HP SURVEY INSTRUMENT
CALIBRATION AND SELECTION
PRINCIPLES OF RADIATION
DETECTION AND QUANTIFICATION
CHAPTER 4
January 13 – 15, 2016
TECHNICAL MANAGEMENT SERVICES
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BACKGROUND RADIATION LEVELS
AROUND THE WORLD
Natural sources account for most of the radiation we
all receive each year. The nuclear fuel cycle does not
give rise to significant radiation exposure for
members of the public, and even in two major
nuclear accidents – Three Mile Island and Fukushima
– exposure to radiation has caused no harm to the
public. Radiation protection standards assume that
any dose of radiation, no matter how small, involves a
possible risk to human health. This deliberately
conservative assumption is increasingly being
questioned.
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Radioactivity of some natural and other materials
1 adult human (65 Bq/kg)
4500 Bq
1 kg of coffee
1000 Bq
1 kg of brazil nuts
400 Bq
1 banana
15 Bq
The air in a 100 sq metre Australian home (radon)
Up to 3000 Bq
The air in a 100 sq metre European homes (radon)
Up to 30,000 Bq
1 household smoke detector (with americium)
30,000 Bq
Radioisotope for medical diagnosis
7E7Bq
Radioisotope source for medical therapy
100 TBq
1 luminous Exit sign (tritium)
1 TBq
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Shielding Calculations
The simplest method for determining the effectiveness
of the shielding material is using the concepts of halfvalue layers (HVL) and tenth-value layers (TVL). One
half-value layer is defined as the amount of shielding
material required to reduce the radiation intensity to
one-half of the unshielded value.
HVL = ln 2/μ = 0.693/μ
The symbol µ is known as the linear attenuation
coefficient and is obtained from standard tables for
various shielding materials.
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The basic equation using HVLs is
I = Io (1/2)n
Where; I = shielded exposure rate
Io = unshielded exposure rate
n = # of HVLs = shield thickness (cm)/HVL thickness (cm)
However, if the beam is broad, photons can be
"randomized" and scattered into the area one is trying
to shield. The secondary photons are accounted for by a
build up factor, B, in the attenuation equation as
follows:
I = BI0e-ux
where B is the buildup factor.
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Tables of dose build-up factors can be found in the
Radiological Health Handbook.
The buildup is mostly due to scatter. Scattered
radiation is present to some extent whenever an
absorbing medium is in the path of radiation. The
absorber then acts as a new source of radiation.
Room walls, the floor, and other solid objects near
enough to a source of radiation may make scatter
appreciable.
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When radiation scattering is a significant factor, the
inverse square law may no longer be valid for
computing radiation intensity at a distance.
Measurement of the radiation is then necessary to
determine the potential exposure at any point.
– persons in the area behind a shield where there is no
direct line of sight to the source are not necessarily
protected.
– a wall or partition is not necessarily a "safe" shield
for persons on the other side.
– radiation can be deflected around corners"; i.e., it
can be scattered.
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Shadow Shields
ISO 4037 specifies the characteristics and production
methods of x-ray and gamma-ray reference radiation
for calibrating protection-level dosimeters and rate
dosimeters. The standard limits scattered radiation to
5%. The shadow shield technique is a widely accepted
test used to determine scatter contribution.
Shadow shields are frequently used when calibrating
photon survey instruments.
Shadow cones are applicable to the calibration of
neutron survey instruments.
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Shielding a Detector From Interfering Radiations
Alpha Shielding
It takes very little shielding to absorb alpha radiation
completely. A single sheet of writing paper will suffice
for most alpha emitters. However, radon and thoron
progeny present a challenge. To effectively shield the
8.78 MeV alpha from Po-212 the shield would
somewhat attenuate low energy beta emitters.
Pulse height discrimination is another method of
minimizing alpha interference.
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Shielding a Detector From Interfering Radiations
Beta Shielding
It takes very little shielding to absorb beta radiation
completely. However, bremsstrahlung production is
enhanced by high Z materials, for effective shielding of
beta particles one would use a low Z material, such as
plastic. This would allow the Beta particle to lose its
energy with minimal Bremsstrahlung production. A
material suitable to shield the Bremsstrahlung X-rays
(such as lead) would then be placed on the
"downstream" side of the plastic. In practice a photon
detector may have a simple low Z shield to remove
interference from beta radiation.
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Shielding a Detector From Interfering Radiations
Photon Shielding
Shielding an alpha or beta detector from photon
interference is best done with either pulse height
discrimination or pulse shape discrimination.
Neutron detectors may be shielded from photon
interference through a combination of HV adjustment
and pulse height discrimination.
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Shielding a Detector From Interfering Radiations
Neutron Shielding
Neutron radiation is very often found with a photon
radiation field. For many alpha detectors based on ZnS
or semiconductors minimizing any neutron target
material near the detector can be effective. Any
photon component may be minimized with pulse
height discrimination. Most photon ion chambers and
gas proportional and GM detectors will respond to fast
neutrons. An NaI(Tl) or other similar gamma detector
will have very little direct response to neutrons.
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Ludlum 44-40 Alpha/Beta/Gamma
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DETECTOR: pancake-type, halogen quenched GM
WINDOW: 1.7 + 0.3 mg/cm² mica with stainless steel
protective screen
WINDOW AREA: 15.51 cm² (2.4 in²) active, 12.26 cm²
(1.9 in²) open
EFFICIENCY (4π): 5% for 14C; 22% for 90Sr/90Y; 19% for
99Tc; 32% for 32P; 15% for 239Pu
SENSITIVITY (137Cs gamma): 3300 cpm/mR/hr
BACKGROUND: 25 cpm
GAMMA FRONT TO BACK RATIO: 4 to 1
LEAD SHIELD THICKNESS: 1.3 cm (0.50 in.)
SIZE: 11.4 x 10.2 x 16.5 cm (4.5 x 4 x 6.5 in.) (H x W x L)
WEIGHT: 2.5 kg (5.5 lb)
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Ludlum 44-38 beta, gamma survey
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DETECTOR: 30-45 mg/cm² stainless steel wall halogen
quenched GM
DETECTION RANGE: ±10% up to 500mR/hr with DTC
SENSITIVITY: 1200 cpm per mR/hr (137Cs gamma) with
window closed
BACKGROUND: 20 cpm closed; 25 cpm open
GAMMA ENERGY RESPONSE: response within 20% of
137Cs (662 keV) from 60 keV to 1.3 MeV
BETA CUT OFF: approximately 200 keV (window open)
WINDOW CONSTRUCTION: Low Energy Blocking
Shield: tin shields mounted on aluminum (1353
mg/cm²) with solid aluminum gap in the middle
Low Energy Pass Through Window: solid aluminum
(610 mg/cm²)
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ALMOST EVERY TYPE OF RADIATION DETECTOR
RESPONDS TO
ALMOST EVERY TYPE OF RADIATION
IF you don’t know which radionuclides are in the
survey area you need to determine which
radionuclides are present in order to quantify them.
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End of Chapter 4
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