Revised on
12/30/15 11:25am

Hydroprobe Safety Manual

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Introduction

When used in accordance with instructions, radioactive materials can be used safely in the environment.

The general public is restricted from unnecessary radiation exposure during hydroprobe use, storage, and transportation by virtue of the operating procedures, locked storage, transportation limitations, and legal restrictions imposed by State and Federal regulations.

Operator protection is obtained through training, good gauge design, and following radiological safe work practices (i.e., time, distance, and shielding).

Types of Radiation
Various elements, both naturally occurring (Radium) and reactor produced (Cesium and Americium) are unstable and are slowly decaying to a more stable state. The act of decay produces emissions of energy upon disintegration of the atoms. These emissions are either electromagnetic radiation (gamma rays) or are actual particles (alpha, beta). Other emissions are produced from various radioactive materials; however, we are concerned with only the alpha and gamma radiations and resultant neutrons for purposes of the nuclear soil gauging.

These emissions are detected by appropriate detectors (Geiger Mueller tubes) for gamma rays and (Boron Tri-fluoride or Helium tubes) for neutron measurements. The resultant signals are displayed electronically as an index of soil density and moisture.

All sources are supplied in a sealed stainless steel capsule, doubly encapsulated, and further welded into a stainless steel source rod or located permanently in the gauge housing.

Sources are manufactured by a number of manufacturers' specifications that have been approved by the State of California, Department of Health Services, Radiologic Health Branch.

Sources should never be removed from their mountings and no attempt should ever be made to repair them yourself. Only the manufacturer should perform these source manipulations.

Soil Source Gauges
The most common soil gauge sources are:

  1. Cesium-137 for gamma emission
  2. Americium-241/Beryllium for neutron emission
  3. Radium-226/Beryllium for combined gamma and neutron emissions

Gamma Radiation
Gamma radiation is high energy electromagnetic energy capable of penetrating several inches of most material. It is useful for the total mass measurement of heavy materials and is used to determine the total density of soil.

Gamma radiation is emitted in several energy levels by a sealed Radium source or in a single energy level by a Cesium source. The Cesium level is 0.66 million electron volts (MeV) and requires less shielding that the multi-level output of the Radium source. In addition, the fixed spectrum emission is superior for soil density determination purposes.

Gamma sources are relatively easy to shield with dense material like lead, depleted uranium, tungsten, etc.

Neutron Radiation
Neutron radiation consists of small, non-charged particles emitted from the source at an average energy level of 5 MeV. This is known as fast neutron emission. Neutron detectors see only slow, or thermal neutrons; therefore, the fast neutrons must slow down or they will be ignored by the detectors. Neutrons slow down by colliding with other objects (especially light elements like hydrogen) much like a rifle bullet ricocheting from rock to rock.

A simple analogy is that of a golf ball colliding with a bowling ball. The golf ball would rebound with little loss of energy. However, two golf balls colliding would produce a strong loss of energy in each of them, or a transfer of energy from one to the other.

This is what happens when a fast neutron hits a hydrogen atom. The neutron is markedly slowed down. After a few collisions with hydrogen atoms, a fast neutron is reduced to the slow or thermal energy that the moisture detectors in the soil gauge can detect.

Neutron emission occurs when an alpha particle emitter (Americium, Plutonium, or Radium) is mixed with Beryllium powder in a tightly compressed pellet. The alpha particles strike the Beryllium atoms to produce fast neutrons of an average energy of 5 MeV. The suffix Be is attached to an alpha source name to identify the type of neutron source (RaBe, AmBe, PuBe).

Neutron sources are more difficult to shield. Use of hydrogenous moderators may provide shielding but reduces the measuring capacity of the gauge. It is impossible to moderate the neutrons with heavy plastic shielding and still expect the ground moisture to then moderate more neutrons for measurement. Neutron shielding is further complicated in that the thermal neutrons are captured by the moderating material with a resultant emission of gamma radiation of fairly high energy.

Protection Standards

Radiation protection standards apply to radiation workers or the general population. Standards for the general population are of importance since they serve as a basis for many of the considerations applicable to the siting of nuclear facilities and the design and implementation of environmental surveillance programs.

Occupational Dose Limit
The occupational dose limit for radiation workers is 5000 millirem/year to the whole body.

