Review on X-ray Detectors Based on Scintillators and CMOS

26 Pages·2010·2.33 MB·English

Recent Patents on Electrical Engineering 2011, 4, 000-000 1
Review on X-ray Detectors Based on Scintillators and CMOS Technology
Jose G. Rocha1* and Senentxu Lanceros-Mendez2
1Algoritmi Research Center, University of Minho, Campus de Azurem, 4800-058 Guimaraes, Portugal. 2Center of
Physics, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
Received: May 27, 2010; Accepted: June 29, 2010; Revised: September 3, 2010
Abstract: This article describes the theoretical basis, design and implementation of X-ray microdetectors based on
scintillating materials and CMOS technology. The working principle of such microdetectors consists in the absorption of
X-rays by scintillators, which produce visible light. The visible light is then detected and converted into electric signals by
means of photodetectors. In order to understand such detectors, several issues related to its implementation are presented
in this article, namely:
Production of X-rays and interaction between them and matter - the first step necessary to the detection of X-rays is that
they must be absorbed by some material, in this case by a scintillator;
Radiation detectors - there are several types of detectors, namely: pn junctions, photoconductors, based on thermal effects
and scintillators;
Fabrication of scintillator arrays - after the X-ray radiation is absorbed by a scintillator, this material emits visible light
whose intensity is proportional to the total energy of the absorbed X-rays;
Optical interfaces between scintillators and photodetectors - the visible light generated by scintillators must arrive to the
photodetectors, so, it is necessary to have an interface between the scintillators and the photodetectors that ideally does not
introduce losses;
Photodetectors and interface electronics - the visible light is absorbed by the photodetectors and converted into electrical
signals, which are finally converted into digital images by means of interface electronics. The article presents some
promising patents on X-ray detectors based on scintillators and CMOS technology.
Keywords: X-rays, scintillators, digital radiography, CMOS imaging.
1. INTRODUCTION radiographic films, scintillators are not essential, but they are
generally used. In some imaging systems, such as com-
In the years that preceded the discovery of X-rays, some
puterized tomography and nuclear medicine imaging, an
physicists observed high-voltage electric discharges in
essential characteristic of the scintillators is used to
vacuum tubes. In 1895, the German physicist Wilhelm
advantage: they produce a flash of light proportional to the
Konrad Röentgen studied the same phenomenon in a
energy of each X-ray photon that interacts with them. A
Crookes tube [1] operating at high voltage, in a darkened
system for providing multiple images in an imaging unit is
room. Suddenly, he observed fluorescence (brightness) in a
disclosed by Erwen et al. in 2010 [2].
barium platinocyanide screen placed a few meters from the
tube. He quickly concluded that the fluorescence was caused
1.1. X-Ray Nature
by an invisible, unknown radiation, which could completely
pass through solid materials such as paper and wood, once An X-ray beam belongs to a group of radiations of the
they did not prevent the fluorescence when placed between same nature as infrared, visible light, ultraviolet, radio waves
the tube and the screen. He also verified that the radiation and other types of radiation energy [3]. All these energy
could be stopped by denser materials, such as lead. forms are classified as electromagnetic radiations, that is,
fluctuations of electric and magnetic fields. The electric and
Röentgen discovered the X-rays and at the same time, he
magnetic fields change perpendicularly to the propagation
discovered the first radiographic image detector. Barium
direction, as well as between themselves. Its speed is
platinocyanide is not used anymore, but the principles of
constant and equal to 299 792 458 ms-1. This is known as
fluorescence or scintillation are applied in about 95% of
speed of the light in vacuum.
modern X-ray detectors. In some detectors, like traditional
All types of electromagnetic radiation differ only in the
frequency of oscillation, (cid:3), and/or in the wavelength, (cid:2).
*Address correspondence to these authors at the Algoritmi Research Center,
These two quantities are related by the constant speed of the
University of Minho, Campus de Azurem, 4800-058 Guimaraes, Portugal;
Tel: [phone]; Fax: [phone]; light, c:
E-mail: [email protected]; Center of Physics, University of Minho,
Campus de Gualtar, 4710-057 Braga, Portugal; Tel: [phone]; (cid:2)=c/(cid:1). (1)
Fax: [phone]; E-mail: [email protected]
1874-4761/11 $100.00+.00 © 2011 Bentham Science Publishers Ltd.
2 Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 Rocha and Lanceros-Mendez
The electromagnetic radiation can also be understood as a - A glass cover to keep the device in vacuum conditions.
set of particles that travel in space at the speed of light,
- An anode that converts the energy of the electrons into
called quanta or photons, each one having a fixed amount of
X-rays.
energy given by:
- A case with a window transparent to X-rays, for the
E(cid:1)=h(cid:2), (2) outgoing of the X-rays.
(cid:1)34 When an electrical current flows through the filament,
where h=6.63(cid:2)10 Js is the Planck’s constant. They also
electrons are released by thermionic emission. They are then
have momentum given by:
sped up by a potential difference, of the order of tens or
p=E(cid:1)/c. (3) hundreds of kilovolts, which is applied between the filament
and the anode. The high-sped electrons, when striking in the
An electromagnetic radiation is fully characterized by its anode, lose their energy in the form of heat and electro-
wavelength, its frequency or its energy, once these quantities magnetic radiation. A small part of this radiation is in the
are related. The X-rays are usually characterized by their spectral range of the X-rays, as will be shown in the
following paragraphs.
energy, measured in electron-Volt (eV), where 1eV is the
energy that an electron acquires when it is speed up by a 1.2.2. X-Ray Spectrum
potential difference of 1 Volt and is approximately
If electrons are speed up by a potential of 35kV, when
1.6(cid:2)10(cid:1)19J. The X-rays used in medical imaging usually they strike in a molybdenum anode, the spectrum of Fig. (2)
are in the range from 15keV to 120keV. is produced. This spectrum is composed by two parts: the
continuous spectrum of the braking radiation and the
1.2. Production of X-Rays characteristic radiation spectrum of the molybdenum anode.
There are some devices used for the production of X-ray 1.3.3. Continuous Spectrum
radiation, namely the X-ray tube, the synchrotron, and the
When a high-speed electron collides with the anode,
free electron laser. In medical imaging, the X-ray tube is the
several processes can occur: the most probable one is that the
most widely used source, whereas in radiotherapy, the
electron suffers a small elastic scattering, which corresponds
synchrotron is also used. The working principle of the X-ray
to transference of energy to the anode, which normally
tube is described in the following sections. Tutomu et al.
appears under the form of heat. In the energy range used in
described the function of X-ray tube in patents no.
medical imaging, about 99% of the electron energy is
US7773726 (2010)[4].
converted into heat. Its dissipation is one of the biggest
1.2.1. X-Ray Tube technical problems of X-ray tubes.
When electrons are speed up to more than 5keV and Occasionally, the electron passes very close to an atomic
strike directly in the surface of a target, X-rays are emitted. nucleus, where it suffers a deflection, mainly due to the
The X-rays are mainly originated in the fast deceleration of nuclear charge. The interaction of the electron with the
the electrons, when they interact directly with the nuclei of atomic nucleus results in a change of its kinetic energy,
the atoms of the target. This principle of production of X- which results in the emission of a photon in the spectral
rays is called bremsstrahlung, a German word which means range of the X-rays. The energy lost by the electron can have
braking radiation. Fig. (1) shows a schematic diagram of an a wide range of values, which justifies the continuous
X-ray tube. spectrum. The emission of X-rays can also occur after some
elastic scattering interactions, which means that the X-rays

