Some aspects of the energy exchange in an ionizing particle track for organic solid detectors
This work studies the radioluminescence energy yield of organic solid scintillators. The energy that is necessary to produce a scintillation photon is calculated for different ionizing radiations. The analysis of the results is based on the study of the physical processes proceeding during the early...
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| Zitieren: | Some aspects of the energy exchange in an ionizing particle track for organic solid detectors / N.Z. Galunov, O.A. Tarasenko // Вопросы атомной науки и техники. — 2014. — № 5. — С. 83-90. — Бібліогр.: 30 назв. — англ. |
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irk-123456789-804892015-04-19T03:02:41Z Some aspects of the energy exchange in an ionizing particle track for organic solid detectors Galunov, N.Z. Tarasenko, O.A. Ядерно-физические методы и обработка данных This work studies the radioluminescence energy yield of organic solid scintillators. The energy that is necessary to produce a scintillation photon is calculated for different ionizing radiations. The analysis of the results is based on the study of the physical processes proceeding during the early stages of the ionizing particle energy exchange. It bases on the concept of a decisive influence of the polarization, which appears in an organic molecular medium, on the recombination of hot charge carriers that results in reduction of the radioluminescence energy yield. The possible reasons of the energy losses are also analyzed. Исследуется энергетический выход радиолюминесценции органических твердотельных сцинтилляторов. Проведен расчет энергии, которая расходуется на создание одного фотона сцинтилляционного импульса при разных видах возбуждения. Анализ результатов основывается на изучении физических процессов, протекающих на ранних стадиях размена энергии ионизирующей частицы. В основе этого анализа лежит концепция об определяющем влиянии поляризации органической молекулярной среды на рекомбинацию горячих носителей заряда, что приводит к уменьшению энергетического выхода радиолюминесценции. Анализируются возможные причины энергетических потерь для ионизирующих излучений разных типов. Дослiджується енергетичний вихiд радiолюмiнесценцiї органiчних твердотiльних сцинтиляторiв. Проведено розрахунок енергiї, яка витрачається на створення одного фотона сцинтиляцiйного iмпульсу при рiзних видах збудження. Аналiз результатiв арунтується на вивченнi фiзичних процесiв, якi вiдбуваються на раннiх стадiях розмiну енергiї iонiзуючої частинки. В основi цього аналiзу лежить концепцiя про визначальний вплив поляризацiї органiчного молекулярного середовища на рекомбiнацiю гарячих носiїв заряду, що спричинює зменшення енергетичного виходу радiолюмiнесценцiї. Аналiзуються можливi причини енергетичних втрат для iонiзуючих випромiнювань рiзних типiв. 2014 Article Some aspects of the energy exchange in an ionizing particle track for organic solid detectors / N.Z. Galunov, O.A. Tarasenko // Вопросы атомной науки и техники. — 2014. — № 5. — С. 83-90. — Бібліогр.: 30 назв. — англ. 1562-6016 PACS: 29.40.Mc, 72.20.Jv, 77.22.Ej http://dspace.nbuv.gov.ua/handle/123456789/80489 en Вопросы атомной науки и техники Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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Ядерно-физические методы и обработка данных Ядерно-физические методы и обработка данных |
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Ядерно-физические методы и обработка данных Ядерно-физические методы и обработка данных Galunov, N.Z. Tarasenko, O.A. Some aspects of the energy exchange in an ionizing particle track for organic solid detectors Вопросы атомной науки и техники |
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This work studies the radioluminescence energy yield of organic solid scintillators. The energy that is necessary to produce a scintillation photon is calculated for different ionizing radiations. The analysis of the results is based on the study of the physical processes proceeding during the early stages of the ionizing particle energy exchange. It bases on the concept of a decisive influence of the polarization, which appears in an organic molecular medium, on the recombination of hot charge carriers that results in reduction of the radioluminescence energy yield. The possible reasons of the energy losses are also analyzed. |
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Article |
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Galunov, N.Z. Tarasenko, O.A. |
| author_facet |
Galunov, N.Z. Tarasenko, O.A. |
| author_sort |
Galunov, N.Z. |
| title |
Some aspects of the energy exchange in an ionizing particle track for organic solid detectors |
| title_short |
Some aspects of the energy exchange in an ionizing particle track for organic solid detectors |
| title_full |
Some aspects of the energy exchange in an ionizing particle track for organic solid detectors |
| title_fullStr |
Some aspects of the energy exchange in an ionizing particle track for organic solid detectors |
| title_full_unstemmed |
Some aspects of the energy exchange in an ionizing particle track for organic solid detectors |
| title_sort |
some aspects of the energy exchange in an ionizing particle track for organic solid detectors |
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
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2014 |
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Ядерно-физические методы и обработка данных |
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http://dspace.nbuv.gov.ua/handle/123456789/80489 |
| citation_txt |
Some aspects of the energy exchange in an ionizing particle track for organic solid detectors / N.Z. Galunov, O.A. Tarasenko // Вопросы атомной науки и техники. — 2014. — № 5. — С. 83-90. — Бібліогр.: 30 назв. — англ. |
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Вопросы атомной науки и техники |
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AT galunovnz someaspectsoftheenergyexchangeinanionizingparticletrackfororganicsoliddetectors AT tarasenkooa someaspectsoftheenergyexchangeinanionizingparticletrackfororganicsoliddetectors |
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2025-07-06T04:30:05Z |
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2025-07-06T04:30:05Z |
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1836870487253188608 |
| fulltext |
SOME ASPECTS OF THE ENERGY EXCHANGE IN AN
IONIZING PARTICLE TRACK FOR ORGANIC SOLID
DETECTORS
N.Z.Galunov, O.A.Tarasenko∗
Institute for Scintillation Materials, STC “Institute for Single Crystals”, National Academy of
Sciences of Ukraine, 60 Lenin Avenue, 61001, Kharkov, Ukraine
(Received May 16, 2014)
This work studies the radioluminescence energy yield of organic solid scintillators. The energy that is necessary to
produce a scintillation photon is calculated for different ionizing radiations. The analysis of the results is based on
the study of the physical processes proceeding during the early stages of the ionizing particle energy exchange. It
bases on the concept of a decisive influence of the polarization, which appears in an organic molecular medium, on
the recombination of hot charge carriers that results in reduction of the radioluminescence energy yield. The possible
reasons of the energy losses are also analyzed.
