RADON
atomic number atomic weight
86 (222)
R n
[Xe]4f145d106S26p6
electronic configuration
Radon is radioactive chemical element group VIII of periodical system, atomic
number 86, belong to inert gases. It doesn’t have stable isotopes, known
radioisotopes with mass numbers 196-228. Most stable radioisotopes of radon is
222Rn, with half-life time T 1/2 3.8days, relative atomic mass 222,0176; α--emitter –
decay product of Radium isotopes.
Naturally occurs radioactive chains of isotopes transforming into each other
described by formula
A = 4n + m, where m – constant, n- changed by one during alpha-decays, and
remain constant during beta – decay. For naturally occurring series m assigns 0
(series of Thorium 23290 Th), 2 (series of uranium 23892U), and 3 (series of Actinium
22789 Ac), and artificial series of Neptunium 23793Np. Below are charts of radioactive
series a-232Th -Thorium, b-237Np-Neptunium, c-238U-Radium, and d-235U-Actinium.
On charts below axis marked numbers of protons – Z, and neutrons N. Chemical
symbol of each element can be figured out from protons number – Z. T1/2 time
shown in seconds. What is interesting, some elements like 22789Ac, having two
channels of decay.
All naturally occurs three series contain one gaseous member (an isotope of Rn) and
end in a stable isotope of Pb. The radium series starts with 238U. Uranium and its
first five daughters are solids that remain in the soil, but the fifth daughter 226Ra
decays into 222Rn. This daughter, called radon, is a noble gas, not bound chemically
in the material where its parents resided. The half-life of 222Rn (3.82 d) is long
enough for much of the gas to work its way out into the atmosphere. Radon is also
generated in the other two series. However, these isotopes of radon are of lesser
radiological importance. The thorium series generates 220Rn, which is also called
thoron. 220Rn has a half-life of 56 s and therefore has a much greater chance to
decay before becoming airborne. The actinium series produces 219Rn, also called
actinon, after several transformations from the relatively rare original 235U nuclide.
Its T 1/2 is only 3.92 s, and its contribution to airborne radon is insignificant. From
practical point of view, only radon from the radium series can be considered for the
study.
THE FOUR DECAY CHAINS
Name of series
Thorium
Neptunium
Uranium
Actinium
Mass numbers
4n
4n+1
4n+2
4n+3
Long-lived nuclide
232Th
(244Pu)
209Bi
(237Np)
238U
235U
(247Cm)
Half-life
(billions of years)
14
(0.08)
20100000000
(0.00214)
4.5
0.7
(0.0156)
End of chain
208Pb
205Tl
206Pb
207Pb
232Th-244Pu-Thorium series chart, decay time in seconds
Z-protons number, N-neutrons number
209Bi -237Np-Neptunium series chart
B
238U-Radium series chart
C
235U-227Ac-Actinium series chart
D
Radon – density 9.73kg/m3 (273 K, 0,1MPa) soluble in water: 0.23m3/kg (293 К, 0,1
МПа), Atomic radius -220pm, covalent radius 145 pm.
Radon is a colorless gas, 7.5 times heavier than air and more than 100 times heavier
than hydrogen. The gas liquefies at −61.8 °C (−79.2 °F) and freezes at −71 °C (−96
°F). On further cooling, solid radon glows with a soft yellow light that becomes
orange-red at the temperature of liquid air (−195 °C [−319 °F]).
Radon atoms possess a particularly stable electronic configuration of eight
electrons in the outer shell, which accounts for the characteristic chemical inactivity
of the element. Radon, however, is not chemically inert. For example, the existence
of the compound radon difluoride, which is apparently more stable chemically than
compounds of the other reactive noble gases, krypton and xenon gases. Radon’s
short lifetime and its high-energy radioactivity cause difficulties for the
experimental investigation of radon compounds.
Radon is rare in nature because its isotopes are all short-lived and because its
source, radium, is a scarce element. The atmosphere contains traces of radon near
the ground as a result of seepage from soil and rocks, both of which contain minute
quantities of radium. (Radium occurs as a natural decay product of uranium present
in various types of rocks.)
