A proportional counter, also known as the proportional detector, is an electrical device that detects various types of ionizing radiation. The voltage of detector is adjusted so that the conditions correspond to the proportional region. In this region, the voltage is high enough to provide the primary electrons with sufficient acceleration and energy so that they can ionize additional atoms of the medium. These secondary ions (gas amplification) formed are also accelerated causing an effect known as Townsend avalanches, which creates a single large electrical pulse. Gaseous proportional counters usually operate in high electric fields of the order of 10 kV/cm and achieve typical amplification factors of about 105. Since the amplification factor is strongly dependent on the applied voltage, the charge collected (output signal) is also dependent on the applied voltage and proportional counters require constant voltage.
This is a subtle, but important difference between ionization chambers and proportional counters. An ionization chamber will produce a current that is proportional to the number of electrons collected each second. This current is averaged and is used to drive a display reading in Bq, or μSv/h. Proportional counters do not work in this way. Instead, they amplify each of the individual bursts of ionisation so that each ionising event is detected separately. They therefore measure the number of ionising events (which is why they are called counters). In nuclear instrumentation, boron-triflouride proportional counters (BF3) are widely used as source range detectors.
BF3 Proportional Counter
Since the neutrons are electrically neutral particles, they are mainly subject to strong nuclear forces but not to electric forces. Therefore neutrons are not directly ionizing and they have usually to be converted into charged particles before they can be detected. Generally every type of neutron detector must be equipped with converter (to convert neutron radiation to common detectable radiation) and one of the conventional radiation detectors (scintillation detector, gaseous detector, semiconductor detector, etc.).
Ionization chambers are often used as the charged particle detection device. For example, if the inner surface of the ionization chamber is coated with a thin coat of boron, the (n,alpha) reaction can take place. Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission.
Moreover, isotope boron-10 has high (n,alpha) reaction cross-section along the entire neutron energy spectrum. The alpha particle causes ionization within the chamber, and ejected electrons cause further secondary ionizations.
Another method for detecting neutrons using a proportional counter is to use the gas boron trifluoride (BF3) instead of air in the chamber. The incoming neutrons produce alpha particles when they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in nuclear reactor. It must be noted, BF3 counters are usually operated in the proportional region.
Source Range Detectors
The source range detectors monitor neutron flux (reactor power) at the lowest shutdown levels and provide indication, alarms, and reactor trips. Source range instrumentation usually consists of two or four source range channels, each with its own separate detector, cable run, and electronic circuitry. The detectors utilized are usually high-sensitivity boron-triflouride (BF3) proportional counters. In general, proportional counters are capable of particle identification and energy measurement (spectroscopy). The pulse height reflects the energy deposited by the incident radiation in the detector gas. As such, it is possible to distinguish the larger pulses produced by alpha particles (produced by (n,alpha) reactions) from the smaller pulses produced by beta particles or gamma rays.
These BF3 detectors produce a pulse rate output proportional to the thermal neutron flux seen at the detector. These channels are typically used over a counting range of 0.1 to 106 counts per second, but vary based on reactor design. These excore detectors are usually located in instrument wells in the primary shield (concrete shield) adjacent to the reactor vessel.
The source range instrumentation monitors and indicates the neutron flux level of the reactor core and the rate by which the neutron flux changes during a reactor shutdown and the initial phase of start-up. They are very important for monitoring of subcriticality during fuel reload, when subcritical multiplication takes place. The neutron flux is indicated in counts per second (cps). The rate of change of the neutron population is indicated as startup rate (SUR), which is defined as the number of factors of ten that power changes in one minute. Therefore the units of SUR are powers of ten per minute, or decades per minute (dpm).
There are two principal problems in the source range instrumentation:
- Discrimination. During the reactor shutdown and the initial phase of start-up, it is required to distinguish the relatively small number of pulses produced by neutrons from the large number of pulses produced by gamma radiation. Thus gamma discrimination is of particular interest during shutdown after the reactor core reaches significant level of fuel burnup. This condition produces a high gamma field and a low neutron flux around the detector. Proportional counters allow for discrimination but they must be calibrated. The discriminator excludes passage of pulses that are less than a predetermined level. The function of the discriminator is to exclude noise and gamma pulses that are lower in magnitude than neutron pulses (alpha pulses respectively). Many power plants have found it necessary to place source range proportional counters in lead shielding to reduce gamma radiation at the detectors. This increases the low end sensitivity of the detector, and it may extend the detector life.
- Dead Time. This instrument can indicate up to a maximum neutron count rate of 106 cps. Higher count rates are influenced by phenomenon known as dead-time. The dead-time is the period during which the detector is busy and cannot accept and process pulses. This phenomenon can have serious consequences, since dead-time distorts outputs at high activities or high dose rates.
There are some power plants that have made provisions for moving the source range detectors from their operating positions to a position of reduced neutron flux level, once the flux level increases above the source range.
Special Reference: Standard Review Plan for the Review of Safety Analysis Reports for Nuclear Power Plants: LWR Edition. NUREG-0800, US NRC.