are not always emitted from the surface of the anode. This
factor, besides justifying the continuous spectrum of the
emission of X-rays, also justifies its continuous spatial

distribution.

1.2.4. Characteristic Spectrum

Overlapped to the continuous spectrum, characteristic
peaks normally appear which are the result of the interaction
of high-speed electrons with the ones of the atoms of the

target. If a high-speed electron has enough energy, it can
ionize an atom, removing one atomic electron from its
orbital. In this case, the orbital in question will have a lack of

an electron, i.e., a hole. Usually, this happens in the layers
close to the nucleus. The hole will then be filled by another
Fig. (1). Schematic diagram of an X-ray tube.
electron of a higher shell, which jumps to it. In this jump, the
electron loses energy by releasing a photon. If the
Basically, an X-ray tube is composed by: phenomenon occurs in the layers close to the nucleus, the
released photon will be in the spectral range of the X-rays.
A filament that is heated by a low-voltage electric
current. This filament delivers electrons by thermionic
emission.
X-ray Detectors Based on Scintillators and CMOS Technology Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 3
As an example, a high-speed electron removes an The mass absorption coefficient μ/(cid:3) is related to the
electron of the K shell of an atom. An electron from the L or cross-sections of the interaction processes of the X-rays with
M shells will fill the existing hole in K shell. As the energy matter, according to:
difference between electrons of L or M shells and K shell is
constant for each chemical element, all photons released in μ/(cid:2)=NAV (cid:1)(cid:3)i , (7)
A
this way have the same energy. This explains the spectral i
peaks, where each one of them corresponds to a well-defined
where (cid:1) is the atomic cross-section of the interaction process
jump. The peaks shown in the spectrum of Fig. (2) corres- i
i, A is the relative atomic mass of the atom with which the
pond to jumps between L, L and K shells in molybdenum.
I II interaction occurs and N is Avogadro’s number
AV
(6.022(cid:1)1023). For energies below 100 keV, the dominant
interaction is the photoelectric absorption [5], which can be

represented by:

(cid:2) + atom (cid:3) atom+ + e(cid:2) (8)

where (cid:2) represents a photon and e(cid:2) an electron. In the
medium range of energies (close to 1 MeV), the Compton
effect is dominant. The Compton effect is basically the

scattering of a photon when interacting with an electron [5]:

(cid:2) + e(cid:2) (cid:3) (cid:2) + e(cid:2). (9)

At very high energies (far above 1 MeV), the cross-section
for the production of electron-positron pairs is the most
significant [5],

(cid:2) + nucleus (cid:3) e+ + e(cid:2) + nucleus. (10)

In this case, e+ represents a positron.

