Safety Code 34. Radiation Protection and Safety for Industrial X-Ray Equipment

1. Introduction

1.1 Background

Radiation Equipment

Machines have been specifically designed to generate x rays for the purpose of investigating the integrity of structures or components through radiographic images; this application is called nondestructive radiography or industrial radiography. Three types of ionizing radiation sources are typically available for such purposes:

1. Radioactive materials that are gamma ray emitters such as iridium-192 (¹⁹²Ir), cesium-137(¹³⁷Cs) and cobalt-60(⁶⁰Co).
2. Neutrons that are produced in reactors or by other means (particle accelerators, radionuclides), and this application of neutrons is specifically re- ferred to as neutron radiography.
3. X-ray tubes that are characteristic of conventional x-ray machines.

Industrial radiography machines which are x-ray tube based can produce dose rates in air of about 2 Gy per minute at one metre ⁽⁷⁾. They may be highly portable or mobile and convenient for use at temporary job sites. Sometimes they can be operated by a single worker in a wide range of conditions such as at aircraft hangers, pipeline construction and deployment, fabrication facilities, offshore platform operations, bridges, or construction sites. At temporary work sites, the working conditions coupled with frequent manipulation of such high-intensity radiation sources present much potential for radiation exposure to occur. Both the worker(s) and other persons proximal to the work area can be exposed to high radiation fields which, potentially, can result in radiation accidents that could lead to serious personal injuries or death. In other work situations, x-ray tube based devices may be installed in a shielded enclosure equipped with safety components which significantly reduce radiation risks. To date industrial radiography is an established practice that provides benefits concurrent with radiation risks.

In another industrial application there are systems specifically designed to focus intense beams of high-energy electrons that melt and bond metals under vacuum conditions, and these electron-metal interactions can produce x rays as a byproduct of the bonding process. Such systems are called electron beam welders. By design, electrons are emitted from heated filaments and accelerated to several hundred kilovolts before impacting on metallic materials which are generally placed in a vacuum. In some designs, however, the electron beam passes through a series of orifices, each of which is individually evacuated while the target material is positioned within close range (few centimeters) of the last orifice. Beam currents and high voltages are typically in the range of 20-200 mA and 120- 450 kV, respectively. A device operating at 150 kV and 50 mA, for example, would yield an estimated electron dose rate in air of ~ 0.5 Gy per second at 50 cm, while the scattered x-ray radiation field at 1 metre would approximate 1 Gy per hour for a 1-cm primary electron beam incident on a copper target ⁽⁸⁾. The welding process requires the use of highly focused beams, a requirement that not only reduces the number of electrons striking the metals being bonded but also lowers the contribution of byproduct x rays potentially scattered. The operation of electron beam welders presents a potential for exposure to x rays and electrons.

Collectively, in this document, x-ray tube based machines used for industrial radiography and electron beam welders are classed as "industrial x-ray equipment." They present potential risks of exposure to x rays and electrons. The radiation protection objective, therefore, is to keep the risks ALARA while maximizing benefits.

Radiation hazard

X rays and electrons are types of ionizing radiation. In general when ionizing radiation traverses matter the interaction is probabilistic, that is, there may or may not be an interaction. In the case of a medium composed of cells of living organisms, the interaction with individual cells may be direct or indirect. At the cellular level, direct interaction with DNA or other constituents can cause damage. In the indirect mechanism, reactive ions are formed due to the breakdownof the water molecules present in the cells; such ions can interact with any cellular constituent thereby leading to potential damage. Various possibilities exist for the fate of cells exposed to ionizing radiation:

1. Damaged cells are completely repaired by the body's inherent repair mechanisms.
2. Damaged cells die during their attempt to reproduce. Thus tissues and organs in which there is substantial cell loss may become functionally impaired. There is a "threshold". dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome. Exceeding the threshold dose increases the level of harm. Such outcomes are called deterministic effects and occur at high doses.
3. Damaged cells survive the radiation insult, but are misrepaired and are able to undergo subsequent divisions. These cells, with the progression of time, may be transformed by external agents (e.g., chemicals, diet, radiation exposure, lifestyle habits, etc.). After a latency period of years, they may develop into leukemia or a solid tumor (cancer). Such latent effects are called stochastic (or random).

Germ cells are present in the ovaries and testes and are responsible for reproduction. Should they be modified by radiation, hereditary effects may occur in the progeny of the individuals exposed to radiation. Radiation-induced hereditary effects have not been observed in human populations yet they have been demonstrated in animals. Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and, during certain periods in early pregnancy, may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high.

Exposure to ionizing radiations has the potential to cause early or late adverse health effects. This is why the radiation risks associated with industrial x-ray equipment need to be managed

1.2 Objective

The objective of this Safety Code is to present information for the radiation protection and safety of individuals operating, using and servicing industrial radiography x-ray equipment at permanent installations or at temporary job sites, and of persons proximal to such work areas. The owners of industrial x-ray equipment, the organizations or individuals carrying out industrial radiography, and clients who hire such organizations or individuals are responsible for ensuring that all safety procedures are followed and that the work is done in a manner that does not pose undue risks to any person.

This document provides basic requirements and guidance intended for the radiation protection and safety of industrial radiographers, other users, service personnel and the public. It does not discuss industrial radiography techniques or electron beam welding processes or other requirements (e.g., electrical or explosive).

1.3 Scope

This Safety Code applies specifically to industrial x-ray equipment operating at energies up to 6 MeV for use in industrial radiography and in material melting-and-bonding applications. It therefore covers x-ray tube based equipment, electron beam welders and low energy (≤ 6 MV) accelerators.

It may have limited application in x-ray photon-based scanning systems used for cargo surveillance purposes. The characteristics of such systems include:

1. large enclosures or modular configurations which comprise shielded walls or panels, which can facilitate human intrusion, and which may be used in temporary work sites;
2. intense x-ray beams produced by high voltage generators (greater than 450 kVp) or by accelerators; and
3. various safety mechanisms to limit exposures to the operators and the public.

This Safety Code excludes:

1. cabinet x-ray systems;
2. radioactive-based equipment that emits gamma rays or neutrons for use in industrial radiography;
3. systems specifically designed for the production of neutrons which may be used in industrial radiography; and
4. facilities that utilize high-intensity gamma ray or electron sources for food irradiation or material sterilization or material property-modification purposes, applications which require a dose on the order of kilogray.

1.4 Definitions

Some of the terms used in this document are defined in the glossary.