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Office of Homeland Security & Emergency Coordination
Radiation Safety Division (RSD)
United States Department of Agriculture

Nuclear Gauges








A nuclear gauge (also called a nuclear density gauge, a soil moisture gauge, or a moisture density gauge) is a tool used for civil engineering projects and construction that measures soil density or soil water content . Nuclear gauges are also used for industrial, mining, and scientific purposes. In USDA, nuclear gauges are used for building roads and dams, and for collecting scientific data on soil water content in agricultural and forestry settings.

For measuring soil density, a radioactive source emits gamma radiation into soil, and a sensor measures the radiation that is reflected by the soil. By calculating the percentage of radiation that returns to the sensor, the gauge can be calibrated to measure the soil density. The source of the gamma radiation is typically a cesium (Cs-137) sealed source of about 8 to 10 milli-Curies.

For measuring water density, the radioactive source emits neutrons and then measures the returning scattered neutrons. Hydrogen, a major constituent of water, characteristically slows down neutrons; the sensor’s measurements can therefore be used to calculate the water content of the sample. The source of the neutron radiation is typically americium (Am-241) an x-ray emitter, in combination with the element beryllium, causing the emission of neutrons; this arrangement is called an americium-beryllium neutron source. It is typically a source of about 40 to 50 milli-Curies.

Devices can be built to measure either soil density or soil moisture content, or both.



Authorized Users

There are two categories of authorized users: Permit Holders and associate users. A nuclear gauge Permit Holder is a person whose training and experience have been reviewed and approved by the Radiation Safety Division, who is named on the Permit, and who uses or directly supervises the use of nuclear gauges. More than one Permit Holder may be assigned to a single nuclear gauge. Any USDA employee seeking to acquire a nuclear gauge must have a Permit issued by the Radiation Safety Division.

An associate user is a person listed on the Permit who is authorized to use or transport a nuclear gauge under the Permit Holder’s supervision. The P ermit Holder must be the supervisor of the associate users listed on the Permit. The Permit Holder may delegate specific tasks to associate users (such as, doing leak tests, keeping records), but the Permit Holder is responsible for the safe use of the nuclear gauge.

A gauge Permit lists the gauge approvals, the actual gauges as inventory items, the permanent storage location, the approved uses, and the associate users. Once approved, a Permit Holder has access to the Permit tracking data base, known as RSMS, (Radiation Safety Management System). Using that system, the Permit Holder may view various aspects of the Permit, and can get the most recent leak test results and the DOT transportation papers for the gauge. The Permit Holder’s primary responsibility is to ensure that nuclear gauges, as listed on the Permit inventory, are used safely and according to regulatory requirements, The Permit Holder must also see that procedures and engineering controls are used to keep radiation doses as low as reasonably achievable (ALARA).



All individuals listed on the nuclear gauge Permit must attend a manufacturer’s one-day nuclear gauge training, or a manufacturer’s web-based training, or the equivalent. DOT HazMat Refresher Training for Nuclear Gauges is also required every three years.


Acquiring a Nuclear Gauge

Before a purchasing agent can place an order to buy a nuclear gauge on behalf of a Permit Holder, the Radiation Safety Division must review and approve the information on the purchase order. A nuclear gauge may also be transferred from another USDA employee, who has a nuclear gauge Permit. The Radiation Safety Division must review and approve such gauge transfers between Permit Holders, in order to review and update Permit and inventory data for both people. If a Permit Holder at a location leaves permanently, and the gauge is to be maintained at that location, it must be transferred to another Permit Holder at that location. A nuclear gauge may also be transferred from a person or organization outside of USDA that is authorized to have the gauge under a state or NRC license. Before a nuclear gauge can be transferred from outside of USDA, the Radiation Safety Division must review the authorizing license and must approve the gauge transfer.


Receipt of a Nuclear Gauge

Upon receipt of a new nuclear gauge, the Permit Holder must submit the following to the Radiation Safety Division:

  • “Sealed Source Inventory Record” (RSD-28), or equivalent information;
  • manufacturer's initial leak test results (if available);
  • manufacturer's information sheet and specifications for the device and source.

Upon receipt of a nuclear gauge that is not new, but transferred from another user, similar, equivalent information is required.