1.3.1. Photoelectric Effect
Fig. (2). X-ray spectrum produced by a molybdenum anode and a
potential difference of 35 kV. Atomic electrons can absorb the total energy of a photon,
whereas for free electrons, due to momentum conservation,
1.3. Interaction Between X-Rays and Matter such is not possible. The absorption of a photon by an atomic
electron requires a third collision entity, which in this in case
The operation of any X-ray detector depends basically on
is the nucleus of the atom. The cross-section for the absorp-
the way the radiation interacts with the material that cons-
tion of a photon of energy E in the K shell of an atom is
titutes it. Notwithstanding several interaction mechanisms of
particularly high ((cid:1) 80% of the total cross-section), due to
X-rays and matter are known, only three of them are impor-
the proximity of the third collision entity, the atomic nucleus,
tant in their measurement: photoelectric absorption, Comp-
which absorbs the recoil momentum. The total cross-section
ton scattering and production of electron-positron pairs.
for the photoelectric effect in the non-relativistic range far
These processes are based on the partial or total transfer of
from absorption edges is given by the Born approach [6]:
the X-ray photon energy to an atom. As a result, the trajec-
tory and the energy of the photon are drastically modified. K (cid:4)32(cid:6) 4 5 e
(cid:2) =(cid:8) (cid:9)(cid:1) Z (cid:2) , (11)
When a beam of X-rays passes through a body, along x photo 7 Th
(cid:5)(cid:3) (cid:7)
direction, after the distance dx in its interior, the X-rays
intensity decreases. The decrease in intensity is given by: where:
dI =(cid:1)μI, (4) (cid:2)=E(cid:1)/mec2, (12)
dx
is the reduced energy of the photon, E is the photon energy,
where I is the intensity of the beam and μ is the coefficient of (cid:2)
m is the electron mass, Z is the atomic number and c is the
linear absorption of the material. Integrating along the e
thickness of the body, gives: speed of light. (cid:1)Teh is the Thomson cross-section for elastic
collisions between photons and electrons, given by:
(cid:1)μx
I=Ioe , (5)
e 8 2 (cid:2)29 2
where Io is the initial intensity of the beam. Usually, instead (cid:4)Th=3(cid:1)re =6.65(cid:3)10 m , (13)
of μ, the mass absorption coefficient μ/(cid:3) is used (coefficient
of linear absorption per unit of density) and equation (5) From classical theory, the electron radius is given by:
becomes: 2
1 e (cid:2)15
re= =2.81794(cid:3)10 m, (14)
I=Ioe((cid:1)μ/(cid:2))(cid:2)x. (6) 4(cid:1)(cid:4)o mec2
4 Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 Rocha and Lanceros-Mendez
where e is the electron charge and (cid:1) is the vacuum The Compton scattering for an atom as a whole is
o
permittivity (8.85 x 10(cid:1)12 F/m). The fine-structure constant is proportional to the atomic number:
given by:
atom e
(cid:1)c =Z(cid:1)c (18)
2
e (cid:2)3
(cid:4)= =7.29735(cid:3)10 , (15)
The relationship between the energies of scattered and
4(cid:1)(cid:5)ohc
incident photons is given by:
where h=h/2(cid:1) is the reduced Planck constant. '
Close to the absorption edges, the dependence between E(cid:2)= 1 , (19)
E(cid:2) 1+(cid:4)(1(cid:1)cos(cid:3)(cid:2))
the cross-section and the photon energy is modified by a
(cid:3) edge(cid:5) where (cid:4) is the scattering angle of the photon in the
function f(cid:7)E(cid:1),E (cid:8) [6]. For high energies the depen- (cid:3)
(cid:4) (cid:2) (cid:6) referential of the laboratory Fig. (3).
dence between cross-section and energy for the photoelectric
effect is:

K 2 5 41
(cid:3)photo=4(cid:1)reZ (cid:2) (cid:4). (16)

In equations (11) and (16), the dependence between the
cross section and the atomic number is Z5. This is an
indication that interactions between photons and isolated

atomic electrons do not occur. Actually, the dependence
between the cross-section and the atomic number shows that
(cid:2)photo is a more complex function of Z. In the energy range Fig. (3). Kinematic variables of the Compton scattering process.
between 100 keV and 5 MeV, the exponent of Z ranges

