CHAPTER 4 SCINTILLATION STUDIES ON ANTHRACENE,...
Transcript of CHAPTER 4 SCINTILLATION STUDIES ON ANTHRACENE,...
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CHAPTER 4
SCINTILLATION STUDIES ON ANTHRACENE,
NAPHTHALENE AND DOPED NAPHTHALENE
CRYSTALS
4.1 INTRODUCTION
A scintillation material is a kind of converter transforming the
energy of one high energy photon (X-ray, -ray) or particle (proton, electron,
-particle, etc) into a number of UV-Visible photons, which are easily
detectable with a conventional photomultiplier tube, semiconductor detector,
etc. Better understanding of the various scintillation mechanisms has led to
innovative new materials for both gamma�ray and neutron detection, and the
concept of scintillation design and engineering has emerged, whereby
materials are optimized according to the scintillation properties needed by
specific applications. The new scintillators have low cost, and offer a light
yield comparable to that of commercial scintillators, making them good
candidates for large detectors.
Scintillation spectrometers are widely used in detection and
spectroscopy of X-rays and gamma rays (Knoll 1999). Broser and Kallman
(1947) discovered that gamma rays could be detected with high efficiency
using a naphthalene crystal and photomultiplier. Many organic and inorganic
scintillators have become available since then and a complete discussion of
the scintillation counter is given by Birks (1964). Common applications of
scintillation spectrometers include medical imaging, nuclear and particle
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physics, non-destructive evaluation, nuclear non-proliferation, environmental
monitoring, and X-ray diffraction. There are a variety of radiation detector
applications that desire scintillators with high light output, good attenuation
power, and a low level of afterglow, but absolutely require a fast scintillation
decay time. This is particularly true for positron emission tomography (PET)
and gamma-ray/alpha particle discrimination via time-of-flight techniques.
The scintillators in some security inspection systems must have decay
constants less than 50 ns (Rodnyi 2001).
It is natural to expect that the needs of the largest scintillator
applications will tend to drive much of the development of new scintillators.
Medical imaging and high-energy physics have been and continue to be the
dominant consumers of scintillators. Due to a considerable overlap in the
desired scintillator properties for these two applications, they have sometimes
used the same scintillator materials, such as NaI:Tl and BGO. However, the
current trend seems to be toward the use of different materials (Melcher
2005).
Organic molecular crystals are used in particle identification using
pulse shape discrimination technique. An extensive work on the scintillation
property was done in view of time and energy response of grown crystals
using laboratory sources. All the works described here, have been carried out
in Inter-University Accelerator Center, New Delhi, India. Scintillation
properties included the measurements of excitation and emission spectrum,
energy spectrum, timing resolution, pulse shape discrimination and decay
time constant. The crystals have been cut using the inner diameter cutter with
crystal boule contained in glass ampoule. The polished crystals were highly
transparent (Figure 4.1).
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Table 4.1 Properties of naphthalene, anthracene and PPO crystals
Properties Naphthalene Anthracene PPO
Molecular structure
Molecular formula C10H8 C14H10 C15H11NO
Molecular weight 128 178 221
Melting point (oC) 80 217 73
Freezing point (oC) 78 216 72
Boiling point (oC) 218 340 360
Density (g/cm3) 0.9623 1.25 1.06
Excitation max. (nm) 270 345 300
Emission max. (nm) 348 448 405
Scintillation efficiency at 30oC
10.8 100 55
Decay time (ns) 80 28 7
N O
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Figure 4.1 Crystal elements used in the experiment
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4.2 EXCITATION AND FLUORESCENCE STUDIES
In fluorescence, the initial excitation takes place via the absorption
of a photon, and de-excitation by emission of a longer wavelength photon.
Fluors are used as wave shifters to shift scintillation light to a more
convenient wavelength. Occurring in complex molecules, the absorption and
emission are spread out over a wide band of photon energies, and have some
overlap, that is, there is some fraction of the emitted light which can be re-
absorbed. This self-absorption is undesirable for detector applications because
it causes a shortened attenuation length. The wavelength difference between
the major absorption and emission peaks is called the Stoke�s shift. It is
usually the case that the greater the Stoke�s shift, the smaller the self
absorption thus, a large Stokes' shift is a desirable property for a fluor.
