What is a Dosimeter and Why is it Important?

A dosimeter is an instrument used to measure ionizing radiation exposure (via alpha or beta particles, neutrons, gamma rays, or x-rays) in a science called dosimetry. It is an essential tool for people who work in situations where they are exposed to radiation. 

Dosimeters are used to ensure that a harmful dose of radiation is not received over a given period of time. Governing bodies have standards for occupational radiation protection and control. The main goal of a dosimeter is to maintain an occupational dose as low as reasonably achievable (ALARA).

Radiation is extremely harmful to humans and can be deadly when people are exposed to high doses of radiation. Dosimeters inform and alert people when radiation levels are too high so that they can evacuate the exposure area and avoid radiation poisoning.

Types of Radiation Dosimeters 2022

Radiation dosimeters come in different types depending on their uses and applications. Some dosimeters are used to record cumulative radiation exposure over time, whereas some sound an alarm when harmful doses of radiation are present.

Geiger Müeller Counters

Geiger Müeller (GM) counters are extremely sensitive dosimeters that use a gas chamber to detect radiation. One radiation particle is detectable through the gas chamber that then will produce a clicking noise in the electronic speaker.

Clicks are measured as counts per second or minute, with a count representing the disintegration of radiation.

Geiger Müeller counter with a pancake probe |
Source: SPW Industrial

Alpha Radiation Survey Meter

Alpha radiation survey meters are used to detect alpha radiation with a probe using scintillation, or luminescence.

Radiation survey meter | Source: Fluke Biomedical

Dose Rate Meter

Dose rate meters are used to detect ionizing radiation in the environment to determine if radiation doses are low enough for humans to enter, and for how long, where there is known radiation contamination.

Dose rate meter | Source: Grainger

Electronic Personal Dosimeter

An electronic personal dosimeter (EPD) is typically worn by radiation workers, defined as those who work in planned exposure situations. This personal dosimeter is worn on the torso and displays immediate dose and dose rate on the small monitor.

Electronic personal dosimeters use a metal-oxide semiconductor field-effect transistor (MOSFET or MOS transistor) that triggers an alarm when radiation levels exceed regulatory compliance.

Electronic personal dosimeter | Source: Mirion

Personal Radiation Detectors-Extended Range

Personal radiation detectors extended range (PRD-ERs) are worn by first responders since alarm set points can be manually set to match the situations they are entering. These detectors differ from EPDs by monitoring total radiation exposure during wear time and exposure rates. They also have a wider dose rate range.

Extended range personal radiation dosimeters use plastic or crystal scintillators as well as a semiconductor or Geiger-Müller tube (G-M tube) which uses a gaseous mixture to detect radiation. The scintillators are used to read high dose rates while the semiconductor or G-M tubes allow for low dose rate sensitivity.

Personal radiation detectors extended range | Source: Fisher Scientific

Handheld Survey Meters

Handheld survey meters are used to take radiation readings on people and surrounding environments. These dosimeters are typically used to detect radiation in nuclear facilities and radiological facilities to ensure compliance with industry standards.

G-M tube detectors, ion chambers—like G-M tubes, ion chambers are gas filled radiation detectors—and scintillators are used in handheld survey meters to detect radiation. These dosimeters require training to operate.

Handheld survey meter | Source: Soeks

Non-alarming Personal Emergency Radiation Detectors

Non-alarming personal emergency radiation detectors (PERDs) do not alarm when radiation levels are high. Instead, a colorimetric card is used to visually monitor radiation levels. Darker colors mean radiation levels are high.

There are some disadvantages to this type of dosimeter, mainly because it does not alarm, but also because the colorimetric cards can be misinterpreted. However, these dosimeters are robust and can be used in harsh conditions or as a back-up dosimeter.

Non-alarming PERD | Source: Fire Supply Depot

Personal Dosimeter

A personal dosimeter is worn by an individual over a specified period of time. The dosimeter is usually sent to a facility that examines the radiation dose, but they can also be read at the site of a hot zone. These dosimeters are very accurate.

There are four types of personal dosimeters:

1. Film badges, the most common type of radiation dosimeter, use film and filters to detect radiation dose levels. They are not reusable but give a permanent record of exposure.

   

   
   Film badge | Source: Timstar
   
   

2. Optically stimulated luminescence (OSL) dosimeters use aluminum oxide to detect radiation dose levels. These dosimeters are ideal for pregnant women because of their increased sensitivity.

   

   
   OSL dosimeter | Source: ARPANSA
   
   

3. Thermoluminescent dosimeters (TLDs) use a lithium fluoride (LiF) or a CaD₂ crystal to detect radiation levels. These dosimeters measure radiation by a light that the crystal produces when it’s heated by radiation.

   

   TLD | Source: DevineExpress
   
   

4. Direct-ion storage (DIS) dosimeters are electronic dosimeters that attach to a breast pocket. DIS dosimeters use ion chambers and an electronic element to detect radiation dose levels, however they are not equipped with an alarm. DIS dosimeters can operate at high radiation doses.

   

   
   DIS dosimeter | Source: Mirion
   
   

Personal emergency radiation detectors (PERDs) 

Personal emergency radiation detectors (PERDs) are used in hazardous environments with high dose rates of radiation. These dosimeters are typically used in emergency response applications because they can be used in cold zones, warm zones, and hot radiation zones.

PERDs are worn on the body and detect photons. If the radiation dose exceeds the preset range, an alarm will sound, alerting the individual to harmful exposure rates or an accumulated dose that exceeds radiation exposure standards.

PERD | Source: Mirion

Personal Radiation Detectors

Personal radiation detectors (PRDs) are used to detect low doses of radiation and were developed to help law enforcement detect and intercept nuclear or radiological terrorism threats.