Transfer or Disposal  

A nuclear gauge cannot be given away, placed on excess property lists, thrown away, or abandoned. Any of these actions is a serious violation of NRC Federal Regulations. If a Permit Holder leaves USDA without arranging for proper transfer of a gauge, local management or the LRPO (Location Radiation Safety Officer) must maintain control of the nuclear gauge until the name of a new Permit Holder candidate can be submitted to the Radiation Safety Division.

The proper disposal of nuclear gauges is required. The original manufacturer will usually accept the return of the gauge. (Other nuclear gauge manufacturers may also accept the transfer of a nuclear gauge). A nuclear gauge may also be transferred to another USDA employee with a nuclear gauge Permit, or to any non-USDA person or organization that is licensed by the state or the NRC to have the gauge.

All gauge transfer methods must be done with the assistance and approval of the Radiation Safety Division. The Radiation Safety Division must verify the license or permit of the recipient and assure that the leak test requirement is current. A nuclear gauge can be transferred only upon the written approval of the Radiation Safety Division.

Upon transfer of the gauge, a letter of receipt from the gauge recipient must be obtained and sent to the Radiation Safety Division. The gauge transfer is not considered complete until the Radiation Safety Division has received this letter. At that time, the Radiation Safety Division will update the permit and inventory data for the Permit Holder, and release the Permit Holder from responsibility for the nuclear gauge.

Theft or Loss

As soon as it becomes known that a nuclear gauge has been stolen, lost or, misplaced, the Permit Holder, or the individual in possession of the nuclear gauge, must immediately call the Radiation Safety Division for assistance. Finding the gauge must be an urgent priority, which may involve law enforcement and the resources of the NRC.




Safe Use of a Gauge

Use the gauge according to the manufacturer’s instructions and recommendations.

Perform routine cleaning and maintenance according to the manufacturer’s instructions and recommendations.

Before removing the gauge from its place of storage, ensure that each gauge source is in the fully shielded position and that the source rod is locked in the shielded position.

Make sure the gauge is locked in the transport case.

When transporting the gauge in a vehicle, block and brace it to prevent shifting or movement, and lock the portable gauge to the vehicle.

Do not touch the unshielded source rod with any part of your body, and do not place hands, feet, or other body parts in the radiation field from an unshielded source.

Unless absolutely necessary, do not look under the gauge when the source rod is being lowered into the ground. If you must look under the gauge to align the source rod with the hole, follow the manufacturer’s procedures to minimize radiation exposure.

For measurements with the unshielded source extended more than 3 feet beneath the surface, use piping or tubing to line the hole together with a trial dummy-probe to test and verify that the hole is accessible and free of obstructions

After completing each measurement in which the source is unshielded, immediately return the source to the shielded position.

Always maintain the gauge under constant view and immediate control when it is not in storage. At job sites, do not walk away from the gauge when it is left on the ground even if it may seem safe to do so. Protect yourself and the gauge from the danger of moving heavy equipment. Always keep unauthorized persons away from the gauge.

When the gauge is not in use at a temporary job site, it must be securely locked in the operator's vehicle (or other appropriate locked storage location).

Return the portable gauge to its proper locked storage location at the end of the work shift.


Non-Routine Maintenance

All servicing or cleaning of a nuclear gauge, involving exposure of the radioactive sources, must be performed by the manufacturer or by an authorized representative of the manufacturer.

In Case of an Accident

If the source fails to return to the shielded position, or if the gauge is damaged in an accident,

  • Secure the area within 15 feet of the gauge (by roping off the area, for example).
  • Maintain the area of the accident site under constant surveillance.
  • Do not allow anyone to enter the area until emergency assistance arrives and radiation levels are known.
  • However, do not delay necessary first-aid to the injured, and move them from the area only when medically safe to do so.
  • A radiation survey must be conducted as soon as possible.
  • Do not use a vehicle or equipment involved in a gauge accident until it is evaluated for contamination.
  • Contact the Radiation Safety Division, the Location Radiation Protection Officer, Location Management, and appropriate Agency personnel. RSD will contact radiation safety personnel, as necessary, to assist in the accident clean-up.

Sign-out Sheet and Logbook

The Permit Holder is required to keep a gauge sign-out sheet which should remain at the permanent storage location. A single sign-out sheet may be used for several gauges stored at a location. The Permit Holder or an associate user listed on the Permit must sign out the gauge when it is taken out of the storage location, with the following information:

  • date of sign-out,
  • the name(s) of the authorized users who will be responsible for the gauge, and
  • the temporary job site where the gauge will be used.

