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2/11/2009 - Do you trust your Gamma Ray Constant?
Ionactive Gamma Ray Constant - Example Still (1)

Many of us rely on the use of gamma ray constants or formulas in order to determine a dose rate (say in μSv/h) from a given level of Activity (say in MBq) from a particular radionuclide at a certain distance. Some of this data appears in tables, whilst some is used by a range of both free and commercial computer programs.

I believe that the reliance placed on this data is sometimes treated as absolute by the end user. However, what does the data really mean and can it be applied to your particular situation. Is the answer you are provided with an absorbed dose rate in air, or an equivalent dose in tissue? A number of similar questions can be summed up as "what is actually getting the dose?" and "where exactly is it located?"

Ionactive Gamma Ray Constant - Example Still (2)

These questions will be explored in some up and coming resource which is being developed for us by Grallator Limited. Some snap shots of the resource, which as usual will be richly colourful and animated, are shown in this blog entry.

Ionactive Gamma Ray Constant Resource - Example Still (3)

31/10/2009 - Potential Overexposure of Members of the Public

This is taken from the IAEA event log.

The Incident

 

A non-radiation worker who was working in the immediate vicinity of five fixed nuclear gauges used to monitor coal flow through coal chutes at a non-nuclear power plant was exposed to the radiation beam emitted from the fixed nuclear gauges.

The exposure to the gauges occurred as he worked near each one in sequence. The licensee preliminarily subsequently determined that the worker likely received 6.5 mSv (650 mrem), which is in excess of the public dose limit of 1 mSv (100 mrem.) Additionally, it was determined that five other non-radiation workers may have received doses in the range of 1.4 mSv to 4 mSv (140 to 500 mrem). The U.S. Nuclear Regulatory Commission is currently conducting a special inspection of this event.

The incident log can be read at the following link: IAEA News (You may need to log in as a guest).

Ionactive Comment

These are quite significant exposures by nuclear industry standards - certainly those in the UK, where most occupational exposures are less than 1mSv annually. Exposures to ‘non radiation workers', approaching that which in the UK would require classified radiation worker status, is quite significant.

We have little information on the gauges involved but the picture below shows a typical example which might be similar.



This gauge has a 2GBq Cs-137 source - exposures from the collimated beam, should you be in the way, is about 700 μSv/h (note this is less than you would receive from a completely unexposed source). It is therefore possible for exposures in the 1-5mSv range to be obtained, but the workers would really need to be quite near those gauges.

Furthermore, depending on the type of collimator fitted (which would relate to the size of the area being measured), it might be that the exposure is non-uniform across the body (in which case the quoted exposure levels may or may not reflect whole body exposure).

29/10/2009 - Reading and exploring Ionactive Tweets

As regular readers will know, Ionactive has been exploring the use of Twitter for some time – we really like it. However, like all ‘social networking sites’, many businesses believe them to be an irrelevance or a time waster and therefore their IT departments block access. To some extent we can understand why – we cannot condone excessive use in the workplace!

However, we do believe Twitter has a place in the working day when used in the way we use it – and that is the sharing of useful  / interesting information related to radiation protection, health and safety, nuclear issues, physics, science and perhaps the odd irrelevant link too!  We have noted that some of our visitors are unable to read the ‘Twitter Updates’ that appear on our home page and blog pages. This is a pity since the visitor is supposed to be able to use those updates to go direct to the source of the information without following us on the Twitter site (but we would of course like you to follow us all the same).

The problem is that the Twitter Updates come direct from Twitter (no surprise there) and so if the Twitter Domain is blocked by your IT department then you are just going to see an empty box. Well no longer!

Ionactive Tweets no longer come via Twitter

Our excellent web designers – Integralvision – have come up with the perfect solution. Basically our Twitter Updates are picked up by servers at their end using the RSS feed. They then use some jazzy code (I have no idea how it works) to reconstruct the Tweet, ensuring that all the contained links and date information etc are preserved from the original. This is then sent direct to the site from their servers when you load one of our pages.

Therefore you see the information we want you to see, and you can choose to visit our highlighted resources, and all without ever going anywhere near Twitter.  Other advantages include a faster page load time, and if Twitter should go down it does not affect how our pages look (under the old system if Twitter is slow then our own page load was slower, and if Twitter went down completely then you were just left with an empty blank box).

Of course, we would like you to follow us on Twitter – but you do not have to.

23/10/2009 - Undergoing Medical Radiotherapy Treatment?

Some time ago Ionactive commissioned some new radiation safety training resource in the form of a video explaining the inner workings of a medical linac (linear accelerator). The unique video, produced by Grallator Limited for Ionactive Consulting, looks at the linac from the electron gun and all the way through to the treatment head.

The video resource was originally intended for our own training delegates (who attend specific courses related to medical uses of ionising radiation), but as with all our resources, it has been posted on the web in our Radiation Safety Video area.

It was therefore most heartening to receive the following email yesterday – which for obvious reasons I have altered to protect the privacy of the sender (who I have also obtained permission to publish):

Dear Mark,

Subject: Thanks for your linac video

Just a line to thank you and your company for the wonderfully clear video that I've just watched explaining how a linac works.  I've just had 20 sessions under one at hospital for salvage radiotherapy to my prostate area, and it's so good to now have an idea of how the machine works. 

(It also throws some light on why they seem to need unplanned maintenance!). ...

Yours,



After some additional email exchange it transpires that a Google search  picked up the video as an external link on Wikipedia here: http://en.wikipedia.org/wiki/Linear_particle_accelerator

Whilst the video is certainly not aimed at the patient (although this chap above clearly knows a thing or two about physics), it is great to think that watching it helps understand what is going on, how the treatment takes place, and why the linac does what it does.

If you want to stay on this page you can checkout the linac resource directly using the player below.