PACS: 29.40.Mc, 72.20.Jv, 77.22.Ej
1. INTRODUCTION
When employing scintillation detectors, it is im-
portant to know their response to ionizing radiations
with different specific energy loss (dE/dx). This
problem becomes particularly topical for organic scin-
tillators, which are efficient detectors of ionizing ra-
diations with high values of dE/dx (alpha particles,
protons, heavy ions) [1, 2, 3, 4]. These types of radi-
ations create high densities of radiation excitation in
an organic scintillator and are the most dangerous to
human life [5].
Since the 60-ies of the XX century, it was known
that the conversion efficiency of organic scintillators,
or in other words the radioluminescence energy yield
decreases with increasing dE/dx of an ionizing parti-
cle [1, 6, 7]. This process characterized by the energy
losses became known as the ”specific quenching” [1]
due to a misapprehension of the mechanisms which
cause it. We studied this problem since 2008. Firstly
we investigated the dependence of the scintillation re-
sponse of organic scintillators against dE/dx-values
of ionizing radiations [2, 3, 4, 8, 9, 10, 11]. These re-
sults allow us to propose the general description (the
one-step model) of the ionizing particle energy ex-
change that takes into account the polarization pro-
cess as the main factor in recombination of hot charge
states in tracks and spurs generated by a primary
particle [8, 9, 10]. In this model, as a ”step” we
assumed a single act of a hop of a charge carrier,
in other words, the particle track expansion on the
one intermolecular distance. The initial recombina-
tion of hot charge states, initiated by the polariza-
tion, was considered as a faster process. Actually, a
sudden decrease of pairs of hot charge states in the
”frozen” track was investigated. It should be noted
that the parameters of this description that were ob-
tained by fitting the experimental data quite accu-
rately describe the quenching process in the ionizing
particle track.
In this paper we discuss the physical processes
that can determine the energy losses in the track re-
gions for the ionizing radiations with low and high
values of dE/dx. Our assumptions are based on the
detail analysis of the radioluminescence energy yield
of organic scintillators and on the concept of the de-
termining influence of polarization interactions on the
recombination of charge states in the particle track.
Organic single crystals of anthracene, stilbene
and p-terphenyl, and also the plastic scintillator on
the base of polystyrene are the objects of the inves-
tigation. It should be noted that recently we have
developed the new types of organic scintillation de-
tectors, namely, organic polycrystals and composite
scintillators [2, 3, 4]. Microcrystalline grains are scin-
tillating material in these types of detectors. It was
shown that their scintillation efficiency depends on a
size of these grains. Such a detector is effective when
the track length of an ionizing particle (in the case
of a fast neutron, e.g., it is the track of a recoil pro-
ton) does not exceed the linear dimensions of a single
grain [4, 9]. A generality of dielectric, optical and
scintillation properties of the new types of detectors
and classical organic single crystals makes it possible
to extend the physical processes under consideration
on a wider range of organic scintillators.
∗Corresponding author E-mail address: tarasenko@isma.kharkov.ua
ISSN 1562-6016. PROBLEMS OF ATOMIC SCIENCE AND TECHNOLOGY, 2014, N5 (93).
Series: Nuclear Physics Investigations (63), p.83-90.
83
2. THEORY
The ionizing radiations with low values of dE/dx
(< 101 MeV/cm) such as photons of gamma radia-
tion, electrons with energies above 100 eV create sep-
arate local regions of high activation density. Such
regions are called spurs. A typical spur may con-
sists of one or several pairs of charge states, several
excited states. The ionizing radiations with values
of dE/dx ≥ 101 MeV/cm (protons, alpha particles,
heavy ions) generate the spurs those overlap and form
a single region of high activation density, which is
known as the primary particle track [2, 10, 12].
During the excitation by an ionizing particle in
each a single event of interaction with a medium this
particle mainly transmits such a portion of it energy,
which exceeds the ionization potential of molecules.