By the late 1980s, naturally occurring radon gas had come to be recognized as a
potentially serious health hazard. Radioactive decay of uranium in minerals,
especially granite, generates radon gas that can diffuse through soil and rock and
enter buildings through basements (radon has a higher density than air) and
through water supplies derived from wells (radon has a significant solubility in
water). The gas can accumulate in the air of poorly ventilated houses. The decay of
radon produces radioactive “daughters” (polonium, bismuth, and lead isotopes)
that can be ingested from well water or can be absorbed in dust particles and then
breathed into the lungs. Exposure to high concentrations of this radon and its
daughters over the course of many years can greatly increase the risk of
developing lung. Indeed, radon is now thought to be the greatest cause of lung
cancer among nonsmokers in the United States. Radon levels are highest in homes
built over geological formations that contain uranium mineral deposits.
Radon Properties
86
(222)
−71 °C (−96 °F)
−62 °C (−80 °F)
9.73 g/liter (0.13
ounce/gallon)
0, +2, +4, +8
(Xe)4f145d106s26p6
Symbol
Z(p)
N(n)
Isotope Mass
(а. е. м.)
Half-Life
(T1/2)
Channel
Decay
Decay
product
Spin
Excitation Energy, keV
195Rn
86
109
195,00544(5)
6 msec
3/2−#
195mRn
50(50) keV
6 msec
13/2+#
196Rn
86
110
196,002115(16)
4,7(11) msec
[4,4(+13−9) msec]
α
192Po
0+
β+ (rear)
196At
197Rn
86
111
197,00158(7)
66(16) msec
[65(+23−14) msec]
α
193Po
3/2−#
β+ (rear)
197At
197mRn
200(60) keV
21(5) msec
[19(+8−4) msec]
α
193Po
(13/2+)
β+ (rear)
197At
198Rn
86
112
197,998679(14)
65(3) msec
α (99%)
194Po
0+
β+ (1%)
198At
199Rn
86
113
198,99837(7)
620(30) msec
α (94%)
195Po
3/2−#
β+ (6%)
199At
199mRn
180(70) keV
320(20) msec
α (97%)
195Po
13/2+#
β+ (3%)
199At
200Rn
86
114
199,995699(14)
0,96(3) s
α (98%)
196Po
0+
β+ (2%)
200At
201Rn
86
115
200,99563(8)
7,0(4) s
α (80%)
197Po
(3/2−)
β+ (20%)
201At
201mRn
280(90) keV
3,8(1) s
α (90%)
197Po
(13/2+)
β+ (10%)
201At
(<1%)
201Rn
202Rn
86
116
201,993263(19)
9,94(18) s
α (85%)
198Po
0+
β+ (15%)
202At
203Rn
86
117
202,993387(25)
44,2(16) s
α (66%)
199Po
(3/2−)
β+ (34%)
203At
203mRn
363(4) keV
26,7(5) s
α (80%)
199Po
13/2(+)
β+ (20%)
203At
204Rn
86
118
203,991429(16)
1,17(18) min
α (73%)
200Po
0+
β+ (27%)
204At
205Rn
86
119
204,99172(5)
170(4) s
β+ (77%)
205At
5/2−
α (23%)
201Po
206Rn
86
120
205,990214(16)
5,67(17) min
α (62%)
202Po
0+
β+ (38%)
206At
207Rn
86
121
206,990734(28)
9,25(17) min
β+ (79%)
207At
5/2−
α (21%)
203Po
207mRn
899,0(10) keV
181(18) microsec
(13/2+)
208Rn
86
122
207,989642(12)
24,35(14) min
α (62%)
204Po
0+
β+ (38%)
208At
209Rn
86
123
208,990415(21)
28,5(10) min
β+ (83%)
209At
5/2−
Energy configurations Rn0→ Rn+→Rn2+ are 1037,07 и 2064,7 kJ/mol (Kilojoule per
Mole)
α (17%)
205Po
209m1Rn
1173,98(13) keV
13,4(13) microsec
13/2+
209m2Rn
3636,78(23) keV
3,0(3) microsec
(35/2+)
210Rn
86
124
209,989696(9)
2,4(1) hours
α (96%)
206Po
0+
β+ (4%)
