Portable EOD X-Ray "Penetration" what does it mean?

As a bomb technician I have used x-ray for over 30 years and teaching x-ray image interpretation for just as long. One of the technical terms you will hear when it comes to x-ray is "penetration". Many vendors will list on their websites that they can penetrate to this or that thickness. When I really started looking at these claims, I started seeing that what was being put out by the vendors to try and sway the buyer where levels of penetration that were"Interesting" to say the least. I just had a sales guy on a IED group claim that with a pulsed generator he can penetrate up to 80 mm. He did not say what generator he used nor did he say how he accomplished this test he just said "Can penetrate 80 mm". For all of us non metric types that is a whopping 3.5 inches and I am assuming "steel". Now I am not going to call out the vendor this guy works for but lets "analyze" this claim of 80 mm (3.5") of penetration with a pulsed generator.

The 1st thing we need to look at is the manufacture of pulsed x-ray generators and verify what level of penetration they "claim" to be able to accomplish. We all know that penetration of a dense or thick material comes down to your x-ray output (KV and pulses). There is only one main manufacture for pulsed generators and that is Golden Engineering. If we go on the Golden Engineering website you will find they do list the levels of penetration they can accomplish with each system.

As you can see from the Golden Engineering website the maximum penetration they claim is for the XRS4 (370 kv) at 1.75 in/44.6 mm. That is almost 40 mm less than the claim by the sales guy said of 80 mm. The next very logical question you should be asking is "HOW" did golden engineering do this penetration test and what "Standard" did they use. This plays a huge factor in the validity of the testing for penetration. That answer would also make us look at what the sales guy said and how he did his test to get 80 mm of penetration. Lets take a look at 2 methods a vendor could use to verify penetration capability of a portable x-ray system.

TEST 1: Image through steel to see a higher density metal (typically lead).

The Standard Test Piece for x-ray uses this method to verify penetration capability of a system by trying to penetrate a step wedge of steel to see a lead bar behind it. Steel has a density range of 7.75 and 8.05 g/cm3 and lead has a density of density of 11.34 g/cm3. That is a MASSIVE difference to a x-ray machine and my experience is that lead over 3-4 mm you are not going to penetrate.

The problem with this test is it is very easy to see a higher density material (like lead) behind steel. This lead bar will actually block x-ray so seeing it not a really viable test for a bomb technician. This test is not "real world" and for EOD guys does not provide any significant verification of that systems capability to image a potential IED threat and its components behind a high density material. All of the IED components are going to be a much lower density/thickness vs a steel plate and when compared to lead this test is not relevant. It is much harder to image a lower density material behind a higher density blocking material and that is what a bomb technician will be trying to accomplish. While this test is out there it is not a good gauge of what we call "useful penetration".

TEST 2: Useful Penetration

Useful penetration means exactly what it says in that the test determines what thickness of steel you can penetrate and still get useful information (e.g. Image IED components). Useful penetration testing is designed around seeing different gauges of wire behind a steel blocking material. For a bomb technician this test "makes sense" and provides a much more realistic verification of that systems ability to penetrate a thick/dense material and provide "USEFUL" information.

The above link talks about the ASTM F792-OE test object that determines useful penetration. This step wedge is a very good test to determine penetration capability with your portable x-ray. The "PROBLEM" is I am not sure this test (software) works with a constant potential (CP) generator or line scan detector. Before you use this to test a line scan system with a CP generator you need to verify it is in fact viable of measuring a line-scan system. I do not think it is and believe it is specifically designed for a pulsed generator. You can still run the test and visually verify useful penetration and this method can be used to verify both pulsed and CP generators. Vendors would be "wise" to post a image on their website to show the results and verify.

These test are NOT realistic and typically done in optimum conditions

When you are on a incident and trying to x-ray a item you will not be dealing with the same conditions as when you do a penetration test or using a step-wedge. When these penetration test are done they are done under optimum conditions that will provide maximum penetration results. When you are the the field you will not be able to create the same conditions so your level of penetration will typically be reduced significantly. Even if the vendors website say "penetrates 20 mm of steel" the reality is that in the field you will NEVER get those kind of results. This is where you need to be smart when purchasing a x-ray system and not fall victim to a sales pitch. While penetration is a required need for EOD x-ray systems you get to a point where more is not better. You also have to take into considering things like wash out and stand off when you do these test. Any bomb technician that tells me they must be able to penetrate over 1 inch of steel I am simply going to ask "what in the hell are you trying to x-ray that requires you to penetrate over 1 inch of steel?".