The interaction of an ionizing particle with a medium
can be described in terms of successive ”collisions” of
the particle with molecules of the medium [2, 10, 12].
The direct knockout of an electron occurs during the
knock-on activation of a molecule. The kinetic energy
of such an electron is sufficient for ionization and exci-
tation. Secondary high-energy electrons (100 eV and
above [2, 12]) called δ-electrons form short tracks.
During the glancing activations of the molecules of
a medium the events of ionization proceed through
a rapid (10–16...10–15 s) intermediate stage of a for-
mation of short-lived superexcited [13] and plasmon
states [2, 9, 10, 14].
U. Fano [15] advanced the theory of the existence
of plasmons as delocalized many-particle states. He
considered organic molecular systems as the most ob-
vious example of systems in which the generation of
plasma oscillations was possible. The experimental
evidence of the existence of plasmons as quanta of
excited valence electrons is based on the measure-
ments of the spectra of characteristic energy loss of
fast electrons in organic polymers and single crystals
[16, 17]. According to the experimental data the av-
erage energy of plasmons in organic condensed media
is estimated as ⟨Epl⟩ ∼ 20 eV [2, 10, 14, 15].
Superexited states arise from a decay of plasmons
[13]. In its turn, they decay into pairs of charge car-
riers. Further recombination of charge pairs can lead
to molecular excitation and then, to a luminescence
of molecules of an organic scintillator [2, 14].
In a molecular organic system, which consists of
initially neutral molecules, the polarization energy
that is necessary to produce a molecular polaron by a
factor of 102 is greater than the energy of intermolec-
ular interaction. The time of the electronic, molecu-
lar and in some cases of the lattice polarization for-
mation is less than the time of electronic excitation
energy transfer in an organic crystal [2, 18]. There-
fore ionizing radiations generate quasi-free electrons
in an organic molecular medium [1]. Such the elec-
tron rapidly localizes on any one molecule and pro-
duces a negative quasi-ion M−. The molecule, which
has lost an electron, in its turn, becomes a positive
quasi-ion M+. The polarization time of the electron
orbitals of the molecules of an organic crystal is esti-
mated as τe ≈ 10–16...10–15 s [18, 19]. The molecular
π-orbitals of the neighbouring nonionizing molecules
have a negative charge. They attract to the positive
quasi-ion M+, but their skeletons repel from it. The
polaron M+
p is formed. In the case of the negative
quasi-ion M− the molecular π-orbitals of the neigh-
bouring molecules repel from it, and the skeletons of
these molecules attract to it. The polaron M−
p is
formed.
According to [18], the polarization surrounding,
which originate around a quasi-ion, includes about
103 molecules. For example, for the organic crystals
of polyacenes the radius of the stable polarization
surrounding rc around a surplus charge is equal to
13...16 nm [19]. The value of rc is defined as [2, 19]:
rc =
e2
4π⟨ε⟩ε0kT
, (1)
where ⟨ε⟩ is an average relative permittivity of a crys-
tal, k is the Boltzmann constant, T is a temperature,
and ε0 is the dielectric constant. In the case when the
distance between the polarons M−
p and M+
p is less
than rc these polarons form the molecular-polaron
pair, or bipolaron (M−
p , M+
p ) [2, 18, 19].
For a very strong external electrostatic field
(E ≈ 106 V/cm) the scattering time τ of a
charge carrier becomes very small value, estimated
as 10–14 s. For example, according to the results of
the direct calculations of the drift velocity of charge
carriers in an anthracene crystal based on the experi-
mental data, it was obtained τ = 8 · 10–14 s [19]. The
value of τ is comparable with the time of molecular
polaron formation [18]. In [20] it was shown that the
carrier, whose initial motion is opposite to the field of
the parent ion, is thermalized in the time no less than
2 · 10–13 s. Therefore, the processes occurring after
generation of charge carriers in a time comparable or
shorter than 10–13 s are the processes involving ”hot”
charge states [2, 14, 19].
The action of the external electric field on the
transport and recombination of excess hot states is
equivalent to the action of the internal local field cre-
ated by a bipolaron (M−
p ,M+
p ). The value of this
field depends on the initial distance R0 between M−
p
and M+
p . Table 1 shows the calculated values of the
local field strength Eloc created by the pair of polaron
states M−
p and M+
p , which are located on the lattice
sites of anthracene crystal. The value of ε=3.2 [18]
was used. Table 1 shows that the values Eloc corre-
spond to the situation of strong external fields, which
promote a major decrease in the charge carrier scat-
tering time τ .
Thus, the polarization mechanisms are able to
accelerate the process of transport and recombina-
tion of hot charge states. They should be taken
into account when describing the energy exchange
processes, especially for the case of high densities of
radiation excitation.