210At
210m1Rn
1690(15) keV
644(40) nano sec
8+#
210m2Rn
3837(15) keV
1,06(5) microsec
(17)−
210m3Rn
6493(15) keV
1,04(7) microsec
(22)+
211Rn
86
125
210,990601(7)
14,6(2) hours
α (72,6%)
207Po
1/2−
β+ (27,4%)
211At
212Rn
86
126
211,990704(3)
23,9(12) min
α
208Po
0+
β+β+ (rear)
212Po
213Rn
86
127
212,993883(6)
19,5(1) msec
α
209Po
(9/2+)
214Rn
86
128
213,995363(10)
0,27(2) microsec
α
210Po
0+
β+β+ (rear)
214Po
214mRn
4595,4 keV
245(30) nano sec
(22+)
215Rn
86
129
214,998745(8)
2,30(10) micros
α
211Po
9/2+
216Rn
86
130
216,000274(8)
45(5) microsec
α
212Po
0+
217Rn
86
131
217,003928(5)
0,54(5) msec
α
213Po
9/2+
218Rn
86
132
218,0056013(25)
35(5) msec
α
214Po
0+
219Rn
86
133
219,0094802(27)
3,96(1) sec
α
215Po
5/2+
220Rn
86
134
220,0113940(24)
55,6(1) sec
α
216Po
0+
β−β− (rear)
220Ra
221Rn
86
135
221,015537(6)
25,7(5) min
β− (78%)
221Fr
7/2(+)
α (22%)
217Po
222Rn
86
136
222,0175777(25)
3,8235(3) d
α
218Po
0+
223Rn
86
137
223,02179(32)#
24,3(4) min
β−
223Fr
7/2
224Rn
86
138
224,02409(32)#
107(3) min
β−
224Fr
0+
225Rn
86
139
225,02844(32)#
4,66(4) min
β−
225Fr
7/2−
226Rn
86
140
226,03089(43)#
7,4(1) min
β−
226Fr
0+
227Rn
86
141
227,03541(45)#
20,8(7) sec
β−
227Fr
5/2(+#)
228Rn
86
142
228,03799(44)#
65(2) sec
β−
228Fr
0+
229Rn
86
143
229,0426536(141)
12(1) sec
β−
229Fr
230Rn
86
144
β−
230Fr
0+
231Rn
86
145
β−
231Fr
RADON ENVIRONMENTAL MEASUREMENT METHODOLOGY AND
TECHNIQUE
Radon and tritium decay radiation measurements are most difficult and challenging. In
case of tritium, in process of disintegration it emits maximum 16keV electron that can be
processed by electronics.
The problem is, the electron energy often is not enough to stay over background radiation.
Signal/noise ratio is in continuous challenge.
The radon radioactive decay measurements usually not a big challenge with known chain
of reactions, and precise timing.
If radon is in a confined volume, and environmental conditions known, the measurement
precision is not a problem. Problems are coming from environmental sourcing, humidity,
weather, water, soil and rock formation.
Time-resolved radon measurements are a powerful method for detailed examination of
the radon concentration over time. They enable precise evaluation of radon exposure
patterns and support the development of effective protective measures against radon. By
using suitable measurement techniques and careful data analysis, the sources and
influencing factors of radon load can be better understood and controlled. Continuously
measuring radon monitors collect data of radon concentration in real time and record the
data at set time intervals. The measurement duration can vary from hours to several weeks
depending on the target of the examination. After the measurement period, the recorded
data is read out and analyzed. The time-resolved data helps to identify fluctuations that
may result from daily or seasonal changes, usage and ventilation behavior, weather
conditions or specific events.
