People often ask these radiation protection questions

Set of radiation protection related questions and answers often asked by RPS / RPO delegates and by people via email or social media

  • Can I become contaminated by radiation?
  • Can I use density ratios when working out radiation shielding thickness?
  • Can radiation be passed from person to person?
  • What materials can block ionising radiation?
  • What are the four main practical principles of radiation protection?
  • What is radiation Skyshine - and does it matter to me?
  • What is a dirty bomb? What is its radiation significance?
  • What is the difference between sealed and unsealed radioactive source?
  • What is RPS training?
  • What type of radiation is most harmful to health?

Can I become contaminated by radiation?

'Radiation' cannot be passed from person to person, just as visible light cannot be passed from person to person.

However, radioactive material (which emits radiation) can pass between individuals in the form of radioactive contamination. Radioactive contamination is a physical substance - and like all substances can move around in the environment. Therefore when considering if it is possible to be contaminated with radioactive material consideration has to be given to its physical properties and environmental factors around it (including possible exposure pathways)

Often in the media the word 'radiation' is used quite carelessly. This extends to ionising and non-ionising radiation which is why mobile phone (e.g. 5G) radiation is often associated with nuclear processes (it is not). So here we are talking about radioactive material which emits ionising radiation.

Can I use density ratios when working out radiation shielding thickness?

This FAQ relates to photon radiation (i.e. gamma or x-rays). For a full understanding of information contained here is it worth ensuring you understand the concept of TVT (10th value thickness). You may wish to read our glossary entry “10th Value Thickness (TVT)

Basically, the TVT is the thickness of a shielding material required to reduce the pre-shielded dose rate to 1/10 of that value. To choose a TVT you need to know the radioactive material (gamma emitter) you are considering, and the shielding material available. For x-ray systems you need to know the kV of the x-ray system, and the shielding material available.

[All figures that follow are used to explain the concepts, they must not be used for official shielding requirements. If you need advice on radiation shielding, then please contact Ionactive].

Where density ratios work (or might work)

Let’s first consider where density ratios generally work, and why you might want to use them. For this we will consider a 10 MV linear accelerator photon beam through concrete. In looking at these examples we will consider the TVT “as is” rather than the “1st-TVT” and “Equilibrium –TVT” as used in formal calculations. Furthermore, broad beam vs narrow beam is not considered, nor is the maths behind partial TVTs (i.e. using 10-x etc.) For the purposes of this FAQ this level of additional detail is not required (and in real shielding calculations you might want to use modelling software anyway).

The TVT for 2.35 concrete (i.e. 2.35 g cm-3), for a 10 MV photon radiation, is about 37 cm. Real life measurements have shown that using density ratios gives good practical results. For example, if you have a Barytes block with 3.3 density, then the ratio work pretty well. So, you could calculate a TVT for this Barytes block of 26.3 cm. This works in real life measurements. Or, you might only have 1.9 density concrete blocks, so a density ratio of 2.35/1.9 would indicate a new TVT of nearly 46 cm.

Let’s return to the TVT for a 10 MV photon in 2.35 concrete being 37 cm. The TVT for steel is 11 cm. Does this figure reflect density alone? Assume density of steel is say 7. Then from a density ratio perspective one would expect the TVT for steel, as related to 2.35 concrete, would be [(2.35/7)*37 cm] = 12.42 cm. Quite close, but not actually what was expected.

How about lead? The reported TVT for lead for 10 MV photons is 5.7 cm. So, let’s see how we could scale that from standard density concrete. This would be [(2.35/11)*37cm] = 7.9 cm]. It’s the same order of magnitude, but now note we are departing from a “linear” relationship. To do this calculation without using reported data would overestimate the lead requirement, based on the standard density concrete density.

In summary, for high energy Photons (e.g. x-ray / gamma rays) with concrete, scaling density by ratio for concrete works pretty well. Whilst this scaling will provide steel and lead TVT of reasonable accuracy, already our analysis shows that using specific data of energy vs. shielding material will be much more reliable. The bottom line here – don’t scale density using different types of materials (e.g. steel vs concrete), it does not work very well!