84
Table 1. Calculated values of the local field strength Eloc created by the pair of polaron states M−
p and M+
p
in anthracene crystal
Position of M−
p R0,
◦
A1 Eloc, 10
6 V/cm Position of M−
p R0,
◦
A1 Eloc, 10
6 V/cm
in a lattice1,2 in a lattice1,2
(1/2,1/2,0) 5.24 32.4 (0,2,0) 12.07 6.1
(1/2,–1/2,0) 5.24 32.4 (3/2,3/2,0) 15.72 3.6
(0,1,0) 6.04 24.4 (3/2,–3/2,0) 15.72 3.6
(1,0,0) 8.56 12.2 (2,0,0) 17.12 3.0
(1,1,0) 10.48 8.1 (2,2,0) 20.95 2.0
(1,–1,0) 10.48 8.1 (2,–2,0) 20.95 2.0
(0,0,1) 11.18 7.1 (0,0,2) 22.37 1.8
1 – according to [21],
2 – polaron M+
p is on (0,0,0) lattice site.
3. EXPERIMENTAL
We investigated the single crystals of stilbene,
anthracene and p-terphenyl, the plastic scintillator
on the base of polysterene. The single crystals
of p-terphenyl containing 1,4-diphenyl-1,3-butadiene
(0.1% in the melt) as an addition agent were stud-
ied as well. The cylindrical samples had a thick-
ness of 5 mm. To obtain a wide range of dE/dx we
used the medium energy photons of gamma radiation
(22Na, 60Co, 137Cs, and 152Eu radionuclide sources;
Eγ ∼ 105...106 eV), conversion electrons from a 137Cs
source (Ee=0.622 MeV), alpha particles in the en-
ergy range from 0.7 to 7.5 MeV, fast neutrons from a
239Pu-Be source. The amplitude scintillation spectra
of the samples were measured by a multichannel am-
plitude analyzer AMA-03F. To separate the spectrum
of recoil protons generated by fast neutrons from a
239Pu-Be radionuclide source in an organic scintilla-
tor, we used the method of discrimination of an ion-
izing radiation by a scintillation pulse shape [2].
The photoluminescence energy yield Yph is defined
as [22]:
Yph = Φph × λex
λav
em
, (2)
where Φph is the absolute photofluorescence quantum
yield, λex and λav
em are the excitation wavelength and
the average wavelength of an emission spectrum, re-
spectively.
The average value of the radioluminescence en-
ergy yield for a particle with the initial energy E0
crossing the maximum range in a scintillator is equal
to [22]:
⟨Yr⟩ =
1
E0
∫ E0
0
Yr(E)dE =
L
E0
, (3)
where L is the total energy of the radioluminescence
photons.
Let us use the following notations: Eph(λ
av
em) is
the average energy of a luminescence photon, Ei is
the excitation energy, Yi is the luminescence energy
yield, and δi is the average energy that is necessary to
produce a luminescence photon. The suffix i means
the excitation of i type. Below by ph, γ, e, n, and α we
denote the following types of excitation i: light pho-
tons, photons of gamma radiation, conversion elec-
trons, neutrons and alpha particles, respectively. The
symbol of averaging will be omitted.
The value of δi is equal to
δi =
Eph(λ
av
em)
Yi
. (4)
According to (3) and (4) it is necessary to mea-
sure the total number of photons in a scintillation
pulse to obtain the values of Yi and δi for i type of
excitation with the energy Ei.
The aspects of measuring the amplitude scintilla-
tion spectra, calculating the light yield of the organic
scintillators and the number of photons in a scintilla-
tion pulse for i type of excitation with the energy Ei
were previously described [11, 22]. The values of Fph
and Eph(λ
av
em) were calculated in [22] as well.
Table 2 shows the calculated values of Eph(λ
av
em),
Fph, Yi, and δi for the photoexcitation and for the
excitation by the ionizing radiations with low dE/dx
(photons of gamma radiation and conversion elec-
trons).
According to Table 2, even in the case of the exci-
tation by the ionizing radiations with low dE/dx the
values of the energy yield Yγ and Ye are 10...20 times
larger, or, respectively, the values of δγ and δe are
10...20 times smaller than in the case of the excita-
tion by light photons of the visible range. It should
be noted that the values Yγ and Ye (δγ and δe) for the
same type of a scintillator are not practically differ.
According to Eq. (3), the values of the radi-
oluminescence energy yield for the cases of excita-
tion by fast neutrons and alpha-particles were de-
scribed as Yn = Pn × Eph(λ
av
em)/En and Yα = Pα ×
Eph(λ
av
em)/Eα, where Pn and Pα, respectively, the
number of scintillation photons for the neutron and
alpha-particle excitations. The values of Yn for the
organic solid scintillators under investigation are in
the range from 0.012 to 0.040, and the values of Yα
ranges from 0.0012 to 0.0095. Moreover the scintilla-
tion response for excitation by the ionizing radiations
with high values of dE/dx is nonlinear.
85
Table 2. Values of Eph(λ
av
em), Φph, Yi, and δi (for i = ph, γ and e)
Scintillator Eph(λ
av
em), eV Φph Yph δph, eV Yγ δγ , eV Ye δe, eV
Stilbene 3.08 0.65 0.55 5.60 0.045 68.03 0.042 72.93
p-Terphenyl 3.13 0.48 0.41 7.63 0.053 58.82 0.052 59.83
(undoped)
p-Terphenyl (0.1% of
1,4-diphenyl-1,3-butadiene) 2.88 0.97 0.77 3.74 0.072 40.16 0.066 43.52
butadiene)
Anthracene 2.71 0.55 0.50 5.42 0.079 34.48 0.073 37.02
Polystyrene
(1.5% of p-terphenyl+0.02% 2.84 0.77 0.49 5.80 0.026 108.70 0.025 113.42
of POPOP)
Fig. 1 summarizes the data of the average energy
δi that is necessary to produce a scintillation photon
for i type of excitation as a function of dE/dx. The
values of dE/dx were calculated using the online pro-
grams ESTAR, PSTAR, and ASTAR of the National
Institute of Standards and Technology (NIST) [23].