Calibration technique
The noble gas 222Rn exists worldwide in different activity concentrations in the air. To
determine the exposure of humans, measurements of the radon activity concentration are
performed in houses and at places of work, because radon is the main part of natural
radiation exposure (UNSCEAR, 2000).
The measurement devices for the determination of the radon activity concentration at the
moment are only traceable when calibrated in the range above 1000 Bq/m3 (IEC 61577-1,
2006, IEC 61577-2, 2000, IEC 61577-3, 2002). For lower activity concentrations, the
calibration factor is extrapolated and the measurement uncertainty of the commercial
devices is in addition too large due to the limited statistic of counts in their active volumes
in the standard calibration time of 24 h.
But the arithmetic mean of the radon activity concentration in houses in Germany for
example is around 50 Bq/m3. So it is obvious that there is a missing link in the metrology
at the moment. All measurements in this range are usually taken by commercial radon
devices without a traceable calibration. To close this gap, constant in time reference
atmospheres below 1000 Bq/m3 must be generated to perform long-term calibrations
(t≥5-10 d), homogeneous reference atmosphere should be created by a certified activity in
a certified volume; for example, using precisely known emanation of 222Rn from 226Ra
activity standard. These long-term calibrations in the constant reference atmospheres
reduce the measurement uncertainty of the commercial radon device significantly and
lead to a calibration factor with a combined relative standard uncertainty below 2%.
Indoor radon exposure
As nuclides in the Uranium and Thorium decay series, radon (222Rn) and thoron (220Rn)
occur everywhere on Earth. Indoor radon exposure has been epidemiologically proven to
be the second cause of lung cancer after smoking (WHO, 2009). And as reported by
UNSCEAR, the inner exposure of radon and thoron is one of the most important sources
of natural radiation to humans (UNSCEAR 2000). Therefore, the accurate measurement
of radon and thoron gas is of great importance, and the quality accuracy and quality
control depend on the reliable metrological system of radon/thoron activity concentration.
Different methods have been developed for the metrological system of radon activity
concentration, which can be divided into two types, namely with sources and without
sources. The radon metrological method with sources (Collé et al., 1990; Linzmaier and
Röttger 2013; Röttger et al., 2014; Mostafa et al. 2016, 2017; Mertes et al., 2020) is based
on the 226Ra activity reference and stable radon emission, and it was adopted in many
radon chambers in early stage. Nevertheless, it relies both on the accuracy of 226Ra activity
determination and the stability of radon emanation. The radon metrological methods
without sources include the radon primary standard system based on frozen radon source
and defined solid angle method (Picolo 1996; Picolo et al., 2000; Sabot et al. 2016a,
2020), and the multi-wire ionization chamber (MWIC) system (Busch et al., 2002;
Linzmaier and Röttger 2014). Both methods can directly determine 222Rn activity or radon
activity concentration accurately combined with a precise volume measurement, and
usually, a secondary standard is needed for radon reference transfer.
Due to the short half-life of 220Rn (55.6 s), there are still difficulties in thoron activity
concentration measurement, and studies did not show enough interest in the accurate
measurement of thoron in the past. A few reference methods of thoron activity
concentration are developed nowadays, including the 232Th/228Th activity reference
method (Möre et al., 1996; Qiu 2006; Röttger et al., 2010; Tang et al., 2012; Buompane et
al., 2013; Wang et al., 2017; Rinaldi et al., 2022), the Lucas scintillation chamber method
(Tokonami et al., 2002; Zhang et al., 2020; Sakoda et al., 2015), and the gas direct
detection method (Sabot et al. 2015, 2016b; Ambrosino et al., 2020). Similar to radon, the
232Th/228Th activity reference method is based on the stable emission of thoron gas, but is
more difficult to realize due to the short half-life of 220Rn as well as the temperature and
humidity influence. The Lucas scintillation chamber method is based on direct
measurement of thoron gas and its progeny. Nevertheless, the accuracy is limited by the
distribution uniformity of thoron progeny, and it is hardly used as a reference (Zhao et al.,
2012). To solve the problems of thoron reference standard, Sabot et al. developed the gas
direct detection method, which is based on directly recorded alpha spectra of thoron gas in
a small chamber and is hardly influenced by humidity and the thoron progeny
distribution. After that, Ambrosino et al. developed a similar direct measurement system
based on the same technical route. Due to its significant advantages and wide applicability
to both radon and thoron gas, this direct measurement method seems to be an excellent
choice for in-situ calibration of online measurement instruments and could be used for
radon/thoron standard transfer.