between 4 and 5.
When (cid:4) = 180o (backscattering) the energy transferred to
(cid:3)
As a consequence of the photoelectric effect in an inner the electron reaches its maximum value. The relationship
shell of an atom (K shell, for example), some secondary between the energies of scattered and incident photons is
effects can occur. If the hole left after the interaction then given by:
between the photon and the electron is filled by another
'
electron of a higher shell, the energy difference can be E(cid:1)= 1 . (20)
released under the form of a characteristic X-ray photon, E(cid:1) 1+2(cid:2)
which in turn can be absorbed by another electron of the
The scattering angle of the electron, relatively to the
same or neighboring atoms. If the energy is higher than the
direction of the incident photon is given by:
binding energy of the electronic shell of the atom in
q[7u]e. stTiohne, aennoetrhgeyr eolfe cttrhoens ec aAn ulgeaevr e etlheec tarotonms (isA ungeecre sesfafericlty) cot(cid:2)e=(1+(cid:3))tan(cid:2)2(cid:1). (21)
smaller than the energy of the primary electron. For example,
Due to momentum conservation, this angle cannot be
if the photoionization occurs in K shell with binding energy
larger than (cid:2)/2. In Compton scattering only a portion of the
E , and the hole is filled by an electron of L shell, whose
K
photon energy is transferred to the electron. The cross-
energy is E , the amount of energy E (cid:1) E will be released.
L K L
section of energy scattering can then be defined as:
This energy, in turn, can be transferred to another electron of
L shell. If EK (cid:1) EL > EL, this electron can leave the L shell as E'
an Auger electron, with kinetic energy of EK (cid:1) 2EL. (cid:2)cs= (cid:1)(cid:2)ce (22)
E
(cid:1)
1.3.2. Compton Effect
and the absorption cross-section is defined as:
The Compton effect describes the scattering of photons
when interacting with free electrons. In practice, one can use (cid:2)ca=(cid:2)ce(cid:1)(cid:2)cs. (23)
the relations established here for atomic electrons although
the theory is developed for free electrons. In the mathe- Equation (23) is important in absorption processes and is
matical model of this interaction process, the binding energy related to the probability that the kinetic energy
of electrons to atoms is normally disregarded. The total Ecin=E(cid:2)(cid:1)E(cid:2)' be transferred to an electron.
cross-section, for each electron, for the Compton scattering
Compton scattering occurs not only with electrons, but
is given by the Klein-Nishina formula [8]:
also with other charged particles. However, the interaction
(cid:13) (cid:17) with atomic electrons is the most important for the operation
e 2(cid:16)(cid:9)1+(cid:8)(cid:11)(cid:4)2(1+(cid:8)) 1 (cid:6) 1 1+3(cid:8) (cid:16), (17)
(cid:3)c=2(cid:1)re (cid:14)(cid:15)(cid:16)(cid:10)(cid:20)(cid:8)2 (cid:12)(cid:22)(cid:5)(cid:21)1+2(cid:8) (cid:2)(cid:8)ln(1+2(cid:8))(cid:7)(cid:23)+2(cid:8)ln(1+2(cid:8))(cid:2)(1+2(cid:8))2(cid:18)(cid:19)(cid:16) of radiation detectors.
1.3.3. Production of Electron-Positron Pairs
where (cid:1) is given by equation (12) and r by equation (14).
e
The production of an electron-positron pair in the electric
field of the nucleus of an atom is only possible if the energy
X-ray Detectors Based on Scintillators and CMOS Technology Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 5
of the photon exceeds a certain threshold, given by the rest Notice that the production of an electron-positron pair
masses of the electron plus the positron, multiplied by c2, cannot happen in vacuum, due to momentum conservation.
athded eadto tmo .t hFer oremc otihl ee nceorngsye rtvhaatti oisn t roafn sefneerrrgeyd taon dth em noumcelenutus mof, The photon has a momentum p=E(cid:1)/c, and as E(cid:2)(cid:1)2mec2
the energy threshold for the production of an electron- (equation (25)), p(cid:1)2mec, that is, the electron and the
positron pair is [8]: positron would move at speeds superior to the one of the
light, which is impossible. Therefore it is necessary that
2
E(cid:2)(cid:1)2mec2+2 me c2. (24) another particle, in whose field the formation of pairs occurs,
mnucleus to receive the difference of momentum.
As the electron mass (m) is far smaller than the mass of
e
the nucleus (m ), the expression of the energy threshold 1.3.4. Other Processes of Interaction Between X-Rays and
nucleus
can be approximated by: Matter
Beyond the three previously described processes, there
E(cid:2)(cid:1)2mec2. (25) are others whose interest for the implementation of detectors
is smaller, namely:
On the other hand, if the interaction takes place in the
electric field of an electron, the expression for the energy Coherent or Rayleigh Scattering
threshold becomes:
In opposite to the Compton scattering of photons in
E(cid:2)(cid:1)4mec2. (26) individual electrons, the Rayleigh scattering occurs only in
electrons that are part of an atom. Once the recoil momentum
The probability of the production of an electron-positron in Rayleigh interaction is absorbed by the atom as a whole,
pair occurring in the field of the electron is, however, far the loss of energy of the photon is insignificant and the
smaller than the probability of occurring in the field of the scattering angle is small. Due to the fact that the effect in the
nucleus [8]. energy or direction of the photon is minimal, usually it is a
common procedure to ignore Rayleigh scattering in the
In the case where the nuclear charge is not shielded by
calculations of interactions between X-rays and matter [10].
atomic electrons (the photon passes close to it) and
Photonuclear Absorption
1
(cid:2)< ,
1/3 The absorption of a photon by an atomic nucleus
(cid:1)Z
normally results in the emission of one or more neutrons
the cross-section for electron-positron pair production is
and/or protons. This interaction can contribute 5% to 10% to
given by [9]:
the total cross-section of interaction with photons in a
relatively narrow range of energies, normally between 5
(cid:3)pair=4(cid:2)re2Z2(cid:5)(cid:9)7ln2(cid:4)(cid:1)109(cid:7)(cid:10). (27) MeV and 40 MeV, depending on the nucleus in question [11
(cid:6)9 54 (cid:8)