The ranges of the δph-values and the δγ-values (see
Table 2) are indicated as well. In the case of neu-
tron excitation we calculated the dE/dx-values for
the maximum energy of recoil protons, which were
generated by the corresponding fast neutrons with a
set of energies En. The complicated dependence of
Yα for high values of dE/dx (for Eα
∼= 1 MeV) is
caused by the effect of an ion recharge [2, 11, 24].
10-1 100 101 102 103
101
102
103
electrons
protons
alpha particles
A
ve
ra
ge
E
ne
rg
y
pe
r P
ho
to
n
i, e
V
dE/dx, MeV/cm
light photons
gamma
ph
= 3.74 7.63
=34.48 108.7
Fig.1. The values of δi as a function of dE/dx of
the ionizing radiation. Squares, circles, triangles
and rhombus represent δi-values for the single crys-
tals of stilbene, p-terphenyl (undoped), p-terphenyl
containing 0.1% of 1,4-diphenyl-1,3-butadiene,
and anthracene, respectively. Stars denote the
polystyrene scintillator
The data presented in Fig. 1 indicate that further
increase in dE/dx from protons to alpha-particles
causes growing the δi-value by a factor of 15-30. At
first sight, this increase of the energy losses, or δi,
with growing dE/dx, seems paradoxical. Indeed, as
the value of dE/dx increases, the separate local re-
gions of the high activation density, or spurs, more
and more overlap and charge states, which are cre-
ated, unite in the single region of excitation. The
increase in the probability of recombination should
lead to the increase in the number of excited states,
and hence, to the increase in the total number of
photons in a scintillation pulse. However, we can see
from the experiment that the situation is radically
opposite.
4. DISCUSSION
A multi-stage nature of the conversion of the pri-
mary energy of an ionizing particle to the total energy
of photons of a scintillation pulse leads to much less
value of the radioluminescence energy yield in con-
trast the photoluminescence one. It is evident from
the above-mentioned results, these losses increase
with dE/dx of an ionizing particle. Let us consider
the possible physical processes proceeding during the
early stages of the ionizing particle exchange energy
that may be responsible for these losses for the cases
of low and high values of dE/dx, separately.
4.1. Low dE/dx
Ionizing radiations with low dE/dx create in a
medium separate local spurs, which consist of one or
several charge pairs [2, 12]. A secondary high-energy
electron (δ-electron), which is created in such a spur,
have a sufficient kinetic energy to escape from this
spur. Neighbouring spurs for the ionizing radiations
with low dE/dx do not overlap. Therefore, the prob-
ability that the electron, which has escaped from one
spur, reaches another one is extremely small. Most
likely, it will thermalize. This electron will not par-
ticipate in the process of charge state recombination,
and in a subsequent molecular luminescence. So, it
may be concluded that the electron escape from a
single spur can be a reason for the energy losses for
the ionizing radiations with low dE/dx. We estimate
the probability of this process on the example of the
polystyrene scintillator.
In the general case the total number of δ-electrons
in the energy range from Emin to Emax is equal to
[2]:
(Nδ)total =
2πnee
4
me
z2
v2
[
1
Emin
− 1
Emax
]
, (5)
where ne is an electron density in a medium, e is an
86
electron charge, ze and v are the charge and velocity
of a primary particle, respectively.
In the absence of an external electric field, δ-
electrons whose range is more than the radius of
the stable polarization surrounding rc (1) are able
to escape from a spur. For polystyrene rc is equal
to 15.4 nm [25]. According to Ashley [26], an elec-
tron with the energy Er=450 eV has such the range
in polystyrene. Let us take Er=450 eV as the lower
boundary of the electrons, which will escape from the
spur with the radius rc=15.4 nm. The ratio of the
number of δ-electrons with the energy Eesc ≥ Er to
the total number of δ-electrons range from Emin to
Emax gives a probability of the δ-electron escape Ξ.
From (5) it is easy to obtain that
Ξ =
Emax − Er
Emax − Emin
× Emin
Er
. (6)
Let us calculate the Ξ-value for the case of excita-
tion of an organic scintillator by conversion electrons
with the energy Ee=622 keV. This value corresponds
to the experiment. For the electron excitation we
have Emax = Ee = 622 keV. Fig. 2 presents the re-
sults of calculation for the case when Emin is varied
from 10 eV to Er=450 eV.
The commonly accepted lower boundary for δ-
electrons is equal to 100 eV [2, 12]. Taking into
account the polarization mechanisms we obtain the
value of Ξ ∼= 22% (see Fig.2) for Emin=100 eV. This
value may be used as the estimation for the analysis of
the effect of the δ-electron escape from a spur on the
energy losses in the ionizing particle track with low
dE/dx. A week distinction in the densities and elec-
tron ranges makes possible extending this estimation
to all organic solid scintillators under investigation.