In recent years, a large number of online radon measurement systems have been
developed and installed in China due to the requirement of atmospheric radiation
monitoring and the NORM effluent in-situ measurements, which leads to a great demand
for field calibration of radon concentration (Zhang et al., 2020). To realize the field
calibration of atmospheric radon monitors and the thoron activity concentration standard
transfer, a new-designed measurement devices are developed with gas direct
measurement, and comparison experiments, as well as uncertainty analysis.
Radiation measurements
The ionization process consists of removing an electron from an initially neutral atom or
molecule. For many materials, the minimum energy required for this process is about 10
electron volts (eV), and this can be taken as the lower limit of the range of ionizing
radiation energies. The more common types of ionizing radiation are characterized by
particle or quantum energies measured in thousands or millions of electron volts (keV or
MeV, respectively). At the upper end of the energy scale, the present discussion will be
limited to those radiations with quantum energies less than about 20 MeV. This energy
range covers the common types of ionizing radiation encountered in radioactive decay,
fission and fusion systems and the medical and industrial applications of radioisotopes.
The term heavy charge particle refers to those energetic particles whose mass is one
atomic mass unit or greater. This category includes alpha particles, together with protons,
deuterons, fission fragments, and other energetic heavy particles often produced in
accelerators. These particles carry at least one electronic charge, and they interact with
matter primarily through the Coulomb force that exists between the positive charge on the
particle and the negative charge on electrons that are part of the absorber material. In this
case, the force is an attractive one between the two opposite charges. As a charged particle
passes near an electron in the absorber, it transfers a small fraction of its momentum to
the electron. As a result, the charged particle slows down slightly, and the electron (which
originally was nearly at rest) picks up some of its kinetic energy. At any given time, the
charged particle is simultaneously interacting with many electrons in the absorber
material, and the net result of all the Coulomb forces acts like a viscous drag on the
particle. From the instant it enters the absorber, the particle slows down continuously
until it is brought to a stop. Because the charged particle is thousands of times more
massive than the electrons with which it is interacting, it is deflected relatively little from a
straight-line path as it comes to rest. The time that elapses before the particle is stopped
ranges from a few picoseconds (1 × 10−12 second) in solids or liquids to a few nanoseconds
(1 × 10−9 second) in gases. These times are short enough that the stopping time can be
considered to be instantaneous for many purposes, and this approximation is assumed in
the following sections that describe the response of radiation detectors.
Several characteristics of the particle-deceleration process are important in understanding
the behavior of radiation detectors. First, the average distance traveled by the particle
before it stops is called its mean range. For a given material, the mean range increases
with increasing initial kinetic energy of the charged particle. Typical values for charged
particles with initial energies of a few MeV are tens or hundreds of micrometers in solids
or liquids and a few centimeters in gases at ordinary temperature and pressure. A second
property is the specific energy loss at a given point along the particle track (path). This
quantity measures the differential energy deposited per unit pathlength (dE/dx) in the
material; it is also a function of the particle energy. In general, as the particle slows down
and loses energy, the dE/dx value tends to increase. Thus, the density with which energy is
being deposited in the absorber along the particle’s track tends to increase as it slows
down. The average dE/dx value for charged particles is relatively large because of their
short range, and they are often referred to as high dE/dx radiations.
