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Radiation Protection and Environmental Monitoring

Helen Redmon and Joe Braun

Purpose

 The purpose of this lecture is to provide nuclear technology fundamentals to serve as a basis for further discussion and development of nuclear power basics

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Types of Radiation

 Alpha particles – Helium particle without the electrons contains 2 neutrons and 2 protons – High energy particle causes excitation and ionization of surrounding material- high LET (linear energy transfer). – Due to its relatively high charge (+2) travels only 3- 4 cm in air but due to its large size it wont penetrate a piece of paper – Dose to soft tissue is the concern-inhalation greatest hazard – Most common exposure for the general public is Radon gas – Not much of a concern in nuclear power generation unless there is leaking fuel.

Types of Radiation Cont.

Beta particles – – – – – –

Same mass as an election Charge of -1 or +1 for positron Variable energy Travels ~4 meters in air and .5cm in soft tissue per MeV External skin hazard is primary concern Most common exposure to population is I131 which is used in medical treatments – Sr90, Co60 and Cs 137 and H3 are a concern in Radiation Protection in nuclear power generation.

Types of radiation cont.

Gamma photons – Have no mass- a high energy photon from the nuclear of the atom (similar to x-rays) – Has 3 different matter interactions and is dependant on MeV, Photoelectric Effect, Compton Scattering, and Pair Production in ascending order. • Photoelectric effect for low energy photons imparts all of its energy to the target electron. The energy is used to free the electron and give it kinetic energy • Compton scattering occurs when the photon has sufficient energy where only some it used to free the target electron. A new photon then continues on in a different vector with decreased energy

Types of radiation cont.

 Gamma photon cont • Pair production is a rare process when a photon with a energy of greater than1.02MeV passes a target nucleus and a beta particle and a positron (anti-beta particle) are produced. When the two meet later then are annihilated and the matter is converted back into energy.

– Highly penetrating in air and low density tissue and penetration is dependant on MeV, density and thickness of absorber. – Best described by HVL (½ value layer) of medium since a certain percentage will penetrate the entire absorber • Ex. HVL for lead is 2.5cm

Types of radiation cont.

Neutron – – – –

Have same mass a proton or hydrogen atom Have no electrical charge Extremely highly penetrating in dense material Slowing of neutrons done with low Z material (material with high hydrogen content) • Ex. Water, wax, concrete and plastic

– 2 main pathways for depositing dose to tissues from fast neutrons (1MeV-20MeV) are elastic and inelastic scattering

Types of radiation cont.

Neutron cont – Elastic scattering billiard ball effect-50% of energy lost per interaction – Inelastic scattering where some energy is deposited in target nucleus and then decays by photon emission – 80% of fast neutron dose to humans is from elastic scattering interactions (humans are mostly made up of water)

Radiation monitoring-gas filled probes

The Six-Region Curve for Gas-Filled Detectors – All gas-filled detectors (including G-M pancake detectors) detect radiation with different characteristics based on the relative applied voltage between the anode and the cathode. – Ion chambers, which respond evenly to different energies of radiation, operate in the second region of the curve. Relatively low efficiency is characteristic of this region.

Radiation monitoring-gas filled probes

– As the detector operating voltage is increased, the detector enters the proportional region (region 3), where • pulse height is proportional to the energy of the photon or particle that initiated the pulse. • can discriminate between different energies of radiation by analyzing the pulse height. – As the detector operating voltage continues to increase it will enter region 5, the Geiger-Mueller region. • all pulse heights are equal and efficiency is relatively high, although counting efficiency will vary based on energy.

Radiation monitoring-gas filled probes

IONIZATION CHAMBER REGION – At a certain voltage, the force of attraction between the ions and the electrodes is sufficient to cause all of the electrons produced by incident radiation to collect on the anode. – Subsequent moderate increases in the voltage does not cause further increases in the electron flow. The number of electrons collected at the anode is equal to the the amount of ionization occurring in the chamber. – In this region the different types of radiation can be distinguished from each other because of the different pulse heights produced in the external circuit. – Meter used at Argonne-R0-20 Dose rate

Radiation monitoring

 PROPORTIONAL REGION –

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As the voltage continues to increase past the saturation level, the primary ions are accelerated toward the anode with enough force to cause additional ionization to occur. This multiplication or avalanche of electrons moving toward the anode is called gas amplification. Typical detectors operating in this region have multiplication factors of 100,000 though factors up to 10,000,000 are fairly common. These detectors, like those which operate in the ionization chamber region, can distinguish between alpha, beta, and gamma radiation. Meters used at Argonne- DABRAS and TENELEC

Radiation monitoring-gas filled probes

 GEIGER REGION – At some point as the voltage increases, the gas amplification is so extensive that an avalanche of electrons spreads along the entire length of the instrument’s anode and all pulses are the same size regardless of the type of radiation. – A detector operated in the Geiger region can not distinguish between the different types of radiation. – Pulses in this region are much larger than those in any of the previous regions and in fact, the production of only one primary ion pair results in an easily measurable pulse (about 1 volt). – Meters used at Argonne Eberline ASP-1 with high sensitivity, energy compensated G-M tube (nightstick) and Ludlum 3 ratemeter with 44-9 GM probe