0 100 200 300 400
20
40
60
80
100
E
min
, eV
Es
ca
pe
p
ro
ba
bi
lit
y
, %
Fig.2. The escape probability Ξ of δ-electrons from
a spur. See the text for details
In our opinion, the random recombination of
charge state pairs in a single spur is another pos-
sible cause of the energy losses occurring in the en-
ergy exchange process of an ionizing particle with low
dE/dx. In the spur with one pair the geminate re-
combination takes place when an electron recombines
with its parent ion. Such a recombination gives a
singlet exciton with a probability close to 100% [2].
The random recombination of charge states prevails
in spurs with several pairs [17]. The random recombi-
nation gives singlet (S) and triplet (T ) states in the
ratio 1 to 3. Taking into account the T − T anni-
hilation, the total yield of S-states is approximately
equal to 0.4 [27].
We will use the experimental data [28] for the
distribution of the energy absorbed in a spur of an
electron in water depending on the spur size. Ac-
cording to this data, 56% of the total energy of the
electron is used to produce the spurs with one or
two pairs of radicals and 44% is used for the spurs
containing three and more pairs. Let us extend this
result to the case of solid organic scintillators and as-
sume that only the geminate recombination proceeds
in spurs with one or two pairs, but the random re-
combination takes place in spurs with three and more
pairs. Considering both the geminate recombination
and the random one it is easy to obtain that the yield
of S-states is 0.736.
For the case of the excitation by conversion elec-
trons with the energy Ee=622 keV we will compare
the experimental value of the average energy de that
is necessary to produce a radioluminescence photon
(see Table 2) and the calculated value δcal taking into
account the above-discussed mechanisms of the en-
ergy losses in a single spur. If the energy δpair =
⟨Epl⟩ ∼= 20 eV is the average energy that is necessary
to create a pair of charge states then approximately
22% of δ-electrons would escape from spurs and will
not take part in the recombination process. It means
that, actually, the energy necessary for creation a re-
combining pair is δ∗pair=(1/0.78)δpair. Allowing for
the yield of S-states (0.736) we can obtain the energy
of creation of one S-state: δ∗S = δ∗pair/0.736. Dividing
the δ∗S-value by the absolute photofluorescence quan-
tum yield Φph we can obtain the calculated value of
the energy δcal that is necessary to produce a radiolu-
minescence photon. Table 3 demonstrates the results
of such the calculations. It should be noted that the
δcal-value (see Table 3) is an estimation value. These
calculations ignores possible distinctions in the values
⟨Epl⟩,Ξ, and in the yield of S-states for each individ-
ual scintillator.
Table 3. Values of δe and δcal for organic solid
scintillators
Scintillator δe, eV δcal, eV
Stilbene 72.93 53.60
p-Terphenyl (undoped) 59.83 72.58
p-Terphenyl (0.1% of 43.52 35.92
1,4-diphenyl-1,3-butadiene)
Anthracene 37.02 63.34
Polystyrene (1.5% of 113.42 45.25
p-terphenyl+0.02% of POPOP)
4.2. High dE/dx
An ionizing particle with high dE/dx forms a
track in which a density of charge states is high. In
such a track region the fast random recombination of
charge state pairs dominates. This recombination is
87
accelerated by the polarization interactions. An in-
fluence of the polarization on the process of charge
state recombination can be characterized by compar-
ing the radius of the stable polarization surrounding
rc in organic systems under discussion with the actual
distance d between charge states generated in the par-
ticle track. Such an analysis was performed in [25].
It has been shown that for the case of excitation by
alpha particles with the energies Eα ≤ 10 MeV and
for the initial radius r0 of a cylindrical track (r0 from
10 to 50 nm) the average distance between pairs d is
always less than rc. It means that the polarization
surroundings of the adjacent pairs overlap. There-
fore, the polarization effects can initiate a very fast
recombination of the charge states in the entire vol-
ume of the alpha particle track [25].
In the case of the neutron excitation, in contrast
to the alpha particle one, the situation is more com-
plicated. Here, the average distance d between charge
pairs can be less than rc, comparable with rc, and
greater than rc according to the value of En. In the
latter case it means that the polarization surround-
ings of adjacent pairs do not overlap. Therefore, the
charge states of the single pair have a possibility to
become widely separated from each other both not
to interact, and not to be involved in the polariza-
tion surroundings of adjacent pairs. In this case, the
influence of the polarization on the fast recombina-
tion of hot charge states should not be as strong as
in the case of the excitation by an alpha particle [25].
Let us consider the charge pair recombination in
the alpha particle track more closely using the re-
sults have been obtained in [25]. For this purpose
let us normalize the calculated values of the average
distance d between the centres of charge pairs (see
Fig.1 in [25]) for the p-terphenyl single crystal on
it minimum value, which corresponds to the energy
Eα = 1 MeV. The values of dnorm as a function of
Eα are presented by squares in Fig.3.