Ok, so far so good! How about lower energy photons?

Now what about lower energy photons? Of the order of a few 100 keV, and certainly less than 511 keV. How does density scaling then impact? In this consideration we are of course specifically looking at different material comparison (e.g. brick shielding vs. lead shielding).

So now consider 100 kV x-ray beam. Most literature will report a TVT for lead as about 0.92 mm.

Following the discussion above, let us consider density ratios alone and first try and estimate the equivalent shielding for standard 2.35 density concrete. Based on the previous discussion our concrete (100 kV, 2.35) TVT should be [(11/2.35)* 0.92 mm] = 4.3 mm. However, this figure significantly underestimates the thickness of standard concrete required to achieve a TVT. The actual TVT for 2.35 concrete for 100 kV x-rays is more like 59 mm (based on reported values in literature and Ionactive workplace measurements).

At low levels of kV, and of course keV, the same effect can be seen in other shielding materials.

Let’s try density ratios between lead and steel. Using the 0.92 mm TVT for lead with 100 kV x-rays, we might assume that the steel required is about [(11/7) * 0.92 mm] = 1.45mm steel. The actual figure is more like 6.7 mm steel. So another big underestimate!

The conclusion here is simple – never try to calculate lead equivalence, or any other material equivalence related to lead for low energy photons (i.e. typically in the range of x-rays of a few hundred KV, or gamma emitting radioisotopes of a few hundred keV) using density. It does not work! And importantly, you will underestimate the shielding requirements for materials of a lower density then lead. Drastically so. [Note that for the high energy photons discussed earlier, scaling from lower density concrete to higher density non-concrete materials actually overestimates the shielding required. So, it does err on the side of caution, but becomes excessively expensive for no good reason].


The point of this FAQ is not to explain the physics in any detail – we like to keep this quite practical. However, at higher energies (typically >> 1 MeV photons) the interaction with a shielding material causes ‘pair production’ - a positron / electron is created (positive electron) very near the nucleus of the shielding material. The positron almost instantly interacts with an electron causing annihilation radiation to be created (i.e. two photons with 511 keV moving in opposite directions). The incident radiation upon typical shielding at high energy is lowered below 511 keV on the first interaction. Therefore, the electron density in the shielding (which is related to physical density) leads to density ratios in different material predicating TVT pretty well (you could see this for concrete, steel and lead with 10 MV photons). At much lower energy the shielding effect is dominated by the 'photo-electric' effect and 'compton scatter' which is much more sensitive to material composition, and so density ratios for different shielding material becomes ever more significant.

Can radiation be passed from person to person?

This question is often asked but needs some clarification before providing an answer. Radiation cannot be passed from person to person, just as visible light cannot be passed from person to person. However, if you mean radioactive material (which emits radiation) then its possible to pass this between individuals. Radioactive material is a physical substance which may take the form of a solid, liquid or gas. Therefore, if the radioactive material is mobile in the environment then it can be passed from person to person.

What materials can block ionising radiation?

The type of material depends on the type of ionising radiation in question.

Alpha Particles - single sheet of paper, the outer dead layer of skin of your body, clothing

Beta Particles - depends in energy of beta particle, but 12mm of a low density material like perspex or plastic will stop all betas

Gamma Ray / X-ray - shielding performance depends on energy of the radiation for a given thickness and density. Higher density shielding such as lead / tungsten will provide the best shielding performance for the least thickness. If space is of little concern high density concrete, normal concrete or even water can be used (but these shields will be thicker).

What are the four main practical principles of radiation protection?

The following four principles are valid for ionising radiation (although not all will apply for individual cases as this will depend on the type of radiation source involved).

Time - Minimise the time of exposure.

Distance - Maximise the distance from the source of radiation.

Shielding - Use radiation shielding where practicable.

Containment - For unsealed radioactive materials use containment to minimise risk of contamination spread and avoid inhalation of ingestion hazards.