General Population Dose Limit
The dose limit for individual members of the public is 100 millirem/year.

Prenatal Radiation Dose Limit  The embryo/fetus is more sensitive to radiation than an adult due to the rapidly dividing cells. Therefore, the dose limit is 500 millirem for the entire gestation period and no more than 50 millirem in any one month for females who have declared their pregnancy

Units of Radiation Measurement

Activity (Unit: Curie)
The Curie (Ci) is defined as the activity of that quantity of radioactive material in which the number of disintegrations per second is 3.7E10 (a number nearly the same as the number of disintegrations per second from 1 gram of radium).

Since a Curie is a large amount of radioactivity sub-units of a Curie, a millicurie (mCi, 1E-3 Curie) or microcurie (µCi, 1E-6 Curie), are commonly used to express the amount of activity.

Exposure ( Unit: Roentgen)
The Roentgen (R) is defined as 2.58E-4 coulomb/kg air. This unit is special in that it is defined only for X or gamma radiation in air.

Absorbed Dose (Unit: rad)
The rad is the special unit of absorbed energy. It is defined as that amount of ionizing radiation that deposits 100 ergs/gram of material. For most applications, it can be assumed that
1 Roentgen = 1 rad.

Dose Equivalent (Unit: rem)
The rem is the unit of dose equivalent. The dose equivalent accounts for the difference in biological effectiveness of different types of radiation. It is the product of the absorbed dose (rad) times the quality factor (QF) of the radiation. The QF for x, gamma, and beta radiation is 1, for alpha radiation 20, and varies with energy from 2-11 for neutrons.

Property of Neutrons

The neutron is a very common particle, since it is a basic constituent of the nucleus along with the proton. It is almost identical to the proton in mass and size, but carries no charge. Normally, it remains locked in the nucleus along with the proton. The number of neutrons and protons is a characteristic number for any given nuclide and is known as the mass number. 

Sources of Neutons
There are no significant naturally occurring neutron emitters. Radionuclides that emit neutrons can be produced artificially, but all, except Californium-252, have half-lives that are too short to be useful. 

Aside from the spontaneous fission of Cf-252, the only way to produce to neutron sources is through nuclear reactions, that is, the bombardment of beryllium with alpha particles. Suitable alpha particle sources are Polonium-210, Radium-226, Plutonium-239, and Americium-241. 

Neutron Interactions with Matter 
The neutron carries no charge and has a mass only slightly larger than that of the proton. Because the neutron is not charged, it does not lose its energy by ionization. A neutron travels through the medium without interaction until it collides with an atomic nucleus. The maximum energy transfer that can result occurs when neutrons collide with the nuclei of hydrogen atoms (protons) that are of almost equal mass. 

Collisions between neutrons and light elements found in tissue at neutron energies of a few MeV and lower are elastic; that is, the kinetic energy of the colliding bodies is conserved during the collision. In heavier elements, some of the kinetic energy of the neutron may be transferred to the internal energy of the nucleus. In this case, referred to as an inelastic collision, the kinetic energy that can be imparted to the atom will be reduced. The excited nucleus will release the energy of excitation in the form of a gamma photon or other particle. Inelastic collisions have significance in the attenuation of neutrons but do not play an important role in the production of damage in living matter. 

Attenuation of Neutrons 
The total reduction in the number of neutrons remaining in a neutron beam following the penetration of a given thickness of matter is called attenuation. The half-value layer (HVL) is the amount of the specific absorber necessary to reduce the beam intensity by one-half of its original value. HVL's are absorber specific since they are dependent on the density of the material.

The attenuation of neutrons with energies less than a few MeV is most effectively accomplished with hydrogen.

Example: Calculate the attenuation due to hydrogen in a water shield, 1.5 m thick, for 8 MeV neutrons, if the HVL for hydrogen in water is 9.25 cm.

The number of HVLs contributed by the hydrogen in the shield is 150/9.25 = 16.2.
The attenuation is (1/2)16.2 or 1.3E-05.

Radiation Protection Standards

Introduction
Radiation protection standards apply to radiation workers or the general population. Standards for the general population are of importance since they serve as a basis for many of the considerations applicable to the siting of nuclear facilities and the design and implementation of environmental surveillance programs.

Occupational Dose Limit
The occupational dose limit for radiation workers is 5000 millirem/year to the whole body. 