CR-39 detectors
In the field of radiation dosimetry, polyallyl diglycol (PADC) carbonate, (CR-39 detector),
is the most applicable detector in the solid state nuclear track detectors (SSNTDs) family
because of its excellent optical properties . Therefore, CR-39 detectors are used in various
branches of science and technology, such as cosmic ray studies, radiation biology, radon
monitoring, neutron radiography, particle identification, radiation dosimetry, X-
rays reflectivity, and low linear energy transfer (LLET) radiation like ultraviolet and
plasma detection. In principle, CR-39 detector is insensitive to low doses of LLET
radiation such as X-rays, gamma rays, and electrons . CR-39 detectors have been preferred
to detect perpendicularly incident heavy ions, with efficiency up to 100%; in some cases
such as in high energy experiments, CR-39 detectors have been integrated with other types
of detectors.
When the CR-39 detector is irradiated with heavy ions, permanent latent tracks are
produced along their trajectories as a result of breaking of molecular bonds of the CR-39
detector's material while these ions lose their energies. The core radii of the latent tracks
in the CR-39 detector are within several nanometers, depending on the heavy ion species
and energies. After chemical etching, the latent tracks are enlarged and visualized under
an optical microscope as dark circles. For a low track density and a short etching time
(tracks are not overlapped), track diameters or track profiles are counted with good
detection efficiency up to 100% and with low systematic error. However, in the case of a
high track density and a long etching time, the tracks are overlapped in such a way that
manual or automatic detection cannot distinguish individual tracks. Neither track density
nor diameter can be precisely counted (saturation regime).
Following several observations in homes affected by high indoor radon concentration, gas
flux from bedrock and soil hosting radium-bearing minerals is considered the major
source of radon; while building materials or well waters were noted as possible additional
secondary issues. Radon is soil and rock may migrate through soil pores, faults and
fractures in bedrock by gas-phase diffusion and advection into the shallow region between
the soil and the building foundation. In the process of mapping indoor radon risk, an
important step is to define geological units that are associated with indoor radon
concentration, collecting indoor radon data measured over a minimum period of three-six
months using passive alpha track detectors (CR-39).
The contribution of building materials to indoor radon and thoron concentration is usually
low, however, certain materials of natural origin such as granite, concrete and by-products
of different industries account for an unusually large fraction of the total radon and thoron
concentration in the indoor environment.
Applications of radiation interactions in detectors
A number of physical or chemical effects caused by the deposition of energy along the
track of a charged particle are listed in the first column of the table. Each of these effects
can serve as the basis of instruments designed to detect radiation, and examples of specific
devices based on each effect are given in the second.
Applications of radiation interactions in detectors
results of interaction of
incident radiation
detector category
active or
passive
single
quantum
sensitivity
mode type (for
active
detectors)
sensitized silver halide
grains in photographic
emulsion
radiographic film
passive
no
nuclear emulsion
passive
yes
trapped charges in
crystalline materials
thermoluminescent
dosimeter
passive
no
memory phosphor
passive
no
damaged track in
dielectric materials
track-etch film
passive
yes
radioactivity induced by
neutrons
activation foil
passive
no
vaporized superheated
liquid drop
bubble chamber
active and
passive
yes
pulse
Applications of radiation interactions in detectors
results of interaction of
incident radiation
detector category
active or
passive
single
quantum
sensitivity
mode type (for
active
detectors)
ion pairs in a gas
ion chamber pocket
dosimeter
(integrating)
no
current-mode ion
chamber
active
no
current
proportional tube
active
yes
pulse
Geiger-Müller tube
active
yes
pulse
mobile electron-hole
pairs in semiconductor
silicon diode
active
yes
current and
pulse
coaxial germanium
detector
active
yes
pulse
prompt fluorescence in
transparent materials
scintillation detector
active
yes
current and
pulse
Cerenkov radiation
Cerenkov detector
active
yes
pulse
One category of radiation-measurement devices indicates the presence of ionizing
radiation only after the exposure has occurred. A physical or chemical change is induced
by the radiation that is later measured through some type of processing. These so-
called passive detectors are widely applied in the routine monitoring of occupational
exposures to ionizing radiation. In contrast, in active detectors a signal is produced in real
time to indicate the presence of radiation. This distinction is indicated for the examples in
the table.