What is radiation Skyshine - and does it matter to me?

Radiation skyshine is where radiation (predominately gamma or x-rays) scatter within air (and other objects) above a facility which is either open topped (no shielding) or has reduced shielding. Radiation from the bottom of the facility will scatter upwards to the top of the facility, and some of this will then scatter downwards towards the ground - the other side of the facility shielding. It may be that direct measurements from the source of radiation through the shield are negligible. However, as you move away from the facility a peak dose rate will be measured some distance from the shielding (maximum skyshine) before reducing with further distance from the facility. It matters to you as many have ignored this phenomenon and only discovered higher dose rates some distance from the facility many months later.

A more rigorous explanation of Skyshine can be found here: Skyshine – radiation scattering around and over shielding.

What is a dirty bomb? What is its radiation significance?


The term ‘dirty bomb’ is generally used in the media and is supposed to be emotive. The term means a device consisting of radioactive material and explosives combined together in some way. The term ‘dirty’ implies that the device contains radioactive material, whereas bomb means the device has material that will cause an explosion upon detonation.

The more ‘technical’ term used by various security services and consultants is ‘Radiological Dispersal Device’. This term, shortened to RDD and sounding more official and scientific, more accurately describes what the dirty bomb is – a device designed to spread radioactive material far and wide causing panic, closure of large areas of infrastructure (e.g. public transport) and potential health effects.

An important consideration is that the RDD (dirty bomb) is NOT a nuclear device. It cannot explode as an atomic / nuclear bomb might. The power of the RDD is governed by the quantity and type of explosive present, the presence of radioactive material has no influence on the explosion itself.

What is the dirty bomb / RDD?

The device is a rather crude / “dirty” way of spreading radioactive material around. When one considers the events of Chernobyl and Fukushima, and the resulting press coverage and anxiety (which lead for example to Germany shutting down many of its nuclear power plants), one can see why an RDD might be a ‘useful’ device for a terrorist group. Furthermore, the demise of Alexander Litvinenko is perhaps the nearest we have come in UK to the aftermath of a dirty bomb. In this case there was no explosion, but the deliberate administration of radioactive Po-210 to a person, with disregard to who else might be harmed by contamination spread, yielded anxiety among many and a hugely costly clean-up. This is different to the accidental spread of radioactive material in the above-mentioned nuclear accidents, or the spread of industrial radioactive material during the infamous Goiânia incident.

Here is a basic mock-up of a dirty bomb containing a capsule of powdered Cs-137 surrounded by explosive material.

Radiation Protection FAQ Dirty Bomb or IDD 01

In all cases public and governmental anxiety, and the clean-up costs (this continues at Chernobyl and Fukushima), plays into the hands of anyone who would want to cause similar levels of disruption.

During the Cold War the principle of MAD was prevalent. Mutual Assured Destruction. Perhaps a modified version of this could be used – Massive Assured Disruption. For this is really the point of the RDD / dirty bomb. The radiation health effects of such a device, when compared to a nuclear explosion, are negligible.

Where is the worst place to be around a dirty bomb?

We once asked this question to a group of police officers we were training on a CBRN course “Where is the worst place to be around an RDD”. Various answers were given including ‘downwind’ and ‘very close’. We said, ‘sat right on top of it when it goes off’. Without getting too quantitative this is a matter of fact. The explosive force will do you more damage than the radiation exposure will. This answer can be slightly modified if you happened to be sitting close to the RDD for some considerable length of time. If the radioactive material were a beta / gamma emitter (something like Cs-137 or Co-60) then you would get maximum potential external exposure since you have the maximum activity of radioactive material all in one place (i.e. inside the RDD). However, the overall risk of radiation induced health effects, as compared to the risk that the RDD will explode at any moment, are again negligible.