General Population Dose Limit 
The dose limit for individual members of the public is 100 millirem/year.

Prenatal Radiation Dose Limit  The embryo/fetus is more sensitive to radiation than an adult due to the rapidly dividing cells. Therefore, the dose limit is 500 millirem for the entire gestation period and no more than 50 millirem in any one month for females who have declared their pregnancy

Health Risks Associated With Radiation Exposure

There are no measurable biological effects below acute exposures of 25–50 rem. The main effect of radiation exposure is cancer.

To put this into perspective, one in five adults will normally die from cancer from all possible causes. Thus, in any group of 10,000 workers, it is estimated that 2,000 workers will die from cancer without exposure to occupational radiation. If this group of 10,000 workers were each exposed to 1 rem of ionizing radiation, it is estimated that 4 will die from cancer due to this exposure.

This means a 1 rem dose may increase an individual worker’s changes of dying from cancer from 20 percent to 20.04 percent.

Methods to Minimize Exposure

Factors in Maintaining ALARA  
The ALARA concept in radiation protection is to keep your radiation exposure as low as reasonably achievable. You can limit your exposure to radiation by using the three methods of (1) time, (2) distance, and (3) shielding.

Time
Reducing the time of exposure is a very practical method of radiation protection. The shorter the time exposed to a radiation field, the lower the total exposure.

Distance 
Distance is a very effective shielding measure and often the least expensive means of radiation protection. As one moves away from the source of radiation the amount of radiation at a given distance from the source is inversely proportional to the square of the distance (inverse square law). For example, a source of radiation that reads 100 mR/hr at 1 foot will read 1 mR/hr at 10 feet.

Shielding 
Shielding is also a practical means of radiation protection. For alpha and beta radiation, very little shielding is required to absorb the emissions completely, while gamma, x-ray, and neutron radiation can be reduced to acceptable levels.

  • Alphas are stopped by a sheet of paper or the dead layer of skin.
  • Betas are stopped by one-inch wood or one-quarter inch plexiglass.
  • X-rays and gamma rays are attenuated by concrete, steel, or lead.
  • Neutrons are attenuated by hydrogen rich materials.

In general, as the density and/or thickness of a shielding material increases, the absorption of radiation emissions by the material also increases. Usually, the higher the atomic number of the shielding material, the higher its density.

Dosimetry

Dosimeters are devices that quantify the amount of radiation to which a person has been exposed.

Thermoluminescent Dosimeters (Whole Body Exposure Monitors)
Thermoluminescent dosimeters, containing lithium fluoride chip and powder cartridges, are used as personnel monitors. Exposure of these materials to ionizing radiation results in the trapping of electrons in energy levels above those occupied normally. When the dosimeter is heated, these electrons are liberated from the traps. As the electrons return to their normal levels, visible light is released. The amount of light released is measured and is proportional to the exposure of the dosimeter to radiation. These materials are x, beta, gamma, and neutron sensitive and exposure is reported as being either deep and/or shallow energy penetration.

The dosimetry reporting company, an independent contractor, will report exposures for each individual in deep and/or shallow dose for the whole body.

Precautions on Use of Dosimetry
The lithium fluoride chips and powder are highly sensitive to heat and moisture. When not in use, store your dosimetry in an area free of ionizing radiation. If you lose, contaminate, get your badge wet or leave it in the sun for an extended period of time, please notify EH&S.

Distribution and Use of Dosimetry 
Dosimetry is issued by EH&S based on procedures used and the type and amount of radioactivity. Please call EH&S at 530-752-1493 for your dosimetry needs.

Badges may be exchanged weekly, monthly, or quarterly, depending upon the type and amount of material used and experimental design.

EH&S documents the dosimetry readings for the State of California, Department of Public Health, Radiologic Health Branch.

Dosimetry Records
All dosimetry records are on file at EH&S. Upon your request, EH&S will supply you with your dosimetry history. If at any time your exposure exceeds the campus guidelines or is unusually high, an EH&S staff member will notify you of the incident.

Soil Moisture and Density Gauges

Soil Density Gauge (Surface Gauges) 
Normal operation of the surface gauge requires the operator to stand within two feet of the gauge for a period of approximately 10 seconds per test. There is little reason to be closer than that distance nor to work longer than this period to obtain the results. It may take longer than 10 seconds to prepare the site; however, the nuclear gauge should be away from the site at this time.