Radiation interactions in matter
There are two major categories: those that carry an electric charge and those that do not.
In the first group are the radiations that are normally viewed as individual subatomic
charged particles. Such radiation appears, for example, as the alpha particles that are
spontaneously emitted in the decay of certain unstable heavy nuclei. These alpha particles
consist of two protons and two neutrons and carry a positive electrical charge of two units.
Another example is the beta-minus radiation also emitted in the decay of some radioactive
nuclei. In this case, each nuclear decay produces a fast electron that carries a negative
charge of one unit. In contrast, there are other types of ionizing radiation that carry no
electrical charge. Common examples are gamma-rays, which can be represented as high-
frequency electromagnetic photons, and neutrons, which are classically pictured as
subatomic particles carrying no electrical charge. In the discussions below, the term
quantum will generally be used to represent a single particle or photon, regardless of its
type.
Only charged radiations interact continuously with matter, and they are therefore the only
types of radiation that are directly detectable in the devices described here. In contrast,
uncharged quanta must first undergo a major interaction that transforms all or part of
their energy into secondary charged radiations. Properties of the original uncharged
radiations can then be inferred by studying the charged particles that are produced. These
major interactions occur only rarely, so it is not unusual for an uncharged radiation to
travel distances of many centimeters through solid materials before such an interaction
occurs. Instruments that are designed for the efficient detection of these uncharged quanta
therefore tend to have relatively large thicknesses to increase the probability of observing
the results of such an interaction within the detector volume.
Interactions of heavy charged particles
The term heavy charged particle refers to those energetic particles whose mass is one
atomic mass unit or greater. This category includes alpha particles, together with protons,
deuterons, fission fragments, and other energetic heavy particles often produced in
accelerators. These particles carry at least one electronic charge, and they interact with
matter primarily through the Coulomb force that exists between the positive charge on the
particle and the negative charge on electrons that are part of the absorber material. In this
case, the force is an attractive one between the two opposite charges. As a charged particle
passes near an electron in the absorber, it transfers a small fraction of its momentum to
the electron. As a result, the charged particle slows down slightly, and the electron (which
originally was nearly at rest) picks up some of its kinetic energy. At any given time, the
charged particle is simultaneously interacting with many electrons in the absorber
material, and the net result of all the Coulomb forces acts like a viscous drag on the
particle. From the instant it enters the absorber, the particle slows down continuously
until it is brought to a stop. Because the charged particle is thousands of times more
massive than the electrons with which it is interacting, it is deflected relatively little from a
straight-line path as it comes to rest. The time that elapses before the particle is stopped
ranges from a few picoseconds (1 × 10−12 second) in solids or liquids to a few nanoseconds
(1 × 10−9 second) in gases. These times are short enough that the stopping time can be
considered to be instantaneous for many purposes, and this approximation is assumed in
the following sections that describe the response of radiation detectors.
Several characteristics of the particle-deceleration process are important in understanding
the behavior of radiation detectors. First, the average distance traveled by the particle
before it stops is called its mean range. For a given material, the mean range increases
with increasing initial kinetic energy of the charged particle. Typical values for charged
particles with initial energies of a few MeV are tens or hundreds of micrometers in solids
or liquids and a few centimeters in gases at ordinary temperature and pressure. A second
property is the specific energy loss at a given point along the particle track (path). This
quantity measures the differential energy deposited per unit pathlength (dE/dx) in the
material; it is also a function of the particle energy. In general, as the particle slows down
and loses energy, the dE/dx value tends to increase. Thus, the density with which energy is
being deposited in the absorber along the particle’s track tends to increase as it slows
down. The average dE/dx value for charged particles is relatively large because of their
short range, and they are often referred to as high dE/dx radiations.