Radiation monitoring-scintillators

Scintillation detectors

– NaI crystal

– ZnS or (ZnS/BC400) probes

Radiation monitoring- scintillator detectors

 Scintillation detectors – The basic principle behind this instrument is the use of a special material which glows or “scintillates” when radiation interacts with it. The most common type of material is a type of salt called sodium-iodide. The light produced from the scintillation process is reflected through a clear window where it interacts with device called a photomultiplier tube. – A scintillator is a material that converts energy lost by ionizing radiation into pulses of light. In most scintillation counting applications, the ionizing radiation is in the form of X-rays, g-rays and a- or b-particles ranging in energy from a few thousand electron volts to several million electron volts (keVs to MeVs). – Pulses of light emitted by the scintillating material can be detected by a sensitive light detector, usually a photomultiplier tube (PMT).

Radiation monitoring- scintillator detectors – The photocathode of the PMT, which is situated on the backside of the entrance window, converts the light (photons) into so-called photoelectrons. – The photoelectrons are then accelerated by an electric field towards the dynodes of the PMT where the multiplication process takes place. – The result is that each light pulse (scintillation) produces a charge pulse on the anode of the PMT that can subsequently be detected by other electronic equipment, analyzed or counted with a scaler or a rate meter. The combination of a scintillator and a light detector is called a scintillation detector.

Radiation monitoring- scintillator detectors

 Photomultiplier tube – The photocathode has the unique characteristic of producing electrons when light strikes its surface. These electrons are then pulled towards a series of plates, called dynodes, through the application of a positive high voltage. – When electrons from the photocathode hit the first dynode, several electrons are produced for each initial electron hitting its surface. This “bunch” of electrons is then pulled towards the next dynode, where more electron “multiplication” occurs. The sequence continues until the last dynode is reached, where the electron pulse is now millions of times larger then it was at the beginning of the tube.

Radiation monitoring- scintillator detectors

– At this point the electrons are collected by an anode at the end of the tube forming an electronic pulse. The pulse is then detected and displayed or counted by the system.

Radiation monitoring-Neutron detectors

 Boron Trifluoride (BF3) Neutron Detectors – A typical BF3 detector consists of a cylindrical aluminum (brass or copper) tube filled with a BF3 fill gas at a pressure of 0.5 to 1.0 atmospheres. The boron trifluoride gas accomplishes two things: • it functions as the proportional fill gas. • it undergoes an n alpha interaction with thermal neutrons: B10 + n ÿ Li-7 + α

Radiation monitoring-Neutron detectors – To improve the detection efficiency, the BF3 is enriched in B10. Typical enrichments increase the B-10 component to 96% (ordinary boron is 20% B-10 and 80% B-11). Aluminum is typically used as the detector (cathode) wall because of its small cross section for neutrons. The anode is almost always a single thin wire running down the axis of the tube.

– PULSE FORMATION BY NEUTRONS • When a neutron is absorbed by the B-10 component of the gas, an alpha particle and a recoil Li-7 nucleus are produced that travel off in opposite directions. The movement of the alpha particle and Li-7 nucleus create primary ion pairs in the gas.

Radiation monitoring-Neutron detectors • The size of the resulting pulse depends on whether the lithium nucleus was left in the ground state or an excited state. When the lithium nucleus is left in the ground state (about 6% of the time), the pulse is larger than if the nucleus were left in an excited state (about 94% of the time) because the alpha particle and Li-7 nucleus have more kinetic energy (2.792 MeV vs 2.310 MeV) with which to create ion pairs.

– OPERATING VOLTAGE AND THE CHARACTERISTIC CURVE • The best way to determine the operating voltage of a BF3 detector is to generate a characteristic curve (count rate versus high voltage).

Radiation monitoring-Neutron detectors • A typical voltage for a BF3 detector might be 1500 to 2000 volts.

– DETECTION OF THERMAL AND FAST NEUTRONS • "Bare" BF3 detectors almost exclusively respond to slow (low energy) neutrons - the probability that a fast (high energy) neutron would be absorbed by boron-10 is very small. • To be able to detect fast neutrons, the BF3 tube can be surrounded by a suitable moderator. The thickness of the moderator (e.g., polyethylene) might range from 1 to 6 inches depending on the neutron energy spectrum and other constraints.

Radiation monitoring

Loose contamination – Smears are taken with fiberglass paper. – Read on proportional counters which read α and β and display results in cpm. US coverts to dpm (disintegration/min) elsewhere (SI units) its converted to Bqs (becquerels/sec). – Recorded in dpm/100cm2 – Counters used at Argonne • Dual Alpha-Beta Radiological Assay System (DABRAS) • Argonne-Tennelec APC-2 Low-Level Counting System • Liquid Scintillation Counting System (LSC)

Radiation monitoring

 Direct Readings – – – – –

Looking for α and β Taken within 1cm of surface Survey speed of 2-5cm/sec Recorded in dpm/100cm2 Meters used at Argonne • NE Electra with DP-6 detector • Eberline E-600 with HP-100B detector • Eberline ASP2e with Eberline PG-2 or Ludlum 44-17 probe (mini-FIDLER) photon energy