  • A busy day can result in 30 tests being taken.
  • A busy work week would include 5 days of this extensive testing.
  • If we multiply this all together:
    • 30 tests/day x 10 second/test = 300 seconds or 5 minutes/day of exposure at 2 feet. 
    • 5 days x 5 minutes = 25 minutes or approximately 0.5 hour.
    • The average exposure level at two feet from the gauge is 0.5 mrem/hour.
    • 0.5 hour x 0.5 mrem/hour = 0.25 mrem accumulated in a busy work week.

Neutron Soil Moisture Gauges (Depth Probes) 
The radiation from depth probes can be higher because of the work requirement of the depth probe. Unlike the surface gauges, the depth probes are carried around by the operator to a greater extent. The sources are the same size, and the shielding is equal, or even better, but the immediate vicinity work requirement is higher.

Depth probes are designed to be carried with a strap or handle. The source area is carried near the lower extremities or ankles.

Density depth probes are used primarily for research and the duty cycle is not high. The use of such a gauge would be infrequent during a total year's time, and radiation accumulation will be low compared to other gauges uses. The major depth probe used will be the hydroprobe for irrigation management. This unit will be used routinely, almost daily, throughout the growing season that may be all year long in some areas.

The gamma output from the hydroprobe is almost negligible. The Americium-241/Be source has a low energy gamma output that is not used for moisture measurement and that is shielded out internally with a small lead sheath. Gamma radiation on the surface of the hydroprobe is approximately 1 mrem/hour which reduces to less than 0.05 mrem/hour at two feet from the gauge.

Thermal neutron output is approximately 0.2 mrem/hour on the surface.

The fast neutron output is approximately 4 mrem/hr on the surface.

The total gamma and neutron radiation at mid-trunk on an individual, with the hydroprobe carried at the side by its handle, is approximately, 0.3 mrem/hour.

The anticipated duty cycle in close proximity to the gauge is approximately 2 hours/day during a full work day. The operator will be driving part of the time, performing some paperwork functions part of the time, and trudging through the fields part of the time.

Multiplying the work day out:

  • 2 hrs/day x 5 days x 0.3 mrem/hr = 3.0 mrem accumulation in a week

It is important that the gauge be carried in its appropriate carrying location in the back of the vehicle at maximum practical distance from the operator, and that all use of the gauge be performed at quickly as possible.

The gauge is at its safest when the probe is in the ground in the process of taking readings. No measurable radiation is detected at the gauge electronics in this operation.

Survey Instruments 
A conventional survey meter will read only the gamma or beta output of the device. Only special neutron meters will read the neutron output.

Possession of a survey meter is not required for gauge use. However, should you have a survey meter, EH&S will calibrate it for you and be sure you are familiar with proper use techniques.

Leak Tests 
All radioactive sources must be tested for contamination periodically. The sources are doubly encapsulated in stainless steel and the likelihood of a leaking source is very remote. However, they still must be leak tested every six months, in accordance with regulations. This is accomplished by a swipe test performed once by EH&S and once by the authorized employee (radiation user). Perform the swipe test as follows, using a cloth swipe:

Surface Gauge

  1. Stand gauge on end; leave shutter closed.
  2. Swipe the cleanout ring. Do not swipe the source rod.
  3. Return the swipe to the EH&S in the envelope provided. Please include: the date, RUA number, source isotope, serial number, and the name of the person performing the test.
  4. A certificate will be returned for your records.

Depth Gauges

  1. Lay the probe on its side. If the source is leaking, contamination will be inside the shield tube.
  2. Swipe the inside of the shield tube.
  3. Return the swipe to the EH&S in the envelope provided. Please include: the date, RUA number, source isotope, serial number, and the name of the person performing the leak test.
  4. A certificate will be returned for your records.

Storage and Posting 
The gauges should be stored in their shipping cases in a locked area with key access only by the licensed operators. EH&S recommends that permanent storage be ten feet from the nearest point of full-time work requirements.

Post a permanent "CAUTION - RADIOACTIVE MATERIAL" sign on the storage area door. These are available from EH&S.

Use Attachment 3, Permanent Storage Location for Nuclear Gauges form, to establish a storage location. Send the copy of the completed form to EH&S.

Attachments