Interactions of fast electrons
Energetic electrons (such as beta-minus particles), since they carry an electric charge, also
interact with electrons in the absorber material through the Coulomb force. In this case,
the force is a repulsive rather than an attractive one, but the net results are similar to those
observed for heavy charged particles. The fast electron experiences the cumulative effect of
many simultaneous Coulomb forces, and undergoes a continuous deceleration until it is
stopped. As compared with a heavy charged particle, the distance traveled by the fast
electron is many times greater for an equivalent initial energy. For example, a beta
particle with an initial energy of 1 MeV travels one or two millimeters in typical solids and
several meters in gases at standard conditions. Also, since a fast electron has a much
smaller mass than a heavy charged particle, it is much more easily deflected along its path.
A typical fast-electron track deviates considerably from a straight line, and deflections
through large angles are not uncommon. Because a fast electron will travel perhaps 100
times as far in a given material as a heavy charged particle with the same initial energy, its
energy is much less densely deposited along its track. For this reason, fast electrons are
often referred to as low dE/dx radiations.
There is one other significant difference in the energy loss of fast electrons as compared
with that of heavy charged particles. While undergoing large-angle deflections, fast
electrons can radiate part of their energy in the form of electromagnetic radiation known
as bremsstrahlung, or braking radiation. This form of radiation normally falls within the
X-ray region of the spectrum. The fraction of the fast-electron energy lost in the form of
bremsstrahlung is less than 1 percent for low-energy electrons in light materials but
becomes a much larger fraction for high-energy electrons in materials with high atomic
numbers.
Interactions of gamma rays and X rays
Ionizing radiation also can take the form of electromagnetic rays. When emitted by excited
atoms, they are given the name X rays and have quantum energies typically measured
from 1 to 100 keV. When emitted by excited nuclei, they are called gamma rays, and
characteristic energies can be as high as several MeV. In both cases, the radiation takes the
form of photons of electromagnetic energy. Since the photon is uncharged, it does not
interact through the Coulomb and therefore can pass through large distances in matter
without significant interaction. The average distance traveled between interactions is
called the mean free pass and in solid materials ranges from a few millimeters for low-
energy X rays through tens of centimeters for high-energy gamma rays. When an
interaction does occur, however, it is catastrophic in the sense that a single interaction can
profoundly affect the energy and direction of the photon or can make it disappear entirely.
In such an interaction, all or part of the photon energy is transferred to one or more
electrons in the absorber material. Because the secondary electrons thus produced are
energetic and charged, they interact in much the same way as described earlier for primary
fast electrons. The fact that an original X ray or gamma ray was present is indicated by the
appearance of secondary electrons. Information on the energy carried by the incident
photons can be inferred by measuring the energy of these electrons.
Photoelectric absorption
In this process, the incident X-ray or gamma-ray photon interacts with an atom of the
absorbing material, and the photon completely disappears; its energy is transferred to one
of the orbital electrons of the atom. Because this energy in general far exceeds the binding
energy of the electron in the host atom, the electron is ejected at high velocity. The kinetic
energy of this secondary electron is equal to the incoming energy of the photon minus the
binding energy of the electron in the original atomic shell. The process leaves the atom
with a vacancy in one of the normally filled electron shells, which is then refilled after a
short period of time by a nearby free electron. This filling process again liberates the
binding energy in the form of a characteristic X-ray photon, which then typically interacts
with electrons from less tightly bound shells in nearby atoms, producing additional fast
electrons. The overall effect is therefore the complete conversion of the photon energy into
the energy carried by fast electrons. Since the fast electrons are now detectable through
their Coulomb interactions, they can serve as the basis to indicate the presence of the
original gamma-ray or X-ray photon, and a measurement of their energy is tantamount to
measuring the energy of the incoming photon. Because the photoelectric process results in
complete conversion of the photon energy to electron energy, it is in some sense an ideal
conversion step. The task of measuring the gamma-ray energy is then reduced to simply
measuring the equivalent energy deposited by the fast electrons. Unfortunately, two other
types of gamma-ray interactions also take place that complicate this interpretation step.
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