HPGe Gamma Ray Spectroscopy

Introduction to HPGe Detectors

The hyperpure germanium detectors (HPGe) are the gamma radiation detectors with the best energy resolution. They are widely used in gamma spectroscopy applications even if they need to be cooled to cryogenic temperatures. We do not go into technical details but we limit to show in the image below, the general construction scheme.

These detectors are quite expensive and definitely out of reach of a “passionate amateur”, but we were lucky enough to be able to access a laboratory and do gamma spectroscopy measurements on some interesting samples. These are samples of contaminated soil from the Chernobyl and Fukushima sites and a sample of Trinitite.
The analysis on these samples has revealed the presence of numerous isotopes that we are going to describe in the next section.

Isotope Data

Isotope data is taken from the site NUCLEIDE-LARA which contains data on the gamma and alpha emissions of most isotopes. This site allows both to select an isotope and display the data on the γ and α emissions, and to enter an energy range (KeV) with the aim to search for isotopes with emissions in the indicated range.

Americium 241

Americium is the chemical element of atomic number 95. Its symbol is Am. Americium is a synthetic metallic element of the actinide family, obtained by bombarding plutonium with neutrons.
The screen shown below refers to the isotope Americium 241.

Cesium 137

Cesium 137 (¹³⁷Cs, Cs137) is a radioactive isotope of the alkali metal cesium which is formed primarily as a by-product of nuclear fission. Cesium 137 undergoes beta (β−) decay and has half-life of about 30 years. Cesium 137 produces gamma emission at 662 KeV and 32 KeV.
The screen shown below refers to the isotope Cesium 137.

Cesium 134

Cesium-134 has a half-life of 2.0652 years. It is produced both directly (at a very small yield because ¹³⁴Xe is stable) as a fission product and via neutron capture from nonradioactive ¹³³Cs (neutron capture cross section 29 barns), which is a common fission product. Cesium 134 is not produced via beta decay of other fission product nuclides of mass 134 since beta decay stops at stable ¹³⁴Xe.
Cesium-134 undergoes beta (β−) decay, producing barium-134 directly and emitting gamma radiation.
Currently the cesium-134/cesium-137 ratio is used to differentiate the areas contaminated by the Chernobyl accident from that of Fukushima, in the most recent incident the concentrations of cesium-134 are greater, due to the relatively short half-life.
The screen shown below refers to the isotope Cesium 134.

Europium 152

There are 35 radioactive isotopes of Europium, of which the most stable are ¹⁵⁰Eu (with half-life 36,9 anni), ¹⁵²Eu (13,516 years) and ¹⁵⁴Eu (8,593 years).
The primary decay mode before the most abundant stable isotope, ¹⁵³Eu, is electron capture, and the primary mode after is beta decay. The primary decay products before ¹⁵³Eu are isotopes of samarium and the primary products after are isotopes of gadolinium.
The screen shown below refers to the isotope Europium 152.

Barium 133

In nature, barium is a mixture of seven stable isotopes. There are twenty-two known isotopes of this element, but most are very radioactive and have half-lives ranging from a few milliseconds to a few minutes : the only exception is the ¹³³Ba with half-life of 10,51 years.
The screen shown below refers to the isotope Barium 133.

HPGe Gamma Spectroscopy of Chernobyl Soil Sample

We have been able to examine a sample of soil coming from Belarus, namely from Chachersk, near Gomel. Chachersk is 190 km north of Chernobyl and has been fully invested by radioactive cloud on 26 and 27 April 1986, shortly after the disaster.
This is 5.6 g contained in the plastic bag visible in the image to the side.

The graph below shows the gamma spectrum of the sample on a linear scale :

 

The graph below shows the gamma spectrum of the sample on a logarithmic scale :

Analysis

The gamma spectrum of the soil sample from Chernobyl shows the evident presence of the Cesium 137 isotope from uranium nuclear fission. The other peaks present are of natural origin (NORM): K40, Bi214 and Th232.

HPGe Gamma Spectroscopy of Fukushima Soil Sample

We were able to examine a soil sample from the Fukushima area invested by radioactive cloud on March 11, 2011, right after the disaster.
It is about 4 g embedded in epoxy resin visible in the image to the side.

The graph below shows the gamma spectrum of the sample on a linear scale :

 

The graph below shows the gamma spectrum of the sample on a logarithmic scale :

The graph below shows the detail of the X-ray emission peaks at 32KeV, it can be seen that the two peaks Ka and Kb are perfectly separate.

The graph below shows the detail in logarithmic scale of the energy range around the main peaks of Cesium 137 and Cesium 134 :

Analysis

The gamma spectrum of the soil sample from Fukushima shows the evident presence of the Cesium 137 isotope and the Cesium 134 isotope both deriving from the uranium fission process, or as a fission by-product or as a neutron activation product. The half-life of Cesium 137 is about 30 years while Cesium 134 half-life is much shorter, this explains the much lower intensity of the emission due to Cesium 134.

HPGe Gamma Spectroscopy of Trinitite Sample

The trinitite (also known as glass of Alamogordo or atomite) is the name given to the glassy residue formed in the desert, near Alamogordo in New Mexico, on the site of the explosion occurred on July 16, 1945, of the first nuclear bomb, code named “Trinity test”, that bomb was based on plutonium.

That glass is mainly made of silicon and feldspar melted by the heat generated by the nuclear explosion, it is usually light green, even if in some samples it has other colors. It is mildly radioactive, but you can handle it without risk given the low level of activity.

In the image above you can see the glass bottle containing the trinitite fragments that were donated to us to analyze the material.

The graph below shows the gamma spectrum of the sample on a linear scale :

The graph below shows the gamma spectrum of the sample on a logarithmic scale :

Detail of the low-energy zone with the peak of Americium 241 :

Detail of the part of medium energies with the peak of Cesium 137:

 

Analysis

From the gamma spectra it is clearly visible the presence of the following isotopes : Cesium 137, Americium 241, Europium 152 and Barium 133. The Americium and the Cesium in particular give rise to evident photo-peaks.

Cesium 137 (¹³⁷Cs, Cs137) is a radioactive isotope of the alkali metal cesium which is formed primarily as a by-product of nuclear fission. We recall that the nuclear bomb that exploded in Alamogordo in the “Trinity Test” had Plutonium 239 as fissile material. During the fission process the “father” nucleus, in this case Plutonium 239, is “fissioned” and produces two lighter nuclei of around half atomic number. Cesium 137 is one of these, and it is also the product of the beta decay of other fission isotopes such as Xenon 137 and Iodine 137. Cesium 137 has half-life of about 30 years. Cesium 137 produces gamma emission at 662 KeV and 32 KeV.

The Americium 241 is the product of the Beta decay of Plutonium 241, in turn obtained from the Plutonium 239 fuel by double neutron capture. The half-life of this isotope is 432 years. This isotope produces strong alpha emission and gamma emission at 60 KeV.

Europium 152 isotope was obtained by neutron activation of the stable Europium 151 and Europium 153 isotopes, already present in the soil of the detonation site. The half-life is 13.5 years.

The Barium 133 isotope was obtained by neutron activation of the stable isotopes of Barium present in the soil of the detonation site. The half-life is 10.5 years.

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