A logbook must be kept with the nuclear gauge documenting every time the gauge is used to include the following information:

  • the date of use,
  • the name of the user, and
  • where the gauge is used.

Log the portable gauge into the sign-out sheet when it is returned to storage.

Permanent Storage Location

The permanent storage location for a nuclear gauge must be within 50 miles of the Permit Holder’s permanent office or work location and must be listed on the nuclear gauge Permit. The following requirements apply to permanent storage locations:

  • Flammable materials must not be stored in the same area as the gauge.
  • A gauge must be stored away from permanent work stations.


Posting of Signs, Labels, and Authorizing Documents

Post a "Caution Radioactive Materials" sign in rooms or areas where nuclear gauges are stored. Post an NRC Form-3 "Notice to Employees,” in areas where employees will see it, for example, on a safety bulletin board.

In addition, the following information must be posted so as to be readily observable or otherwise made readily available, or be readily accessible on the computer: 

  • 10 CFR 19 Notices, Instructions and Reports to Workers: Inspection and Investigations
  • 10 CFR 20 Standards for Protection Against Radiation
  • the material license,
  • any notice of violation involving radiological working conditions, proposed imposition of civil penalty, or order issued by the NRC, and any response, such notice being posted according to RSD instructions..


Temporary Job Sites

A Permit issued by the Radiation Safety Division allows use of a nuclear gauge at temporary job sites without any additional or special approval from the Radiation Safety Division; and, a nuclear gauge may be kept at a temporary field site for up to  1 80 days a year, without amending the permit.

The following requirements apply to temporary storage locations:

  • Flammable materials must not be stored in the same area as the gauge.
  • The storage area shall be posted with a Caution - Radioactive Materials sign,



Nuclear gauge manufacturers provide a carrying case designed, marked, and labeled to comply with Department of Transportation (DOT) regulations. The case is a US DOT Type-A container. The following markings must appear on the carrying case:

Radioactive Material, Type A package, Special Form, 7, UN3332.

The letters, RQ (for reportable quantity) also must appear on the carrying case if the gauge contains 10 milli-Curies or more of Americium (Am-241).

Two diamond-shaped radioactive hazardous material labels, either white or yellow; must also appear on opposite sides the carrying case.

The transport index (Ti) is the radiation dose-rate in milli-Rem per hour, measured at 1 meter from the gauge case, rounded up to the nearest tenth. It alerts shippers and transporters to the radiation hazard. The Ti appears on the HazMat label and on the Shipper’s Declaration. White-I labels do not have a Ti. Yellow-II labels have a Ti ranging from 0.1 to 0.9 . The Ti is measured and assigned by the manufacturer. This original Ti value should be maintained on all replacement paperwork and replacement labels

During transport, the gauge shall be located as far back in the vehicle as possible, away from passengers, and blocked and braced to prevent shifting. The case shall be locked.

If the driver and all passengers leave the vehicle unattended at any time during transport, the case shall be securely locked inside of the vehicle to prevent theft.

For additional guidance on shipping a nuclear gauge, the Permit Holder should contact RSD.

Documents Required During Transport

The following documents shall be carried within reach of the driver, either in a pocket in the driver's side door, or on the seat next to the driver:

  • USDA Radiation Use Permit
  • USDA Radioactive materials license;
  • Transportation Declaration;
  • Emergency Response Guideline.

In case of a transportation accident, emergency responders may need these documents.

A gauge Permit Holder has access to the Permit tracking data base, known as RSMS, (Radiation Safety Management System). Using that system, the Permit Holder may view and print a copy of the DOT gauge shipping papers (also called the bill of lading, or the transportation declaration).


Leak Tests

A sealed source is a contained source that has been constructed and tested to pass specific accident conditions without the release of radioactive material. NRC (or an Agreement State) performs a safety evaluation of each sealed source before authorizing a manufacturer (or distributor) to distribute the sealed source. The safety evaluation is documented in a Sealed Source and Device (SSD) Registration Certificate. Sealed sources in nuclear gauges are encapsulated in stainless steel.