0 1 2 3 4 5 6 7 8 9 100,95
1,00
1,05
1,10
1,15
1,20
1,25
1,30
1,35
0 1 2 3 4 5 6 7 8 9 10
0,002
0,003
0,004
0,005
0,006
1 - d
norm
2 - Y
2
E
a
, MeV
E
a
, MeV
d n
or
m
1
E
ne
rg
y
Y
ie
ld
Y
Fig.3. Squares are the normalized values of the
average distance between charge pairs in the alpha
particle track as a function of Eα for the single
crystal of p-terphenyl, Stars are the values of Yα for
the single crystal of p-terphenyl (undoped)
The experimental values of the energy yield Yα for
the single crystal of p-terphenyl excited by the alpha
particles in the range of Eα from 0.9 to 7.6 MeV are
presented by stars in Fig.3. From Fig.3 we notice a
similar nature of the dependencies of Yα and dnorm
in the range of Yα under investigation. The energy
losses decrease with increase in dnorm, or with de-
crease in a density of the recombining pairs in the
alpha particle track. It causes increasing the radiolu-
minescence energy yield Yα. It should be noted that
the dnorm-value does not depend on the initial radius
r0 of a cylindrical track (see Fig.1 in [25]).
Using the data [25], let us estimate the aver-
age number of charge pairs Nr, which are inside
the sphere of radius rc. For the single crystal of p-
terphenyl rc is equal to 18.5 nm [25]. If the distance
between centres of pairs is d, then the radius of the
sphere containing one pair is d/2. The maximum co-
efficient of the dense packing of spheres is about 0.75.
Table 4 shows the results of such estimations for the
single crystal of p-terphenyl excited by an alpha par-
ticle with the energy Eα=5 MeV.
Table 4. The Nr-value for Eα=5 MeV and
rc=18.5 nm
r0, nm d, nm Nr
10 3.32 1038
20 5.26 261
30 6.90 115
40 8.35 65
50 9.69 41
The electrostatic field energy of a quasi-ion neces-
sary for formation a polarization surrounding, Epol,
is about 1...1.5 eV [2, 18, 19]. It means that the to-
tal energy of a single recombining pair is comparable
with the energy of S1-state (see. Table 2, the value
of Eph(λ
av
em)). Table 4 shows that within a very small
local region of a medium, whose size is determined by
the value of rc, the average number of charge pairs
Nr ≫ 1, and pairs of charge states can recombine
simultaneously. In such a situation, an immediate
and very strong local heating of a medium becomes
very probable. This local heating should lead to a
dramatic increase in the nonradiative deactivation of
excitation [29]. Such a mechanism of the ”temper-
ature quenching” can be a reason for the effective
primary quenching of the radioluminescence for the
ionizing radiations with high values of dE/dx.
It is reasonable to suggest that the main en-
ergy losses in the particle track with high dE/dx
occur during the stage of the hot charge state re-
combination accelerated by the polarization interac-
tions. During the primary recombination (τ < 10–13)
the concentration of charge states drastically drops.
Their subsequent recombination leads to creation of
S- and T -states in the ratio 3 to 1. However, the con-
centration of S- and T -states in the ”cooled” track
remains high enough. It makes possible the subse-
quent secondary stage of the quenching process. Let
us name this process as the ”concentration” quench-
ing of luminescence [2, 30].
88
The concentration of S1-states, which are the
source of the fluorescence in the S1 → S0 + hν tran-
sition, can be reduced in the processes of the mu-
tual S − S annihilation, S − T annihilation, singlet
exciton fission. In the condition of high activation
density the process of S − S annihilation can lead
to the autoionization mechanism. The quenching of
S1-states by doublet states results in decreasing their
concentration as well. The contribution from each
of the above-mentioned processes to the total energy
losses during the stage of energy exchange of excited
states will be defined by characteristic rate constants
of these processes [30].
Both the primary stage of hot charge state re-
combination and the secondary stage of the energy
exchange of excited states are the concentration-
controlled processes. Nevertheless the secondary
quenching proceeds after the primary one when the
concentration of the states is appreciably lower.
Therefore the influence of the secondary quenching
is not as significant as the influence of the primary
one.
5. CONCLUSIONS
Table 5 summarizes the results of this work. The
values of the luminescence energy yield Yi and the en-
ergy δi that is necessary to produce a luminescence
photon for the photoexcitation as well as for the ex-
citation by different types of ionizing radiation are
presented.
Table 5. Ranges of the values Yi and δi for organic
solid scintillators
Type i Yi δi, eV
of excitation
Light photons ph 0.41-0.77 3.74-7.63
Gamma and
conversion γ, e 0.025-0.079 34.48-113.42
electrons
Neutrons n 0.012-0.040 71.3-250.0
Alpha α 0.0012-0.0095 302.4-2346.2
particles
The main reasons of the energy losses for the ionizing
radiations with low values of dE/dx are the following:
1) An electron escape from a spur that decreases the
number of charge state pairs, which recombine.
2) The random recombination of charge states in a
single spur results in a decrease in the probability of for-
mation of S-states. If one takes into account both the
geminate recombination, and the random one, the total
yield of S-states is approximately equal to 0.4.
The main reasons of the energy losses for the ionizing
radiations with high values of dE/dx are the following:
1) The primary recombination of hot charge states
(the ”temperature quenching” of luminescence), acceler-
ated by the polarization interactions in an organic molec-
ular medium.