PRDs cannot be used in dangerous-radiation zones, but can alert an individual of unexpectedly low levels of radiation exposure.

Personal Radiation Detectors | Source: Dosimeter Shop

Pocket Ionization Chamber

Pocket ionization chambers are simple devices, no larger than a pen, that can be read in real time. These dosimeters are battery operated, so they need to be charged before and after use.

Common names for this type of dosimeter include: self-reading pocket dosimeter, self-indicating pocket dosimeter, and quartz-fiber dosimeter. Pocket ionization chambers do not record cumulative doses of radiation and do not alarm. This type of dosimeter is less accurate than more modernized types of dosimeters.

Pocket ionization chamber | Source: MegaDepot

Radioisotope Identification Device

Radioisotope identification devices (RIID) are complex dosimeters that are used by HAZMAT companies and public safety officials to detect radiation on civilians, vehicles, cargo, packages, or any other suspect materials.

RIIDs use gamma-ray spectroscopy to detect radiation doses. They can also be used to verify readings of other dosimeters, like PRDs or other gamma-ray dosimeters.

Radioisotope identification device | Source: Mirion

Radiological Detection Portal

Radiological detection portals screen humans, cars, and other cargo for radiation around secure facilities.

Human radiological detection portal at Chernobyl

How to Read Radiation Dosimeter Levels

Radiation is transmitted through both waves and particles. Radiation waves can travel through walls whereas particles can be blocked by a piece of paper. Dosimeters detect the cumulation of radiation coming from both waves and particles.

Radiation dosimeters measure the following: 

• Radioactivity—the amount of ionizing beta, gamma, alpha, and neutron radiation released, expressed as the becquerel (Bq) and the curie (Ci).

• Exposure—the amount of radiation in the air, expressed as roentgen (R) and coulomb/kilogram (C/kg).

• Absorbed dose—the amount of radiation absorbed, expressed as radiation absorbed dose (rad) and gray (Gy).

• Dose equivalent—the combination of the absorbed radiation and the effects from that dose of radiation, expressed as roentgen equivalent man (rem) and sievert (Sv). Biological dose equivalents are measured in 1/1000th of a rem (millirem or mrem).

Put simply, 1 R (exposure) = 1 rad (absorbed dose) = 1 rem or 1000 mrem (dose equivalent).

Because alpha and neutron radiation are more damaging, the dose equivalent is larger than the absorbed dose, whereas the dose equivalent is the same as the absorbed dose for beta and gamma radiation.

Radiation Exposure Standards

The US Department of Energy (DOE) and the US Nuclear Regulatory Commission (NRC) have standards for the industries it regulates. The Occupational Safety and Health Administration (OSHA) regulates standards when the NRC does not.

In general, a layperson is limited to 0.1 rem/100 mrem per year, excluding background radiation from natural sources, like radon, and medical scans.

However, those who are considered radiation workers have more detailed standards. Because some areas can handle more radiation than others, there are different standards for different parts of the body.

Let’s start with some important terms for radiation doses:

• Total effective dose equivalent (TEDE) or deep dose equivalent (DDE)—ionizing radiation at a tissue depth of 1 cm

• Shallow-dose equivalent (SDE)—ionizing radiation at a tissue depth of 0.007cm over an area of 10 cm2

• Total organ dose equivalent (TODE)—ionizing radiation to internal and external organs at a tissue depth of 1cm

• Lens dose equivalent (LDE)—ionizing radiation at a tissue depth of 0.3 cm

Here’s a look at the NRC’s breakdown of annual allowances of radiation exposure:

• Whole body (TEDE): 5,000 mrem

• Skin and extremities (SDE): 50,000 mrem

• Any other organ (TODE): 50,000 mrem

• Lens of eye (LDE): 15,000 mrem

There are also separate limits for declared pregnant women's’ (DPW) fetuses:

• 500 mrem

Who Uses Dosimeters?

Dosimeters are worn most commonly by professionals in the industrial and medical sectors and also by radiation emergency workers, like first responders and HAZMAT professionals.

Traditional radiation workers are employed in the following industries:

• Nuclear power

• Radiology

• Oncology

• Nuclear medicine

• Construction

• Maritime

• Public safety

These professionals wear personal radiation dosimeters to collect cumulative radiation dose exposure. Some of these dosimeters alert the wearer of immediate harmful doses of radiation, while others are a part of a cumulative radiation dose monitoring and protection program.

First responders and HAZMAT professionals will assist with a radiation incident. In these cases, radiation control zones are set up to contain the radiation and protect responders from high doses of radiation. 

There are three control zones when handling a radiation incident:

• Cold zone. The cold zone is a contamination free zone that supports operations and strategic planning to eradicate radiation from an emergency incident. Radiation levels are at background doses in this zone.

• Warm zone. The warm zone is the decontamination zone and is established between the cold and hot zones.

• Hot zone. The hot zone immediately surrounds the radiation scene. Appropriate personal protective equipment (PPE) is required here.

Personal radiation detectors extended range (PRD-ERs) are worn by emergency workers in these control zones and will alert the wearer when radiation dose levels are hazardous.

Radiation Detection in Robotics

In recent years, dosimeters have been used on drones and other remote visual inspection (RVI) tools to detect radiation in hazardous and/or confined spaces. This advancement allows drones to be sent into dangerous-radiation zones to detect radiation levels and perform visual inspections instead of humans.

For example, the Flyability Elios 2, was sent into a building outside of the dangerous-radiation zone in Chernobyl to demonstrate the usefulness of the technology in radioactive areas. 

During that mission, engineers asked a Flyability drone pilot to complete an inspection of Reactor 5 to prove that there was no radioactive material present through a visual inspection.

Check out the video footage below from VICE News: