Lead glass: your trusty shield
Introduction
Kodial zero length viewport with x-ray protection lead glass
Kodial zero length viewport with x-ray protection lead glass
Many laboratories and other workplaces contain equipment or materials that emit or produce ionizing radiation such as X‑rays or gamma (γ) radiation. The radiation might be a by‑product of a process, or it might be an integral component of, for example, an analytical technique. In imaging with a scanning electron microscope (SEM), a beam of electrons is rastered across part of a sample in a vacuum chamber. The interaction of the beam with the sample produces X‑rays as a by‑product. If an X‑ray spectrometer is fitted, the spectrum of X‑rays produced can be analysed to identify which elements are present and where; the X‑rays are now the excitation source for the technique known as energy‑dispersive X‑ray spectroscopy (EDX). Facilities such as synchrotrons produce more penetrating radiation such as high‑energy X‑rays or γ‑rays. A widely used strategy for protection from this radiation is to place shielding between the source and the operator to block the radiation. In this article we will discuss the nature of the radiation, the mechanism by which the shielding works and why lead glass is an effective shielding material.
Electromagnetic radiaton
X-rays and γ-rays, like visible light and radio waves, are a type of electromagnetic (EM) radiation and so sometimes exhibit wave‑like qualities and sometimes particle-like qualities. For the current subject, it is more useful to consider the behaviour of particles of radiation called photons. They are not particles of matter, rather they are discrete packets of energy, and they may be classified by how much energy makes up each photon. The convenient unit of energy is the electron volt, eV, equivalent to approximately 1.6 x 10-19 joules. 1 keV is a thousand eV; 1 MeV is a million eV.
Radio photons have relatively low energy, visible light somewhat more. Ultraviolet (UV) has more still, and at some level beyond 10 to 33 eV the photons are energetic enough to pop out an electron from an atom, in other words, to ionize it. Ionization can precipitate damage to the chemical bonds in body tissue and DNA, the blueprint for tissue growth both in you and your children. This is not to say that lower energy photons cannot cause damage: infrared (IR) radiation, as from a cooker grill, can cause thermal damage; anyone doubting this should try my cooking. With more energy than IR, UV photons in sunlight can cause photochemical damage (driven by the light rather than heat). A familiar example is sunburn. UV is fairly easily blocked with sunscreen or suitable headwear – not so with X‑rays and γ‑rays.
Photon energy and attenuation
Let’s get some perspective on photon energy. Sunburn is caused by UV‑B radiation with energy less than around 5 eV. Techniques such as SEM/EDX and X‑ray photoelectron spectrometry might produce X‑rays up to 30 keV. This energy is more than 600x greater than UV‑B. Last time I checked, the Health and Safety Executive (a UK body concerned with safety in the workplace) only needed notifying if your equipment produces radiation greater than 30 keV. Radiation up to 30 keV is effectively blocked by the metal walls of the vacuum chamber, and 10 mm of fused silica will block radiation up to around 20 keV. For total protection up to and beyond 30 keV, lead‑glass shielding is worth considering.
Radiation passing through matter is generally attenuated; that is, the radiation intensity becomes progressively more diminished as it proceeds through the matter. There are several mechanisms at play. A photon might interact inelastically with the electrons of the atoms of the material; inelastic means that the photon loses some energy in the interaction. There may also be elastic interactions (no loss of energy) that by chance divert the photon back into the chamber. The photon might also undergo absorption, with total energy loss, by ejecting an electron altogether from an atom.
Attenuation of light by a medium can be described by a form of the Beer‑Lambert relation that the non‑scientist just might remember from chemistry at school:
I = I0 e -µρd (Eq. 1)
Where
I is beam intensity (photons/s∙cm2)
I0 is incident (initial) beam intensity (photons/s∙cm2)
e is Euler’s constant ≈ 2.718
µ is the mass attenuation coefficient (cm2/g) of the medium
ρ is the density of the medium (g/cm3)
d is the distance travelled through the medium (cm)
This is the only equation that I’ll bother you with in this article, so those of you that dislike them can breathe easy. Your beam starts with intensity I0 which decreases to I as it passes through the medium. The mass attenuation coefficient, µ, is a property of the material and accounts for several quantities, including the number of electrons, the atomic cross section (a measure of how likely an interaction is) and the photon energy. If µ, ρ or d get larger, the attenuation of the beam is increased: I gets smaller. For effective shielding, choose a material with high µ, high density ρ and make the shielding thick. The exponential part of the expression, e-µρd, means that the attenuation rate changes with µ, ρ and d. One consequence of this is that the front end of the shielding (facing the radiation source) attenuates more radiation than does the back end, simply because the intensity is higher when the radiation enters the shielding than when it leaves.