2) The secondary (”concentration”) quenching pro-
ceeding during the subsequent stage of energy exchange
of excited states. The influence of this process is minor
in comparison with the influence of the ”temperature
quenching” of luminescence.
ACKNOWLEDGEMENTS
This work was supported by the State Fund for Fun-
damental Research of Ukraine (project No. F58/06, ”The
effect of large radiation doses on scintillation and optical
properties of novel types of organic detectors”).
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ÎÑÎÁÅÍÍÎÑÒÈ ÐÀÇÌÅÍÀ ÝÍÅÐÃÈÈ Â ÒÐÅÊÅ ÈÎÍÈÇÈÐÓÞÙÅÉ ×ÀÑÒÈÖÛ
ÄËß ÎÐÃÀÍÈ×ÅÑÊÈÕ ÒÂÅÐÄÎÒÅËÜÍÛÕ ÄÅÒÅÊÒÎÐÎÂ
Í.Ç.Ãàëóíîâ, O.À.Òàðàñåíêî
Èññëåäóåòñÿ ýíåðãåòè÷åñêèé âûõîä ðàäèîëþìèíåñöåíöèè îðãàíè÷åñêèõ òâåðäîòåëüíûõ ñöèíòèëëÿòî-
ðîâ. Ïðîâåäåí ðàñ÷åò ýíåðãèè, êîòîðàÿ ðàñõîäóåòñÿ íà ñîçäàíèå îäíîãî ôîòîíà ñöèíòèëëÿöèîííîãî
èìïóëüñà ïðè ðàçíûõ âèäàõ âîçáóæäåíèÿ. Àíàëèç ðåçóëüòàòîâ îñíîâûâàåòñÿ íà èçó÷åíèè ôèçè÷åñêèõ
ïðîöåññîâ, ïðîòåêàþùèõ íà ðàííèõ ñòàäèÿõ ðàçìåíà ýíåðãèè èîíèçèðóþùåé ÷àñòèöû.  îñíîâå ýòîãî
àíàëèçà ëåæèò êîíöåïöèÿ îá îïðåäåëÿþùåì âëèÿíèè ïîëÿðèçàöèè îðãàíè÷åñêîé ìîëåêóëÿðíîé ñðåäû
íà ðåêîìáèíàöèþ ãîðÿ÷èõ íîñèòåëåé çàðÿäà, ÷òî ïðèâîäèò ê óìåíüøåíèþ ýíåðãåòè÷åñêîãî âûõîäà ðà-
äèîëþìèíåñöåíöèè. Àíàëèçèðóþòñÿ âîçìîæíûå ïðè÷èíû ýíåðãåòè÷åñêèõ ïîòåðü äëÿ èîíèçèðóþùèõ
èçëó÷åíèé ðàçíûõ òèïîâ.
ÎÑÎÁËÈÂÎÑÒI ÐÎÇÌIÍÓ ÅÍÅÐÃI�  ÒÐÅÊÓ IÎÍIÇÓÞ×Î� ×ÀÑÒÈÍÊÈ ÄËß
ÎÐÃÀÍI×ÍÈÕ ÒÂÅÐÄÎÒIËÜÍÈÕ ÄÅÒÅÊÒÎÐIÂ
Ì.Ç.Ãàëóíîâ, O.À.Òàðàñåíêî
Äîñëiäæó¹òüñÿ åíåðãåòè÷íèé âèõiä ðàäiîëþìiíåñöåíöi¨ îðãàíi÷íèõ òâåðäîòiëüíèõ ñöèíòèëÿòîðiâ. Ïðî-
âåäåíî ðîçðàõóíîê åíåðãi¨, ÿêà âèòðà÷à¹òüñÿ íà ñòâîðåííÿ îäíîãî ôîòîíà ñöèíòèëÿöiéíîãî iìïóëüñó ïðè
ðiçíèõ âèäàõ çáóäæåííÿ. Àíàëiç ðåçóëüòàòiâ ðóíòó¹òüñÿ íà âèâ÷åííi ôiçè÷íèõ ïðîöåñiâ, ÿêi âiäáóâà-
þòüñÿ íà ðàííiõ ñòàäiÿõ ðîçìiíó åíåðãi¨ iîíiçóþ÷î¨ ÷àñòèíêè.  îñíîâi öüîãî àíàëiçó ëåæèòü êîíöåïöiÿ
ïðî âèçíà÷àëüíèé âïëèâ ïîëÿðèçàöi¨ îðãàíi÷íîãî ìîëåêóëÿðíîãî ñåðåäîâèùà íà ðåêîìáiíàöiþ ãàðÿ÷èõ
íîñi¨â çàðÿäó, ùî ñïðè÷èíþ¹ çìåíøåííÿ åíåðãåòè÷íîãî âèõîäó ðàäiîëþìiíåñöåíöi¨. Àíàëiçóþòüñÿ ìîæ-
ëèâi ïðè÷èíè åíåðãåòè÷íèõ âòðàò äëÿ iîíiçóþ÷èõ âèïðîìiíþâàíü ðiçíèõ òèïiâ.
90
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