A leak test of a nuclear gauge must be performed every six months to assure that the sealed sources are not leaking. The leak test must be done in accordance with the manufacturer's instructions. Using a small standard laboratory wipe, a leak test sample should be collected at the most accessible area where contamination would accumulate if the sealed source were leaking. The Permit Holder (or Associate User) can create a leak test record in the RSMS database, via the “Inventory Module.” The user can then print-out a leak test sheet from that record. The Permit Holder is expected to have the leak test done and returned to RSD for evaluation and reporting results. Manufacturers, consultants, and other organizations may analyze the leak test. However, the Permit Holder must still return the leak test sheet along with the leak test results to RSD. A sealed source fails the leak test if the contamination on the filter paper used for the test exceeds 0.005 micro-Curies. Using the RSMS database , the Permit Holder may view the leak test results.

A gauge Permit Holder has access to the Permit tracking data base, known as RSMS, (Radiation Safety Management System). Using that system, the Permit Holder may view the most recent leak test results.


Dosimetry is required for nuclear gauge Permit Holders and associate users listed on the Permit. It consists of a radiation monitoring badge worn on the clothing. Always wear your assigned dosimetry badge when using the gauge. Never wear another person’s dosimetry badge. Never store your dosimetry badge near the gauge. The dosimetry badge shall be stored away from areas of extreme heat, cold or humidity when not in use.



The Permit Holder must participate in a review of the radiation protection program annually to ensure compliance with NRC regulations, and the USDA radiation safety program. RSD will request an audit from Permit Holders or others involved in the location radiation safety program. The audits promote safety awareness and interaction between Permit Holders, the LRPO, and the Radiation Safety Division, and helps identify problem areas.



The Radiation Safety Division (RSD) inspects each location where nuclear gauges are used. These inspections seek to identify and correct problems with radiation safety compliance. Inspections are usually done by appointment, with RSD Health Physicists acting as the inspectors. Inspectors compare the scope of nuclear gauge activities against the Permits at the location. Inspectors look for additional physical and administrative compliance with the Nuclear Regulatory Commission license requirements. A detailed report is transmitted to the location management, and other appropriate officials. If the report lists recommendations or cites violations, RSD works with the location management to resolve these report findings.

The Nuclear Regulatory Commission (NRC) also routinely conducts inspections at USDA locations. NRC inspections are usually unannounced, so location staff may be required to modify and improvise their schedule during an NRC inspection. The Radiation Safety Division should be immediately notified upon the arrival of an NRC inspector. The results of the inspection are usually reported to RSD. The NRC has a complex hierarchy of citations and violations. Serious violations may be circulated to higher management within USDA, for eventual resolution.

Records Retention

All documents relating to the purchase, transfer, or disposal of a nuclear gauge must be retained by the Permit Holder for as long as the permit is active.

The Radiation Safety Division maintains records of the purchase, transfer, and disposal of a gauge as long as the Department's radioactive materials license is active.

Records of leak tests must be retained for 5 years. Both the Permit Holder and the Radiation Safety Division must retain these records.





General Security (CFR Part 30.34(i))

Briefly, gauge security requires two locks. To be in compliance, RSD recommends that a gauge be stored in a locked cabinet inside of a closet or room that is always locked when no one is present.

A more complete analysis follows:

A nuclear gauge must be stored in a location that includes a minimum of two independent physical controls that form barriers to secure portable gauges from unauthorized removal whenever the portable gauge is not under the control and constant view of the Permit Holder.

The security requirement applies to:

  • storage on vehicles during transport and when parked;
  • storage at temporary facilities (e.g., residence, hotel, job site trailer); and
  • storage at the permanent storage location listed on the permit.

The physical controls must secure the portable gauge from unauthorized removal, and both physical controls must be defeated in order for the portable gauge to be removed.

The physical controls must deter theft by requiring a more determined effort to remove the portable gauge than if a single control were used. The security procedures must ensure that the two physical barriers clearly have greater theft deterrence than a single barrier and the two physical barriers would make the gauge harder to steal.

Using two chains is not the preferred method. To provide adequate security, Permit Holders are encouraged to use other combinations of barriers.

Chain and lock combinations should be heavy duty enough to provide both, a deterrence, and a reasonable delay mechanism. When two chains or cables are used, the second chain or cable should be substantially more robust and more difficult to cut than the first chain or cable.

If possible, the Permit Holder should consider storing the portable gauges inside a locked facility or other permanent structure overnight, instead of storage in a vehicle.