- 15]. The effects of this interaction can be observed in
At low energies, the photon must pass relatively close to measurements of the total attenuation coefficient [16].
the nucleus so that a pair production can take place, which However, there is an irregular dependence between this
means that the photon will see the nucleus uncovered by cross-section and the atomic number or the atomic mass, and
electrons. there are not theoretical models comparable to the ones of
the other cross-sections.
In the case of high photon energy, that is,
Elastic Nuclear Scattering
1
(cid:2)>> ,
1/3 This is an effect analogous to Compton scattering, but
(cid:1)Z
produced by the nucleus. In this process, a photon interacts
and the nucleus is shielded by electrons, the cross-section for
with the nucleus in such a way that it is absorbed and re-
the production of electron-positron pairs is given by:
emitted with the same energy [12].
(cid:3)pair=4(cid:2)re2Z2(cid:4)(cid:5)(cid:8)97lnZ118/33(cid:1)514(cid:6)(cid:7)(cid:9). (28) Inelastic Nuclear Scattering
The nucleus of an atom is excited to a higher energy level
In this case, the fact that the nuclear charge is covered by by the absorption of a photon. The excited nucleus then
electrons must be taken into account. For high energy releases another photon with equal or smaller energy than the
photons, the cross-section for the production of electron- first one [12].
positron pairs tends to a value that is independent of the
Delbrück Scattering
energy of the incident photon, as it can be seen in equation
(28). As the term 1/54 in equation (28) is far smaller than the The phenomenon of scattering of a photon by the electric
other, it can be disregarded, giving the following appro- field of the nucleus is called Delbrück scattering. This
ximated equation: phenomenon can be understood as the formation of a virtual
electron-positron pair in the field of the nucleus followed by
(cid:3)pair (cid:1) 974(cid:2)re2Z2lnZ118/33. (29) its annihilation [17, 18].
6 Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 Rocha and Lanceros-Mendez
1.3.5. Total Cross Section of Absorption of Photons medium. There are several types of detectors, namely: pn
junctions, photoconductors, based on thermal effects and
The mass absorption coefficient, which is related with the
scintillators. Each method has its own advantages and dis-
cross-sections for the interaction processes, according to
advantages.
equation (7), is shown in Fig. (4) for cesium iodide. In this
figure, it can be verified that in the energy range of interest
2.1. Detectors Based in pn Junctions
for X-ray detectors (represented in a gray background), the
most important processes of interaction are the photoelectric The simplest model of radiation detector is based on a pn
effect and Compton scattering. junction, usually implemented in amorphous silicon
substrates. X-ray photons that are absorbed in the depletion
Since Compton scattering has a special importance in the
region of the junction create electron-hole pairs, which are
interactions between photons and electrons, due to the fact
separated by the electric field that exists in this region.
that only a portion of the energy is transferred between them,
Amorphous silicon is often preferred instead of crystalline
the mass attenuation coefficient and the mass absorption
silicon in high-area detectors (up to 50cm(cid:1)50cm) for
coefficient must be distinguished [8]. The mass attenuation
digital radiography. This is mainly due to the enormous
coefficient (μ ) is related to the cross section of the energy
cs difficulty in manufacturing devices of those dimensions in
scattering of Compton ((cid:1) ) (equations (22) and (7)). In an
cs crystalline silicon. The main advantage of this method is that
analogous way, the mass absorption coefficient (μ ) is
ca a little more than 1 eV of energy is enough to produce an
calculated from the cross-section of energy absorption of
electron-hole pair. Unfortunately, the X-ray absorption
Compton ((cid:1) ) (equations (23) and (7)).
ca power of silicon is much reduced. For example, a silicon
The mass absorption coefficient of a composite or detector with thickness of 525 μm would only absorb
mixture of elements can be calculated from the individual approximately 2.2% of 100 keV X-rays. In order to increase
coefficients of the elements from: the absorption to near 50%, the same detector must have 16
mm of thickness, which is not practical.
(μ/(cid:2))c=(cid:1)wi(μ/(cid:2))i (30)
i
2.2. Detectors Based in Photoconductors
where w represents the ratio of mass of element i in the
i
The photoconductor method uses materials with higher
composite or the mixture. In the example of Fig. (4), iodine
absorption power than silicon and whose conductivity
with atomic number 53 and relative atomic mass of 126.9
changes with the amount of absorbed radiation.
contributes with a ratio of mass of 48.8%, whereas cesium,
with atomic number 55 and relative atomic mass of 132.9, The best known photoconductor is probably amorphous
contributes with a ratio of mass of 51.2%. selenium. When compared with silicon, its X-ray absorption
is a little better (the thickness of 525 μm would absorb
2. RADIATION DETECTORS approximately 12.7% of 100 keV X-rays), but it needs about
50 eV for producing an electron-hole pair. Another disad-
The output signal of a radiation detector results from the
vantage is that it needs a relatively high bias voltage (about
ionization and/or excitation of atoms produced by the
10 V/μm) to work properly [19].
previously described interactions, which occur in the detec-
tion medium. The output signal of the detector can be pro- Other promising photoconductors are based on CdTe,
duced by primary ionizations, thermal changes or scintil- CdZnTe, HgI and PbI [20 - 22]. Due to the fact that they
2 2
lations that afterward are converted into electric signals. are constituted of heavy elements, they have high X-ray
Usually, the output electric signal must be close to a value absorption power. However, they show almost the same
proportional to the energy that falls upon the detection disadvantages as amorphous selenium.