A radioactive contamination spreader

We have noted that the aim of the dirty bomb / RDD is to spread radioactive material far and wide (as radioactive contamination). Depending on the physical / chemical properties of the radioactive material, the nature of the explosion, the wind direction and other environmental factors, the type of area (i.e. city / town / grassland etc), the radioactive material will somehow spread far and wide. It may be blown about in the wind, settle out on flat surfaces, enter drains or ventilation systems. The one thing that is guaranteed is that the radioactivity is ‘diluted’ and this is quite an important consideration when looking at potential radiological health effects.

Radiation Protection FAQ Dirty Bomb or IDD 02

Let's look at some basic figures

Suppose the RDD has Cs-137 in the form of a radioactive powder. Assume that the activity of the CS-137 is 1 GBq (that is 1,000,000,000 Bq). Post blast that activity will be spread out such that you will have lower specific activities in the environment. Examples could include 10’s Bq/cm2 over a surface area, 10’s Bq/m3 in air, 10’s of Bq/g of contaminated soil and so on. The point is that however you look at it, if you have dispersed the radioactivity and if you happen to be in some a location “X”, some distance from the blast (i.e. your survived the effects of the explosion), then the radioactivity at X will be much less than the activity within the RDD before it exploded.

It therefore follows that the external radiation hazard will be considerably less than it would be if you were still sitting on top of the unexploded RDD (it is assumed the RDD does not have radiation shielding). Time is a factor here that cannot be ignored, but in the response times we are talking about after an RDD event, it has little effect oveerall. It is true that you might stay at location X longer than you would sitting upon the RDD, but reduction of activity is on your side. All other things being equal, if you are sitting within the vicinity of 1/10000 of the total radioactive content of the RDD, then the dose rate at that point can be approximated to 1/10000 of the dose rate you would be receiving if sat on top of the RDD. Let’s look briefly at some numbers.

External radiation

Dose rate sat 1 cm from 1 GBq of Cs-137 will be about 760 mSv/h (12.7 mSv/ minute). Hardly good for you (!), but not a killer either. Several hours of continuous exposure is survivable (assuming the RDD does not explode!).

Assume activity at location X to be 1/10000 of the RDD total. This is an arbitrary figure but feels about right. If you blow up a small bag of flour, think how that material would physically disperse and how long it would take you to scrape up and place all the flour back in a new bag? The dose rate at X, assuming the radioactive material be treated as a point source (unlikely but worse case) would be 76 micro Sv/h at 1 cm (so assume you are laying on the ground!). Perhaps you are actually standing up, simplifying using the same point source idea, means that your trunk (whole body dose rate) would be about 0.001 micro Sv/h (i.e. background).

There are many simplifications and assumptions here - but this crude analysis shows that the external dose rates, not very far from the site of an exploded RDD which contained a high energy / high activity beta / gamma emitters, are not that great (actually nothing to worry about compared to an explosion!). Perception of radiation risk is a totally different matter and be sure that once the nature of the incident is known the area will be emptied of all persons.

If the radioactive material were only an alpha emitter, or a low energy beta / gamma emitter then the external radiation hazard sat upon the unexploded RDD would be negligible, and even less so some distance away post explosion. We have not looked at the effect of half-life in this example, which for Cs-137 (T1/2 is 30 years) would not be a factor in the early months (or years) of a clean-up. In contrast, the ability to readily detect such contamination will be diminished and this could impact exposures as we will now see.

Internal radiation

Next, we turn to the potential for internal radiation exposure. For this we will consider inhalation of radioactive material only (ingestion, skin irradiation and entry of material through cuts in the skin will not be considered). This is much more complicated due to the number of factors involved, so we will treat this at a much-simplified level. Suffice to say any or all of the following will influence the potential internal radiation exposure:

  • Radioactive material used (in this case Cs-137)
  • Its physical properties (solid, powder, liquid, gas)
  • Its chemical properties (e.g. soluble in water, reactive in air)
  • Its biological properties (i.e. potential transmission through human body)
  • The radioactive substances properties (i.e. alpha, beta, gamma, emission energies and their probabilities)
  • The radiological and biological half life of the material
  • The environmental factors (wind, rain, topography)
  • Potential dose delivered per unit Bq (activity) inhaled or ingested
  • The activity concentration (e.g. in air) at the point of intake
  • The duration of intake

And so on (you get the general idea)

If external exposure assumptions were tenuous, internal exposure are even more so. For this reason, we will look at this from another angle – what intake of radioactive material would yield a certain level of radiation exposure at our point “X”. As you might now guess there are even more factors at play (pick up factors, transfer factors etc). We will simplify and just assume that a certain activity of Cs-137 taken in to the body will yield a certain whole body committed effective dose.

