Control devices that energize a conspicuous visible or audible alarm so that the individual entering
the area and the supervisor of the activity are made aware of the entry.
Entryways that are locked, except during periods when access to the areas is required, with positive
control over each entry.
Continuous direct or electronic surveillance that is capable of preventing unauthorized entry.
Very High Radiation Areas.
Additional measures shall be instituted to ensure that an individual is not able to gain unauthorized or
inadvertent entry into a Very High Radiation Area. (A Very High Radiation Area is an area in which
radiation levels could be encountered at 500 rads (5 gray) per hour at one meter from a radiation source or
from any surface that the radiation penetrates) (10 CFR 20.1602). The requirements of 10 CFR 20.1603 shall
be implemented for all radiation sources, including X-ray machines, which create very high radiation areas.
Protective Installations or Shielded Installations.
PROTECTIVE INSTALLATIONS: This class provides the highest degree of inherent safety because
the protection does not depend on compliance with any operating limitations. This type also has the
advantage of not requiring restriction in occupancy outside the enclosure; the built-in shielding is
generally sufficient to meet the maximum permissible dose requirements for the environs.
The allowable usage exposure level limits exposures to the lesser of 0.5mR (5 mSv) in any one hour or 2
mR (20 mSv) per week (when corrected for workload, utilization, and type of occupancy) for this class
of installation necessitates a higher degree of inherent shielding. For radiation sources of lower
energies, and for smaller enclosures such as cabinets, the initial extra cost of the increased shielding
is usually insignificant compared with the operational advantage.
At higher energies, as in the megavolt region with high workloads, the required additional shielding
will usually make the use of this class extremely expensive compared with the enclosed installation.
SHIELDED INSTALLATIONS: The Air Force describes a shielded installation as any enclosed
radiographic facility designed to limit exposures on the outside of the facility to less than 2 mR in any
one hour and less than 100 mrem per year. The shielding design incorporates the energy of the x-ray
or gamma ray source to be used, as well as the expected workload, use factors and occupancy factors of
An installation shall be classified as protective or shielded when it conforms to all of the following
The source and all objects exposed thereto are within a permanent enclosure, within which no person
is permitted to remain during irradiation.
Each entrance that is used for personnel access to the enclosure/high-radiation area shall have both
visible and audible warning signals as described in paragraph 18.104.22.168.1c(1). At a minimum, a signal
interlock system shall be placed on each door to interrupt power to the control box/tube head, stopping
the irradiation process, when unauthorized access is attempted.
Each of the following shall be provided (except where specifically noted) without regard to the size
and/or configuration of the enclosure:
A time delay/interlock may be locally fabricated in order to meet this require-
ment. Note that the wiring harnesses are similar to the harnesses used with
X-ray Interlock Assembly, NSN 6635-00-292-7637. All time delay interlock
systems installed shall be compatible with all X-ray units commonly