Fig. (4). Mass absorption coefficients of cesium iodide. The energy range of interest for X-ray detectors based in scintillators is represented
in a gray background.
X-ray Detectors Based on Scintillators and CMOS Technology Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 7
2.3. Detectors Based in Thermal Effects This method, being relatively simple, has some limita-
tions in spatial resolution, due to the restrictions imposed by
The working principle of this kind of detector is based on
the height of the scintillator. On one hand, it is desirable to
the Seebeck effect. It consists of the detection of the
have a a large height of the scintillator layer, so that more X-
temperature increase caused by the absorption of X-rays by a
ray photons are absorbed. On the other hand, when
material of high atomic number and high density. The junc-
increasing the height of the scintillator, the amount of light
tion of two thermoelectric materials, with different Seebeck
produced in one pixel that reaches its neighbors is higher,
coefficients, is used to detect the temperature increase. Fig.
that is, the spatial resolution decreases [24].
(5) shows the basic structure of such detector. As target,
copper can be used and as thermoelectric materials, Sb Te A method to increase the spatial resolution, without
2 3
of p type and the Bi Te of n type materials can be used [23]. decreasing the height of the scintillator, consists of using an
2 3
individual scintillator for each pixel, separated by layers of a

reflector material, as is shown in Fig. (7).

Fig. (5). Structure of an X-ray detector based in thermal effects.

This method does not need bias voltages, as the previous
ones, since the output signal appears as an electric voltage. Fig. (7). Schematic representation of a structure based on light
As main disadvantage, it can be pointed out the frequency guides that are used to increase the spatial resolution of the detec-
response, which is very reduced, of the order of tenths of Hz, tors of radiation based on scintillators.
or at most of some Hz.

This method will be detailed in the following sections.
2.4. Detectors Based in Scintillators
This method uses a material that absorbs the radiation 3. X-RAY DETECTORS BASED ON SCINTILLATORS
and converts its energy into visible light, which is then
X-ray detectors based on scintillators, firstly convert X-
detected by a conventional photodetector. A good scintillator
ray energy into visible light, which is then converted into
must be constituted by chemical elements of high atomic
electrical signals by means of photodetectors. Fig. (8) shows
number and have high density, in order to absorb the X-rays.
a schematic diagram of a detector of this kind, where the
It must further produce a high number of visible light
scintillator is the thallium doped cesium iodide (CsI:Tl),
photons for each absorbed X-ray photon.
which is embedded inside reflective layers of aluminum.
This approach seems to be the best, since scintillators Aluminum is a material of low atomic number and low
have high X-ray absorption power, they do not need bias density, allowing the penetration of X-rays. On the other
voltages and they have very high frequency responses hand, it shows high reflectivity for visible light, guiding it to
(usually in the GHz range). the corresponding photodetector.
Fig. (6) shows the structure of an X-ray image detector based The most critical steps of this process that can influence
on scintillators. It consists basically of a scintillating layer the efficiency and the signal to noise ratio of the detector are:
placed on top of a matrix of photodetectors.

Fig. (8). Schematic diagram of an X-ray imaging detector based in
Fig. (6). Basic structure of a X-ray detector based on scintillators. scintillators.
8 Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 Rocha and Lanceros-Mendez
• Transmission of the X-rays through the aluminum
reflective layer.

• Absorption of X-rays by the scintillator and their

conversion into visible light.

• Reflection of visible light by the reflective layers.

• Transmission of visible light to the photodetector.

• Detection of visible light and its conversion into
electrical signals.
Some considerations must be taken into account before
the explanation of each step:

1. The most common noise sources in X-ray imaging

detectors are the photon noise, the fixed pattern noise
and the noise of the readout electronics [25]. Fixed
pattern noise is present in all detector matrices and can
Fig. (10). Signal to noise ratio of the radiation produced by an X-
be cancelled by using gain maps [26]. The readout
ray tube with a molybdenum anode, when powered by 35 kV, 1
electronic noise depends mainly on the electronic con- mA. The signal is measured by a detector of 1 mm2 area, placed at
figuration and usually is less important than the photon
0.5 m from the anode.
noise.

