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