So, let’s state the following - broadly speaking inhaling 150 Bq of CS-137 will present a committed effective dose of 1 micro Sv. (Strictly speaking for 150 Bq to fully enter the lungs / systemic system would require more activity at the point of intake, since the human body is good at clearance - e.g. the nose). Let is not worry about this - we are just playing with some numbers.

We can scale this as follows:

1 micro Sv150 Bq

1 mSv150 KBq

Noting we said earlier that the total source term of the RDD was 1,000,000,000 Bq (1 GBq), then 1 mSv inhaled is of the order of 0.015 % of the total activity. Think back to the bag of flour. What is the activity in air at point X going to be, to deliver a dose of 1 mSv? Think about the physical spread of material.

What about inhaling 10% of the total activity? That would be inhaling 100 MBq (that is 0.1 GBq or 100,000,000 Bq). This would yield a committed dose of 667 mSv. That is a massive radiation dose, but not immediately life threatening, In fact, on this basis inhaling the entire content of the RDD is not life threatening from the internal hazard alone – the combined internal / external exposure would be … rather unfortunate. But hopefully you can also accept is totally unrealistic too!

The bottom line

The bottom line here is that an RDD / dirty bomb is very unlikely to create a situation where lives are lost from acute radiation exposure. Lives lost from statistical radiation induced cancer (stochastic radiation effects) is still a valid concern but orders of magnitude below the figures present here (probably in the background noise).

The RDD is not an effective weapon for causing mass acute radiation casualties (death). Neither is it a good weapon for causing mass statistical lives lost from radiation induced cancer either.

At best it is an emotive weapon, based on perception of radiation risk. It is a potentially powerful economic weapon as the clean up (demanded by governments and public alike) would be at immense cost. The economic effects of closing down and restricting access to several square km of the city of London would be considerable.


The dirty bomb images are taken from a video that was produced for Ionactive by Chris Robbins from Grallator. Find out more about his work here:

What is the difference between sealed and unsealed radioactive source?

Sealed radioactive material is that which is contained or encapsulated in some way so that the radioactive material cannot move within the environment around it. This is sometimes known as a sealed or closed radioactive source. Such sources can only present an external radiation hazard, walking away from the source will reduced radiation exposure to negligible levels. Unsealed radioactive material (or source) is where the material is not contained, and depending on circumstances, can move around the environment. An example could be a radioactive liquid that has been spilled from an open container. This unsealed material is called contamination and could be taken into the body by ingestion, inhalation or through cuts in the skin. Walking away from the container will not necessarily reduce the radiation exposure hazard to zero since this person may already be contaminated.

What is RPS training?

What is RPS training?

The term RPS stands for Radiation Protection Supervisor (RPS). The RPS is a position recognised by the UK Ionising Radiations Regulations 2017 (IRR17). It is required in most cases where an employer wishes to work with ionising radiation (e.g. x-ray systems, radioactive sources and similar).

In order for the employer to appoint one or more of their employees as an RPS, the employee should attend an RPS training course. Usually the RPS course is provided outside the company (i.e. only the nuclear industry, other very large users of radiation and perhaps some hospitals provide their own in-house courses). Pre-Covid RPS courses would normally be held at a training venue arranged by the training provider in the form of public face-2-face training. During the pandemic many courses have been run online using Teams or similar. Ionactive took a different approach and created a popular multimedia online RPS training course.