2. The photon noise, caused by the quantized nature of X- As mentioned before, the photons coming from the X-ray
rays, is the fundamental limitation of the signal to noise tube must follow several steps before being detected. These
ratio of the detector. Several theoretical and experi- steps will be described in the following sections.
mental analyses demonstrate that the intrinsic photon
noise of counting the events from an X-ray beam is 3.1. Transmission of the X-Rays Through the Aluminum
random and follows a Poisson distribution, that is, the Reflective Layer
standard deviation, (cid:1) , is equal to the square root of the
prx
When an X-ray beam penetrates some material, it is
mean number of detected photons, m [27, 28], i. e.:
prx
absorbed according to equation (5). Fig. (11) shows the X-
(cid:1)prx= mprx (31) rays percentage transmitted by aluminum layers of different
thicknesses, between 10 μm and 500 μm. As it can be
Fig. (9) shows the spectral distribution of the photons observed, there is a reduction of the transmissivity with
produced by an X-ray tube with molybdenum anode, when it increasing the thickness of the layer.
is powered by a 35 kV, 1 mA source, measured by a detector
of 1 mm2 of area, placed at 0.5 m from the anode [29].

Fig. (11). Transmissivity to X-rays of different aluminum

thicknesses.

Fig. (9). Spectrum of the radiation produced by an X-ray tube with 3.2. Absorption of X-Rays by the Scintillator and Their
molybdenum anode, powered at 35 kV, 1 mA. The signal is mea- Conversion Into Visible Light
sured by a detector of 1 mm2 area, placed at 0.5 m from the anode.
The absorption of X-rays by the scintillator depends on
the laws described in previous sections. However, scintil-
Fig. (10) shows the corresponding signal to noise ratio, lators are formed by composites or mixtures of some
which is calculated, for each energy, as the ratio between the elements, therefore, the mass absorption coefficient must be
mean number of photons and its standard deviation, for a calculated from the individual elements that form the
unit exposure time. composite, using equation (29). As was seen for CsI:Tl,
X-ray Detectors Based on Scintillators and CMOS Technology Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 9
iodine with atomic number 53 and relative atomic mass of This result shows that the variance (equation (33)) is
126.9 contributes with a fraction of mass of 48.8%, whereas superior to the mean (equation (32)) and the number of
cesium, with atomic number 55 and relative atomic mass of visible photons does not follow a Poisson distribution
132.9, contributes with a fraction of mass of 51.2%. anymore. The difference between the variance and the mean
Thallium concentration in the CsI:Tl is of about 0.02% to of the distribution is known as Swank noise [32, 33].
0.03% [30], and its contribution for the absorption of rays-x
The signal to noise ratio can be calculated, for each value
is negligible.
of energy, by:
Fig. (12) shows the percentage of X-rays that is absorbed
L (E)
by CsI:Tl scintillators with thicknesses between 100 μm and SNR(E)= R = N(cid:1)Tral(cid:1)Absc . (34)
900 μm. As expected, the absorption increases with thick- (cid:2)L2R(E)
ness.
Fig. (13) shows the mean of the number of photons per

second, produced by the scintillator, for the input signal of
Fig. (9).

Fig. (12). Absorption of X-rays by CsI:Tl scintillators of different

thicknesses.

The scintillator will convert X-rays into visible light. In
Fig. (13). Number of visible photons produced per second in the
the case of CsI:Tl, about 65 900 visible photons (wavelength
scintillator as a function of the incident X-ray energy.
close to 560 nm) are produced for each 1 MeV of absorbed
energy, at room temperature [31]. This means that for each
Fig. (14). shows the corresponding signal to noise ratio,
photon of 1 MeV, a random number of visible photons is
obtained from equation (34).
produced, whose mean is 65 900.

Therefore, the average amount of visible light produced,
LR(E), at a given X-ray energy, for a unit time, is obtained
from the product of five factors:

