Published: Apr 30, 2020
A guest blog article from our good friend and colleague Mark Keeping, MSRP. Mark is currently Work Control Centre manager and Radiation Protection Manager for the Imperial College Reactor Centre. Mark has had significant involvement in the de-fuelling and subsequent decommissioning of the light water CONSORT reactor at Silwood Park. You can read more about Mark, including his contact details, at the end of his article.
Contamination monitor window protection grilles
The most important aspect of a radiation monitor is that is detects radiation - that is obvious. But we also need to exclude other emanations which will give undesired false readings. This is where we have to compromise the ability of the detector. In gamma and neutron monitors this is accomplished easily with strong or thick materials due to the (generally) good penetrating ability of the radiation. I’m going to concentrate on portable alpha and beta contamination detectors which tend to be much more fragile.
Whether you are using a Geiger-Muller (GM) or a scintillation detector for contamination, they have a physical weakness: the detection window.
With a scintillation monitor it's the very thin aluminised plastic foil (also known by the brand name Mylar) which enables all but the weakest particles to penetrate but excludes all of the light. If the foil is compromised, you will usually notice a sudden change in count rate - usually upwards, or a sudden drop in some cases. You might obtain a higher count rate if the monitor is pointed toward a light source. You might observe a tiny blemish on the foil, but the only true test is to run a small source all over the detector window under a strong light source (see page 29 of the NPL Good Practice Guide No. 14) If the count rate varies widely as you move the source, you probably have a hole and the foil should be replaced. It’s especially problematic in alpha monitors as they have a very low background of typically better than 0.1cps.
On a GM probe there is a thin glass wall on the tube, or a fine magnesium silicate (mica) end-window. Both are easily broken and you will know you’ve broken it by the sudden drop to zero in count rate. You might even hear a little pop before being showered with the fragments.
For the best count rate efficiency, you need to be quite close to your source - for alpha monitoring 3mm is generally considered the optimum being the compromise between losing counts due to distance and damaging or contaminating your probe. With beta monitoring, you can normally locate contamination at a slightly greater distance, but need to get closer for an accurate reading.
Unfortunately, outside of the test lab, surfaces for monitoring are far from ideal. They will rarely be totally flat or clean, and may have sharp projections, so it’s almost inevitable you will at some time put a hole in your scintillator foil, or bust your GM probe.
Thankfully, manufacturers incorporate a grille to protect your probe window on end-window GM and scintillation probes. (Side-window GM probes are normally held in the body of a probe which has on opening for beta detection).
Most protective grilles have square openings. Typically, an AP2 probe (made by Thermo) has grille with openings of around 1cm square and about 1mm of metal between squares. The monitor window is 49 cm2 in total. It has a good efficiency but poor resistance to stray projecting screw heads. Thermo can also supply a ruggedised version with smaller 3mm openings, but you loose almost a third of the counts due to more metal being between your contamination and the detector. (35% to 25% efficiency at 4𝛑)
An EP15 end-window GM detector active surface is only about 16cm2, but can be protected by a finer mesh with less material as it is smaller and therefore more mechanically resistant. They will detect alpha but the window is thicker than a scintillation probe and the detector area is smaller than an AP2 so they are used more as an all-rounder bench test type instrument than as a survey monitor in alpha-handling facilities. Larger end-window GM probes are rare.
An efficient grille requires the openings to tessellate. There are only three regular shapes that tessellate fully. Triangles, squares and hexagons.
Triangles require the most material so are seldom used.
Squares are commonly used, but if you make the strands of your grille thinner to gain detector efficiency, you loose mechanical strength, so for wider area probes this becomes a problem as the grille is more likely to distort under pressure.
Nature has worked out the best compromise which can be seen in insect compound eyes, snowflakes, The Giant’s causeway and beehives. Hexagons are the best way to achieve strength with the minimum material. Bees know this - they don’t waste precious wax on constructing square or triangular cells. Hexagons tessellate more efficiently than squares. Hexagonal grilles gives the lowest perimeter to area ratio of any of the regular tessellation shapes.
A square of 16cm2 is required to constrain a 4cm ⌀ circle, but a hexagon can do it with just 13.856cm2. This makes the hexagon a more efficient grille shape. (I cheated and used this calculator at Omni Calculator as maths is not my strongest field.)
As you move a hex-grille probe over a surface, it is impossible to move it so that the grille material covers any one point so there is less chance of momentarily shielding a small point of radioactivity with the grille material. Have you noticed that big televised football matches rarely use square nets these days? They use hexagonal netting as it can constructed of thinner threads than the equivalent square celled net. It’s better for the TV cameras to see the action through the back of net onto the field, and the lines break up so you can see more.
The 120 degree angle formed at the corner of a hexagon is most mechanically stable of all angles.
This is why larger area detectors requiring a protective grille will normally be a hexagonal rather than square mesh.
Now having it stronger by being hexagonal, you can make the strands of material that make it up slightly thinner thereby increasing efficiency. You can calculate the openness of various grille shapes using this Boegger Calculator , and by playing with the numbers, you will see that a shaving of a sliver of metal between the cells of a grille will improve the openness of it.
About Mark Keeping, MSRP
Mark started his career at AWE Aldermaston in 1987 with placements at varied locations on the site which included analysis of bioassay samples, personnel dosimetry, and radiological instrumentation. He also visited large x-ray and LINAC facilities during the course of his work.
In 1989 he started on the AWE Health Physics section and during the next nearly 20 years he spent time in nearly all of the designated areas at both AWE sites and provided emergency and shift cover for both sites with on and off-site duties. During this time he used many types of protective equipment including pressurised suit work in high-contamination zones. In 2004 he became a safety representative and took part in safety incident investigations and attended Nuclear Safety Committees.
In 2009 he joined Imperial College London at the site of the Consort light-water research reactor at Silwood Park as a Health Physics Technician. He undertook and passed the NEBOSH general certificate in safety, and then the University of Strathclyde’s radiation protection course in 2013, and joined both the Association of University Radiation Protection Officers and the Society for Radiation Protection where was accepted as a member. In addition to his varied duties at the reactor centre, he became the last person to train successfully to operate the reactor before it was shut down for the last time in 2016. He continued his radiation protection duties whilst taking part in de-fuelling the reactor and was appointed Decommissioning Radiation Protection Team Leader.
He has delivered lectures for modules of the Imperial College MSc course on Nuclear Engineering.
He was appointed Work Control Centre manager in 2018 and was also appointed Radiation Protection Manager in 2019 and now has front-line oversight of contractor staff on the back-end decommissioning and demolition contract until the site is ultimately handed back for delicensing. Mark is a safety representative for Imperial College London and attends the Imperial College Reactor Centre Nuclear Safety Committee.
Mark can be contacted as follows:
Find out more about Mark at Mark Keeping at LinkedIN