Attending a RPS training course does not make that individual an RPS. To become an RPS the employee needs to attend a suitable training course and then be appointed in writing by their manager or other suitable person. The type of training and number of RPS positions required will depend on the nature of the work with ionising radiation and usually the employer would consult with a Radiation Protection Adviser to determine what is required.

In some cases attendees of RPS training are not appointed into post after completion of the course. This is usually due to the employer wishing certain employees to be trained to a recognised standard in radiation safety, but without the need to then take on a RPS position formally.

What happens at an RPS training course?

At the time of writing the exact nature of RPS training may vary if it is being taken online, or live via Teams / Zoom or similar. A good RPS training course will have a little maths and physics - this is unavoidable. However, this is at a level which does not require any previous formal education in either subject. It is more important that the training delegate is interested and willing to take the training rather than come prepared with formal qualifications. The course will also contain some elements of health and safety law including a summary of the Ionising Radiations Regulations 2017 (IRR17). A course will generally cover some of the following matters:

  • Basic introduction to ionising radiations and their biological effects
  • Units used in radiation protection (e.g. for dose and radioactivity)
  • The basic protection principles (e.g. time, distance, shielding and potentially containment)
  • Contingency arrangements (what to do when things go wrong)
  • Radiation monitoring and dosimetry (i.e. workplace and personal monitoring)
  • The content of Local Rules (the 'do' and 'do not').
  • The role of the RPA
  • The role of the RPS

Suitable courses will also usually include a small test so that an 'achievement' certificate can be awarded (this more valuable to the delegate and employer than an 'attendance' certificate.

Whereas some training providers will offer 'sector' (or source) specific RPS training (e.g. x-ray or unsealed radioactive materials), from the very beginning Ionactive has provide courses that cover all areas of ionising radiation use. Whilst the delegate usually ends up with more knowledge than they strictly need, it allows them to consider their own radiation source use in context with other uses (and this helps in the appreciation of radiation risk).

RPS course providers including Ionactive will also often provide onsite bespoke RPS training where enough delegates all need training at once.

What type of radiation is most harmful to health?

In answering this question we will concentrate on ionising radiation (since that is what Ionactive mostly works with). Visit our glossary for a definition of ionising radiation and non-ionising radiation.

The main types of ionising radiation we can consider are:

  • Alpha Particles
  • Beta Particles
  • Gamma / X-rays
  • Neutrons (not directly ionising but will cause ionisation upon interaction).

In order to discuss harm we then need to consider if the potential harm is via external or internal radiation exposure. External exposure is where the body is exposed to ionising radiation from a source that is external to the body (e.g. via an x-ray machine or sealed radioactive beta or gamma ray source outside the body). Internal radiation exposure is where the body is exposed to a source of ionising radiation which is located inside the body (e.g. radioactive material). In some cases certain types of sources can present both an internal and external radiation exposure.

External Radiation Exposure

For external radiation exposure, gamma or x-rays will generally be capable of delivering the most harm. Actual harm will be related to the energy of the radiation, the dose rate (how fast the radiation is delivered) and the total dose. Beta particles of high energy are also capable of superficial harm to the skin and can cause radiation skin burns in extreme cases.

Internal Radiation Exposure

For internal radiation exposure alpha particles will generally be capable of delivering the most harm. A pure alpha emitter (e.g. Po-210, ignoring the very weak and occasional gammas) presents little harm externally. However, if a significant quantity (activity) of Po-210 was ingested or inhaled then a very large radiation dose could be delivered. The actual dose will depend on many factors including the physical and chemical form of the alpha emitter and how the body deals with the contaminant biologically.

In some circumstances a radiation source may present an external and internal radiation hazard. For example, Am-214 emits alpha, beta, gamma, and x-rays.

Hold on, can radiation do good as well?

Whenever considering harm from ionising radiation it is also useful to consider exposures in context. For example, low energy x-rays and certain radioactive substances are used in medical diagnostics. High energy x-rays / gamma rays (and others like protons and neutrons) are used in radiotherapy for the treatment of many cancers. These type of exposures, where carefully administered and controlled, can do more good then harm! It is all about weighing up benefit and risk.