1. Number of X-ray photons that fall upon the detector, N,

which is random,

2. Transmissivity of aluminum, Tr Fig. (11),
al

3. Absorption of the scintillator, Ab Fig. (12),
sc

4. 65 900 photons/MeV, which is the mean of a random
process,
5. Energy of the incident X-ray photon, E, in MeV.
That is:
LR(E)=N(cid:1)Tral(cid:1)Absc(cid:1)65900(cid:1)E. (32)
Fig. (14). Signal to noise ratio associated to the visible photons
In this case, the variance of the produced visible photon
produced by the scintillator.
distribution is given by the product of the average value of
X-ray photons absorbed by the scintillator by the square of
the number of visible photons produced by each X-ray:: By comparing the signal to noise ratio of the scintillator
Fig. (14) with the one of the X-ray tube Fig. (10), it can be
(cid:2)L2R(E)=N(cid:1)Tral(cid:1)Absc(65900E)2 (33) observed that this is practically constant.
10 Recent Patents on Electrical Engineering, 2011, Vol. 4, No. 1 Rocha and Lanceros-Mendez
With this result, the first step of the working principle of values of E and H of one side of the film with the ones of the
the X-ray detector based in scintillators is completely other side. Here, the generalized refractive index of the film
characterized. Walter et al. described the applications of an is represented by u and g is the phase thickness of the film,
i i
X-ray detector in 2010 [34]. given by:
3.3. Reflection of Visible Light by the Reflective Layers gi=2(cid:1)uidcos(cid:2)i , (39)
(cid:3)
The visible light that reaches the reflector can have two
where d is the thickness of the film and (cid:1) is the angle of
independent polarizations. The light that reaches the reflector i
incidence of the light, given by the Snell law:
in an angle (cid:1), measured from the normal, can have either the
electric field vector or the magnetic field vector parallel to uscsin(cid:1)sc=uisin(cid:1)i=uwsin(cid:1)w. (40)
the plane of incidence. In the first case, the polarization is
called p and in the second case, it is called s. In general, the In this case, the reflectivity is given by:
electric field vector has an arbitrary angle with the incidence
2
plane and can be decomposed into two components, one of R= r2= Eq(cid:1)/Eq+ (41)
polarization p and another one of polarization s.
In the case of the CsI:Tl scintillator, the light produced
The refractive index, n, is equal to H/E, where H and E
has a wavelength around 560 nm [31]. The dependence
represent the amplitudes of magnetic and electric fields
between the reflectivity and the angle of incidence of the
respectively. In a similar way, a generalized refractive index,
light, for an aluminum reflector, is shown in Fig. (15).
u, can be defined for each polarization:

up=H/(Ecos(cid:1))=n/cos(cid:1)
us=Hcos(cid:1)/E=ncos(cid:1). (35)

Notice that the directions of electric fields of the light
produced by scintillators are random and can be decomposed
into the two polarizations with equal probability.

In the case of a homogeneous metallic reflector, the

reflectivity of the interface between scintillator and reflector
is given by:
2
R= usc(cid:1)ur , (36)
usc+ur

where u and u represent the generalized refractive indexes
sc r
of the scintillator and the reflector respectively. In the case of
materials that absorb light, the value of the refractive index, Fig. (15). Dependence between the reflectivity of the visible light
n of equation (35), has an imaginary component and must be and the angle of incidence, for a CsI:Tl - Aluminum interface.
substituted by n(cid:1)jk, where k is the extinction coefficient of
the material and j is the complex operator ( (cid:1)1). 3.4. Transmission of the Visible Light to the
Photodetector
In a more generic case of a reflector with multiple Due to the differences of refractive indexes between the
interfaces (for example, when a thin film of silicon dioxide is scintillator and the photodetector, some light produced by the
placed between the scintillator and a silicon wall) [35 - 38], first one is reflected in the surface of the second one. In order
the reflectivity can be calculated from [39]: to minimize this phenomenon, an anti-reflective filter can be
necessary. The simplest way to obtain such filter consists in
(cid:2)(cid:3)(cid:6)(cid:6)EEqq+(cid:1)(cid:4)(cid:5)(cid:7)(cid:7)=12(cid:2)(cid:3)(cid:6)11 (cid:1)11//uusscc(cid:4)(cid:5)(cid:7)Mi,L,M1(cid:2)(cid:3)(cid:6)u1w(cid:4)(cid:5)(cid:7)Eo+ (37) tdheete catpoprl.i cTahtieo np aorfa ma ettheirns ofifl mth ein f itlhme csuanrf abcee oobft atihnee dp hfrootom-
equation (42), which was deduced from equation (37):
ewlhecetrrei,c Efiq+el,d Evq(cid:1)ec, toanrsd oEf o+t hree pinrecsiednetn tt,h ere falmecptelidtu daensd otfr anthse- Es+c=2us1cutf (cid:1)(cid:2)(cid:5)(uscutf+utfuph)cosgtf+j(uscuph+ut2f)singtf(cid:3)(cid:4)(cid:6)E+ph,
mitted waves, respectively. Variable u and u represent the
generalized refractive indexes of the scscintillatowr and the wall Es(cid:1)c=2us1cutf (cid:2)(cid:3)(cid:6)(uscutf (cid:1)utfuph)cosgtf +j(uscuph(cid:1)ut2f)singtf(cid:4)(cid:5)(cid:7)E+ph,
respectively, and:
(42)
(cid:1) cosg jsing /u (cid:3)
Mi=(cid:5) i i i(cid:6) (38) where E+ is the amplitude of the electric field vector of the
(cid:2)juisingi cosgi (cid:4) incident slcight (coming from the scintillator), E(cid:1) is the
sc
is a matrix that contains the details of thin film i, placed amplitude of the electric field vector of the light reflected at
between the scintillator and the wall. Basically, M relates the
i