Transportation Security

As long as the Permit Holder maintains constant control and surveillance while transporting the portable gauges, the Permit Holder need only comply with the DOT requirements for transportation (e.g., labeling, shipping papers, blocking and bracing).

However, if the Permit Holder leaves the vehicle and portable gauge unattended (e.g., while visiting a gas station, restaurant, or store), the Permit Holder must ensure that the portable gauge is secured by two independent controls.

The Permit Holder should not modify the transportation case. This includes drilling holes to mount the case to the vehicle or to mount brackets or other devices used for securing the case to the vehicle.

Permit Holders commonly use a chain and a padlock to secure a portable gauge (in its transportation case) to the open bed of a pickup truck, while using the vehicle for storage. Because the transportation case is portable, a theft could occur if the chain is cut and the transportation case, with the portable gauge, is taken. If a Permit Holder simply loops the chain through the handles of the transportation case, a thief could open the case and take the portable gauge. Similarly, because the transportation case is also portable, it must be protected by two independent physical controls if the portable gauge is inside. A lock on the transportation case, or a lock on the portable gauge source rod handle, is not sufficient because both the case and the gauge are portable.

A vehicle may be used for storage, but it is recommended that this practice only be used for short periods of time or when a portable gauge is in transit. A portable gauge should only be kept in a vehicle overnight if it is not practical to provide temporary storage in a permanent structure.


Security at Temporary Sites

When a job requires storage of a portable gauge at a temporary job site or anywhere else away from the permanent storage location listed on the Permit, the Permit Holder should use a permanent structure for storage, if practical. When storing a portable gauge in a hotel or a private home, the Permit Holder should limit access by storing the gauge in a separate room away from residents and other members of the public.


Examples of Physical Controls

Physical controls used may include, but are not limited to:

  • a metal chain with a lock,
  • a steel cable with a lock,
  • a secured enclosure,
  • a locked tool box,
  • a locked camper,
  • a locked trailer,
  • a locked trunk of a car,
  • inside a locked vehicle,
  • a locked shelter,
  • a secured fenced-in area,
  • a locked garage,
  • a locked non-portable cabinet,
  • a locked room, or
  • a secured building.

Examples of Security at a Permanent Storage Location

Examples of two independent physical controls used to secure a portable gauge when stored at a permanent storage location listed on the Permit:

  1. The portable gauge is stored inside a locked storage shed within a secured outdoor area, such as a fenced parking area with a locked gate, where the gate is kept shut and locked all the time (except for entry or exit);
  2. The portable gauge is stored inside a locked, non-portable cabinet inside a room with a locked door;
  3. The portable gauge is stored in a separate locked area inside a locked mini-warehouse or storage facility.


Examples of Security in a Transportation Vehicle

Examples of two independent physical controls to secure portable gauges in in a transportation vehicle

  1. The gauge is stored in a box that is attached to a vehicle, and the box is secured with:
    • two independent locks;
    • two separate chains or steel cables attached independently to the vehicle so that the box cannot be opened without the removal of both chains or cables; or
    • one locked box and one chain or steel cable is attached to the vehicle so that the box cannot be opened without the removal of the chain or cable; or
  2. The gauge is stored in a locked trunk, van, or other similar enclosure and is physically secured to the vehicle by a chain or steel cable in such a manner that one would not be able to open the case or remove the portable gauge without removal of the chain or cable.


Examples of Security at Temporary Locations

Examples of two independent physical controls to secure portable gauges at temporary locations are :

  1. At a temporary job site, the portable gauge is stored inside a locked building or in a locked non-portable structure (e.g., construction trailer, etc.), and is physically secured by a chain or steel cable to a non-portable structure so that an individual would not be able to open the transportation case or remove the portable gauge without removing the chain or cable. A lock on the transportation case or a lock on the portable gauge source rod handle would not be sufficient because the case and the portable gauge are portable;
  2. The portable gauge is stored in a locked garage, and is within a locked vehicle or is physically secured by a chain or steel cable to the vehicle so that an individual could not open the transportation case or remove the portable gauge without removing the chain or cable; or
  3. The portable gauge is stored in a locked garage, and is within a locked enclosure or is physically secured by a chain or steel cable to a permanent or non-portable structure so that an individual could not open the transportation case or remove the portable gauge without removing the chain or cable.