is also evident the spectrum of beta emission.
The graph below shows the alpha spectrum obtained from a fragment of FiestaWare. There is low resolution due to auto absorption of alpha particles. There are still some peaks and steps in correspondence at the energy of main alpha emitters of uranium chain : U 238 and U 234. There is also the peak for the isotope U 235 : thus the fiestaware chips is realized with not depleted uranium.
Spectrum α Thorium from “Gas Mantles”
Thorium is a chemical element with symbol Th and atomic number 90. A radioactive actinide metal, thorium is one of only two significantly radioactive elements that still occur naturally in large quantities as a primordial element (the other being uranium).
A thorium atom has 90 protons and therefore 90 electrons, of which four are valence electrons. Thorium metal is silvery and tarnishes black when exposed to air, forming the dioxide. Thorium is weakly radioactive: all its known isotopes are unstable. Thorium-232 (232Th), which has 142 neutrons, is the most stable isotope of thorium and accounts for nearly all natural thorium, with six other natural isotopes occurring only as trace radioisotopes. Thorium has the longest half-life of all the significantly radioactive elements, 14.05 billion years, or about the age of the universe; it decays very slowly through alpha decay to radium-228 (228Ra), starting a decay chain named the thorium series that ends at stable lead-208 (208Pb). Thorium is estimated to be about three to four times more abundant than uranium in the Earth’s crust, and is chiefly refined from monazite sands as a by-product of extracting rare earth metals.
In the past, thorium was commonly used as a source of light in Auer mantles and as a material for metal alloys, but these applications declined because of concerns about its radioactivity.
The graph below shows the alpha spectrum obtained from a fragment of gas mantle. There is low resolution due to auto absorption of alpha particles. There are still some peaks and steps in correspondence at the energy of main alpha emitters of thorium chain.
Spectrum β Strontium 90 (90Sr)
Strontium-90 is a radioactive isotope of strontium produced by nuclear fission, with a half-life of 28.8 years. It undergoes β−decay into yttrium-90, with a decay energy of 0.546 MeV. Strontium-90 has applications in medicine and industry and is an isotope of concern in fallout from nuclear weapons and nuclear accidents.
Naturally occurring strontium is nonradioactive and nontoxic at levels normally found in the environment, but 90Sr is a radiation hazard. 90Sr undergoes β− decay with a half-life of 28.79 years and a decay energy of 0.546 MeV distributed to an electron, an anti-neutrino, and the yttrium isotope 90Y, which in turn undergoes β− decay with half-life of 64 hours and decay energy 2.28 MeV distributed to an electron, an anti-neutrino, and 90Zr (zirconium), which is stable. Note that 90Sr/Y is almost a pure beta particle source; the gamma photon emission from the decay of 90Y is so infrequent that it can normally be ignored.
The alpha spectrometer can also be used for the measurements of beta particles, even if the electrons are more penetrating, and then the stopping power of the detector is lower; this means that the more energetic electrons can pass through the sensitive layer of the detector releasing only a part of their energy.
Below, the spectrum obtained from a sample source to 0,1 μCi.
As it can be seen the maximum detected energy is about 1200 KeV due to the fact that the more energetic electrons escape from the detector. The measure is therefore reliable only for the part relative to the beta emission of the Sr-90, while the more energetic emission of Y-90 is detected only partially.
Spectrum β Rubidium 87 (87Rb)
Rubidium is a chemical element with symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group, with an atomic mass of 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other alkali metals, including rapid oxidation in air. On Earth, natural rubidium comprises two isotopes: 72% is the stable isotope, 85Rb; 28% is the slightly radioactive 87Rb, with a half-life of 49 billion years—more than three times longer than the estimated age of the universe.
Although rubidium is monoisotopic, rubidium in the Earth’s crust is composed of two isotopes: the stable 85Rb (72.2%) and the radioactive 87Rb (27.8%). Natural rubidium is radioactive, with specific activity of about 670 Bq/g, enough to significantly expose a photographic film in 110 days. Rubidium-87 has a half-life of 48.8×109 years, which is more than three times the age of the universe of (13.799±0.021)×109 years, making it a primordial nuclide. It readily substitutes for potassium in minerals, and is therefore fairly widespread. Rb has been used extensively in dating rocks; 87Rb beta decays to stable 87Sr with an electron of about 280 KeV.
In the picture below you can see the beta spectrum of rubidium: in about three hours, 250 pulses have been counted with a maximum energy of about 280 keV.
Spectrum β Lutetium (176Lu)
Natural lutetium is composed of two isotopes of which only one is stable, 175Lu (natural abundance 97.41%) while the other, the 176Lu beta decays with half-life of 3,78 × 1010 years (2.59% natural abundance) .
In the picture on the left you see the scheme of Lutetium decay in which the maximum energy of the electron is shown which corresponds to about 600KeV.
As a sample for measurement was used a LYSO scintillator crystal .
Spectrum β Cesium (137Cs)
Cesium-137, is a radioactive isotope of caesium which is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons. It is among the most problematic of the short-to-medium-lifetime fission products because it easily moves and spreads in nature due to the high water solubility of caesium’s most common chemical compounds, which are salts.
In the picture on the left you see the scheme of cesium decay in which the maximum energy of the electron is shown which corresponds to about 500KeV
As a sample for measurement was used a sample source of 0,25uCi.
Spectrum β+ Sodium 22 (22Na)
The isotope Na-22 decays (in 99.95% of cases) with a half-life of 2.6 years, by positron emission or electron capture to the first excited state of 22Ne 1,274 MeV (which then relaxes by emitting gamma photon). The positrons emitted by the source annihilate inside the material that acts as a support to the source, producing two gamma photons of energy 0.511 MeV each.
In the picture on the left you see the scheme of sodium decay in which the maximum energy of the positron is shown which corresponds to about 544KeV
As a sample for measurement was used a Sodium 22 sample source of 1uCi.
Spectrum β Potassium (40K)
Potassium-40 (40K) is a radioactive isotope of potassium which has a very long half-life of 1.251 × 109 years. It makes up 0.012% (120 ppm) of the total amount of potassium found in nature.
Potassium-40 is a rare example of an isotope that undergoes all three types of beta decay. About 89.28% of the time, it decays to calcium-40 (40Ca) with emission of a beta particle (β−, an electron) with a maximum energy of 1.33 MeV and an antineutrino.
In the picture above you see the scheme of potassium decay in which the maximum energy of the positron is shown which corresponds to about 1300KeV.
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