Figure 1 shows the transmittance over a range of photon energies for different thicknesses of fused silica, lead (Pb) and lead glass. The general trend is that higher photon energies are more penetrating, but 1 mm thickness of either pure lead or lead glass effectively blocks all radiation up to 30 keV. Pure lead gives the better protection but has the disadvantage that we cannot see through it. Lead shielding is effective because it has high µ: its high atomic number means lots of electrons for the photons to interact with. Lead also has high ρ: those electrons are densely packed.
Figure 1: Transmittance of three materials across the photon energy range 0 ‑ 30 keV and at different shielding thicknesses
Absorption edges
The trend of higher energy photons being more penetrating does not hold right across the energy range: there are, at certain energies, sudden decreases in transmittance. These are visible on the transmittance curves as the sharp ‘saw tooth’ features and are known as absorption edges. Each of these marks an energy level that matches the energy required to eject an electron from an atom of the shielding medium. For example, three prominent absorption edges on the curves for lead (Pb) correspond to electron binding energies of 2.484 eV, 13.035 eV and 15.200 eV. For any such edge, if the photon had been a few eV lower in energy then that absorption would not take place. Of course, when an electron from a higher shell fills the vacancy, a photon may be emitted in a forward direction, but its energy will be lower than the absorbed original; some other emitted photons will be lost by being emitted back towards the chamber.
Hang on a minute, I hear some of you cry, exponential functions like Equation 1 create smooth curves not ones with sudden discontinuities. I agree, in most cases they do, but only if the quantities in the power to which e is raised all themselves vary smoothly. Here, we have a smooth change in d as the photon traverses the shielding at the materia‑adjusted speed of light. On the other hand, µ conceals those precipitous drops at the electron binding energies. The value of µ is proportional to the atomic cross section which is the original source of the discontinuities (Figure 2). If you locate lead (82‑Pb) on the bottom axis then follow a line vertically upwards (increasing photon energy) then by the time you get to 100 keV (0.1 MeV), it looks to me by eye that the atomic cross section has decreased by a factor of around 10 000x, but not always smoothly. When you get to 10 000 keV (10 MeV) and beyond, it looks more like 50 000x. As the range of the colour‑map values is so large, the smaller individual step changes in the colour map might not in all cases be clear, so the creator* of the map has highlighted them with the coloured curves K to N.
Figure 2: colour map of atomic cross section for elements with atomic number z 1 to 100 and photon energy 0 to ~20 MeV. Source
Lead glass viewports
TSL radiation‑‑shielding viewports have a 6 mm (nominal thickness) lead‑glass plate covering the air side of a CF‑style viewport that has a hermetic window of borosilicate glass. The clips holding the plate are removeable so the plate can be removed when not required or for cleaning. The shielding performance of such plates is commonly quoted as a lead (Pb) equivalent, indicating how thick a piece of lead would need to be to match the shielding of the plate. The 6 mm thickness is equivalent to 1.3 mm of lead for 300 keV photons, rising to 1.7 mm for less energetic 100 keV photons. The lead equivalent thickness is around 25% of the nominal thickness of the plate, but the shielding performance of the lead glass will be better than 25% of that of a 6 mm lead sheet. The reason is again explained by the exponential nature of attenuation: the front part of the shielding (facing the radiation source) does more of the attenuation than the rear part. Making the shielding thinner is, from the photon’s perspective, removing material from the rear. If the shielding you require is non‑standard, please get in touch with our sales team.
Summary
We have considered the various sources of ionizing radiation, commonly under 30 keV, but in some situations considerably higher. We have seen that in the context of radiation shielding, radiation is best considered as discrete particles of energy called photons. Photons can be described by the amount of energy they pack. Somewhere in the energy range 10 ‑ 33 eV is the threshold beyond which photons become capable of causing ionization, potentially causing molecular damage to body tissue or DNA. Radiation traversing a shielding material such as lead glass loses energy by interacting with the electrons of the material. Lead is an effective shielding material because it is dense and has lots of electrons, but we can’t see through it. Glass containing a high percentage by weight of lead oxide can provide effective radiation shielding but remains transparent to light. Higher energy photons tend to be more penetrating as they interact less with matter, but this trend breaks at photon energies corresponding to electron binding energies.
*Thanks to Jarekt on Wikipedia under Creative Commons Licence. No changes were made to the image.
