March 24th, 2022 - Identifying Nuclear and Special Nuclear Material with the SAM 940+

The SAM 940+ uses several unique algorithms to identify Nuclear and Special Nuclear Material (SNM).  This article is provided to help the user verify different forms of uranium, especially when shielding and background radiation can produce interfering anomalies in the spectrum.  The four nuclear materials to be discussed are Natural Uranium, Depleted Uranium, Enriched Uranium and Weapon’s Grade Uranium.  The first two are not classified as SNM but the different forms of enriched uranium are categorized as Special Nuclear Material.

Uranium and SNM

The chemical element uranium (U), has an atomic number 92 and is listed in the actinide series of the periodic table.  Uranium is a naturally occurring radioactive element with no stable isotopes.  It has two primordial isotopes, uranium-238 (U-238) and uranium-235 (U-235) and are found in appreciable quantity in the earth’s crust.  Naturally occurring uranium is primarily U-238 (~99.3%) with a small amount of U-235 (~0.7%).  Natural uranium is identified by the progeny of U-238 as shown in the gamma spectrum below (Figure 1).  The progeny of U-238 (protactinium-234m) is shown at 776 and 1001 keV.  The peak at 1001 keV will be the easiest to recognize since the 776 keV peak may be difficult to see above high background.  At about 98 keV is a large peak formed by a group of X-rays.  

A little higher in energy is a broad peak centered at about 186 keV, which is the primary energy of U-235.  This poorly resolved U-235 peak, along with the two U-238 peaks, are a good indicator of natural uranium (low content of U-235 in natural uranium).

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Fig. 1. Natural Uranium Spectrum (primarily U-238)

When extracting U-235 from natural uranium, depleted uranium (DU) is formed.  A spectrum of DU is shown in Figure 2.  It is noted that the area under the 186 keV region is now reduced even further.  This indicates less U-235 (DU has typically 0.2% U-235).  DU is used in many applications but is not SNM by definition.  The SAM 940+ will Identify both natural uranium and DU as U-238 and DU, since this is the latest ANSI requirement.  High background and shielding may reduce or eliminate the 98 keV peak and make the 776 keV peak difficult to observe but the 1001 keV peak will allow reliable identification as U-238/DU.

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Fig. 2. Depleted Uranium (Blue indicates the level of background)

Uranium Enrichment

When uranium is enriched (meaning the ratio of U-235 to U-238 is increased) there are two designations for the level of enrichment: Low Enriched Uranium (LEU) and Highly Enriched Uranium (HEU).  LEU is less than 20% enrichment and HEU designates the enrichment to be 20% or above.  Figure 3 shows a spectrum of Low enriched uranium.  Notice that the U-235 peak is now clearly discernable above the continuum.  Also, observe that the U-238 peaks are still visible.  Since this spectrum still represents mostly natural uranium, it will be identified as U-238.  Another indication of LEU is the ratio of the 186 keV peak to the 98 keV peak.  As long as this ratio is less than 1 the material is designated as LEU.  If there is shielding involved, the magnitude of the 98 keV peak may be reduced affecting the ratio, however, the peaks indicating U-238/LEU will still be present.

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Fig. 3. LEU Spectrum at 1.9% Enrichment

When the enrichment reaches 20% or more the material is designated as Highly Enriched Uranium (HEU). The spectrum in Figure 4 shows a much larger 186 keV peak with the ratio of 186 keV to 98 keV greater than 1.  The SAM 940+ is not designed to be an accurate enrichment meter but when these two peaks are about equal in magnitude (ratio equals 1) the enrichment will be approximately 20%1.  This ratio gives a fairly good estimate for quickly determining if the material is LEU or HEU, however, shielding will greatly affect this ratio.  Therefore, other clues can be used to determine the enrichment.

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Fig. 4. HEU Spectrum at well over 20% Enrichment 

When the enrichment exceeds 20% (Figure 4.) the U-238 peaks (766 and 1001 keV) become lost in the continuum.  This, along with a strong U-235 peak (186 keV) is a good indication of HEU.  The Nuclear Regulatory Commission (NRC) has given an additional notation for enrichment of 20% or greater as Strategic Special Nuclear Material (SSNM).  The SAM 940+ will identify this material as HEU and U235HE.

Determining Weapons Grade Uranium 

Weapons Grade Uranium (WGU) is defined as uranium with an enrichment of 80% or above.  Figure 5 shows a WGU spectrum of over 90% Enrichment.  This spectrum is exposed to some background radiation including K-40 at 1461 keV (as seen in the spectrum).  Highly enriched material such as WGU has an additional indication above 2600 keV.  This tell-tale indication is the presence of Thalium-208 (Tl-208) at 2615 keV in the spectrum.  This key indicator of WGU will easily penetrate large amounts of shielding since the photon energy of Tl-208 has a mean-free path of 20 cm in lead.   This spectrum was acquired with a HEU source of less than 0.5 mg and yet the 2615 keV peak is clearly seen.  The SAM 940+ will still identify this source as U-235HE/HEU but noting the Tl-208 peak will verify that it is WGU.

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Fig. 5. WGU Spectrum with Tl-208 at 2615 keV

It is important to note that small sources near the detector or high activity weapons at some distances will both require acquisitions of at least 2 to 5 minutes.  This is because the Tl-208 photons are so penetrating that even a large detector will have fractional photopeak collection (this phenomenon is referred to as photofraction).  Some users may tend to use too short an acquisition time and not observe the 2615 keV peak, so we have allowed HEU and WGU to have the same identification (even though it's important to correctly discern enrichment levels).  At this time there is no specific ANSI designation of WGU, therefore, this paper allows the user to easily interpret the various levels of enrichment and subtle differences in the continuum.  

Always be aware of geometrical considerations and the Inverse Square Law.  For those unfamiliar with scintillation detectors and nuclear spectroscopy, you may want to take the course, “The Fascinating World of Scintillation Detector Technology” offered by BNC.

1The ratio of the two photopeaks indicating enrichment may vary somewhat depending on the detector size, detector type (high/low resolution detector), background and shielding.