A bit of history: what is Trinitite ?
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.
Several different types of trinitite have been identified : the most common form is green, black specimens contain iron from the metal tower that supported the bomb above the desert surface, red samples contain copper from the bomb itself and cables that connected it to the control station.
In the image on the side you can see one of our trinitite samples.
In the late forties and early fifties, samples of trinitite were collected and sold to mineral collectors as novelties. Some samples can still be found on the site of the explosion, although most were removed and buryed by the US Atomic Energy Commission in 1952 : it is currently illegal to collect the remaining material on the site, however the material collected before this prohibition it is still in the possession of collectors and it is possible to find samples of this material online : trinitite on eBay.
In the initial image of the Post we see the glass bottle containing the trinitite fragments that have been given to us to perform the analysis of the material.
The sample of trinitite was subjected to gamma spectroscopic analysis with the instrument described in the post DIY Gamma Spectrometry, using the 63x63mm crystal sensor, positioning sensor and sample inside a shielded measurement well. The image on the left shows the detector and the measurement well. To drive the PMT we used the Theremino PMT Adapter and to process the data we have used the proven Theremino MCA.
the sample activity is low but it is easily measurable; however, a background spectrum should be acquired for an appropriate time in order to remove the background component from the sample gamma spectrum. measurement times are long: to obtain adequate data, it is advisable to measure for a time greater than 10 hours.
The background activity, measured within the well is worth about 40 CPS, the measure with the sample records about 70 CPS, the net activity of the trinitite sample is therefore around 30 CPS. This value allows us to make good measurements and at the same time makes the sample absolutely safe to handle.
The results of gamma spectroscopy are shown in the following charts. The first chart shows in green, in linear scale, the gamma spectrum of the sample, superimposed on the gamma spectrum of background, in gray. The second graph shows, in logarithmic scale, the spectrum of the sample without background component.
From the gamma spectra it is clearly visible the presence of the following isotopes : Cesium 137, Americium 241, Europium 152. The Americium and the Cesium in particular give rise to evident photo-peaks.
The presence of these isotopes is the characteristic “signature” of authentic trinitite. Let’s now look at the origin of these isotopes.
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.
This 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 sample has undergone quantitative analysis, using the gamma spectrometer and Beqmoni software in order to establish the activity of the isotopes present in the sample. Quantitative measurements of this type are described in the posts Gamma Activity Measures, Cesium 137 Activity in Contaminated Soil.
Preliminarily we made a measurement by calibrating the spectrometer with a sample source of Cs137, achieving an efficiency of 20.8 Bq / CPS, which means that the value obtained by the detector has to be multiplied by this factor in order to obtain the actual activity of the sample expressed in Bequerel. Subsequently, we placed the sample in contact with the detector and we obtained the spectrum below, in which the main gamma peaks are present:
Thanks to Beqmoni ROI feature, we are able to determine the activity of the individual isotopes, we focus on cesium 137 because it allows us to measure more precisely the activity:
Trinitite sample weight = 12g
Trinitite sample radioactivity = around 30 CPS
Detector efficiency = 20.8 Bq/CPS
Activity Cs137 = 7.4 CPS -> 154 Bq -> 12.8 Bq/g
Naturally, these are the current activities, measured in 2018, however, knowing the half-life of the isotope we are able to calculate the original activity at the time of the explosion. We can easily determine this by knowing the half-life of the isotope : 30 years for Cesium 137. The decay constant and the half-life are also related to each other by the following relation:
λ = ln2 / T1/2 = 0,693 / T1/2
The rule of radioactive decay states that : A(t) = A0 e-λt
Knowing A(t), λ, t one can easily calculate A0
A0 = A(t) / e-λt
Doing the calculations and knowing that t = 2018 – 1945 = 73 years we obtain the following values :
Activity Cs137 = 12.8 Bq/g x 5.1 = 65.3 Bq/g
These values are a good estimate of the original values of radioactivity and are close to other measurements that can be found in scientific literature. Naturally, in order to have more precise measurements it is necessary to use more sophisticated experimental devices such as HPGe detectors (Hyper pure Germanium detector operating at cryogenic temperatures) or mass spectrometers.
Despite the “amateur” equipment we have been able to make a fairly complete and in-depth analysis of the Trinitite sample: a piece of our recent history that marked the beginning, for better or for worse, of the atomic age.
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