A real linac (Elekta Synergy), and the type used in the above described radiotheraphy, is shown in the picture below.

22/10/2009 - Radiation Safety Software
Ionactive Training Resource - Radiation Scatter

Don’t you just love buying something that does ‘exactly what it says on the tin’? Whether its fence paint, carpet cleaner or virus software – it’s great when you purchase something that lives up to expectations (or indeed in many cases exceeds pessimistic, and perhaps unjustified, predictions).  The same can be said for radiation protection software.

Free Radiation Protection Software

Firstly it is worth saying from the outset that there are some fabulous free resources on the web – indeed some of these are so good that it is amazing the creators have decided to offer them for no financial reward. The cream of the crop in my view has to be Rad Pro Calculator – we have featured this resource many times in the past and continue to support it – it just does not get any better and I am sure that many of the features exceed much of what you might end up paying for. RadProCalculator does exactly what it says on the tin – and much more besides.

Grove Software (Home of MicroShield et al)

Then one moves to software that is offered for sale – one might say professional radiation protection packages (although I believe the above mentioned free resource would fit that category absolutely). Ionactive has for some time been a user of Grove Software (radiationsoftware.com), and in particular MicroShield. We have in fact just purchased the most up to date version of this software and continue to believe it to be excellent, easy to use, and an essential tool for our radiation shielding design work. This software uses point kernel integration which is not as sophisticated as probabilistic monte carlo simulation, but for many typical problems it is more than adequate (and in fairness a lot less expensive).

MicroSkyShine (also from Grove Software)

In addition to purchasing MicroShield we have also recently purchased the latest version of MicroSkyShine and RadDecay. RadDecay is a program for displaying radioactive decay,  halflives, radioactive daughter nuclides, probabilities per decay, and decay product energies for alphas, betas, positrons, electrons, X-rays, and gamma rays. In other words it is a useful resource program for the Radiation Protection Adviser.

What I really want to target though is the other program - MicroSkyShine. I believe that scattered radiation, particularly that which is derived from air interaction above a radioactive source, is a surprise to many.  MicroSkyShine is a program which can be used to model this scattered radiation in several different specified geometries.

Ionactive has been involved in a number of Industrial Radiography enclosure design projects – a recent one has involved a bay with an open top – an ideal candidate for MicroSkyShine. Everyone knows that, so far as is reasonably practicable, one would design a radiography bay with a shielded roof – but a roof is not always reasonably practicable. We have been working with a client that needs to move very large pipes in and out of a radiography bay – a fully shielded roof is just not an option.  The bay needed to be designed such that the radiography could be classed as ‘enclosure radiography’ such that unrelated work around its perimeter could take place unhindered.  Designing shielding walls is the easy part (and MicroShield played an important part here). Determining scatter from above the enclosure, and its influence on dose rates in the undesignated work area some distance beyond the enclosure is much more difficult. We will feature specifics about this new client design project shortly in a later blog.

Sky-Shine (Ionising Radiation Air Scatter Problem

Quantifying Sky-Shine is very important – ignore it at your peril – its effect can be to make an open topped radiography bay useless. Whilst ionising radiation scatter and absorption in materials can be explained at the physics level, I do not believe that many truly understand its significance. To investigated this, consider the following scenario – place an un-collimated 370 GBq (10 Ci) Ir-192 source in the middle of a 4m by 4m bay (internal dimensions)  at 1.0 m off the ground.  Assume that the walls are 1m thick, and have a height of 3m - enough shielding to attenuate the direct or scattered gamma ray photons (which do not leave bay as sky-shine) to background levels. Then, crouch behind the ‘safe side’ of one of the walls roughly in line with the source and measure the dose rate at 1m above the ground with your trusty dose rate monitor – what dose rate do you measure? This scenario is depicted in the picture below.


Ionactive Sky-Shine Bunker Example


Then, you move away from the wall – let’s say by 4m – and measure the dose rate at 2m from the ground – what dose rate do you expect? Ok, so radiation does not travel upwards, turn about 90 degrees, travel along the ceiling for while, before turning 90 degrees again and zapping you! But ... if you take dose rate readings for the above described scenario you might be forgiven for thinking so.  You might indeed get quite a nasty surprise. So what are the dose rates for the example outline above? MicroSkyShine can be used to determine these values.

MicroSkyShine Results for Ionactive Bunker Example

In the first case the model predicts that right up against the shield, 1m from the ground, the dose rate will be about 6 μSv/h – and none of that is coming through the wall. Now move back by 4m and measure the dose rate again at a height of 1m – and you get 5 μSv/h ( yes 5 – oh what happened to the inverse square law!!).  Then get into your Cherry Picker and measure the dose rate at 3m above the ground – and you get 8 μSv/h. 

Now move a distance of 7m away from the bay wall – and use your Cherry Picker to raise you up to 7m – and you get 11 μSv/h. And so this goes on – you get the idea.

MicroShyShine Model Dose Rates vs Real World Measurements

The values from MicroSkyShine in the above example are just that – an example. So, what about real world dose rates, how do these compare to modelled values? Well, a detailed analysis of this will feature in a future blog entry where we describe in detail a recent design project for a large open topped radiography bay – of the type depicted in the picture below.

Ionactive Radiography Bay - Real World Sky-Shine evaluation

We have found MicroSkyShine to be excellent at predicting radiation exposure around this open topped radiography bay – we are talking about modelled values being within a few percent of measured values – for such a ‘black art’ in radiation protection we think this is rather good – it does exactly what it says on the tin.

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This is the company blog of Ionactive Consulting Limited, a Radiation Protection Adviser consultancy. Visit here often to read our views on radiation protection and related matters. You can contact our director and RPA directly at mark.ramsay@ionactive.co.uk

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