The Figure 4.2 illustrates the energy transfer process from the host
to guest system. From the host-guest system it is understood that the incident
radiation is entirely absorbed by naphthalene molecule, and that naphthalene
is transparent to the fluorescence emission of guest molecules.
Figure 4.2 Schematic of Host-Guest singlet energy transfer
mechanisms in crystalline complexes
Host molecule Guest molecule
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The crystals were excited with radiation from a FlouroMax-2
equipped with the modified Czerny-Turner spectrometer with continuous
ozone-free xenon lamp of 150 W in the required excitation wavelength at
room temperature.
The excitation spectrum of naphthalene is shown in Figure 4.3. The
excitation wavelength is ~ 270 nm. The fluorescence spectrum was recorded
for the excitation wavelength 270 nm at room temperature. Figure 4.4
represents the fluorescence spectrum of pure naphthalene and the naphthalene
containing 10-2 mol anthracene per mole naphthalene crystal. Anthracene
fluorescence spectrum recorded under similar conditions is also shown in the
Figure 4.4. From the Figure 4.4 it is seen that the fluorescence of naphthalene
is completely quenched by anthracene molecules with a new emission
spectrum peaking at 425 nm. This value is well matched with reported value
(Patil and Patwari 1999). It has been observed that very small addition of
anthracene completely quenches weak violet fluorescence of naphthalene and
an intense blue fluorescence appears from naphthalene doped by anthracene.
The fluorescence spectrum of anthracene doped naphthalene crystals shows
red-shift with respect to the pure naphthalene crystals and blue-shift with
respect to pure anthracene crystals.
This shift indicates a coupling and charge transfer between the
naphthalene and the anthracene molecules. From the host-guest system
(Figure 4.2) it is understood that the incident radiation is entirely absorbed by
naphthalene molecule, and that naphthalene is transparent to the fluorescence
emission of anthracene. The luminescence intensity of the NA crystal is three
times that of pure naphthalene crystal. The chosen doping concentration in the
present investigation yields the crystal having emission wavelength matching
the spectral sensitivity of phototubes without losing the fluorescence intensity
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220 240 260 280 300 320 340-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Wavelength (nm)
Pure naphthalene
Figure 4.3 Excitation spectrum of pure naphthalene crystal
250 300 350 400 450 500 550
c
b
a
Wavelength (nm)
a Pure naphthaleneb Naphthalene:Anthrecene c Pure anthracene
Figure 4.4 Fluorescence spectra of anthracene, naphthalene and
anthracene doped naphthalene crystal
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The luminescence property of the PPO-doped naphthalene crystals
was understood from the fluorescence studies carried out on the samples
obtained from the crystals having different PPO concentrations. The
fluorescence spectrum was recorded for two excitation wavelengths 270 and
290 nm, respectively at room temperature. The 270 nm excited fluorescence
spectra of 0.01, 0.03, 0.05 and 0.07 % PPO doped naphthalene crystals are
shown in Figure 4.5. When crystal was excited with 270 nm, the pure
naphthalene shows its characteristic emission in the region 335-360 nm. For
doped crystals, the fluorescence spectra show both emissions from
naphthalene and PPO. This is because the naphthalene emission not fully
overlapped with the absorption of PPO. It is noticed that at a concentration of
0.03 % of PPO, the emission intensity is maximum at higher wavelength
region. The decrease in fluorescence intensity above 0.03 % PPO is due to
concentration quenching effect.
Figure 4.6 shows the 290 nm excited fluorescence spectra of pure
and doped naphthalene (0.03 %, 0.05 % PPO) crystals. The fluorescence
intensity of pure naphthalene is very less; hence the excited states
corresponding to 290 nm excitations are to be less populated in the crystal.
Since the excitation wavelength is fully absorbed by PPO, the PPO doped
naphthalene crystal shows the PPO characteristics emission only and the
naphthalene acts as a medium.
The Stoke�s shift is 150 nm for anthracene doped naphthalene and
135 nm for PPO doped naphthalene crystal (270 nm excitation). But the
Stoke�s shift for pure naphthalene is ~70 nm.
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250 300 350 400 450 500 5500
200000
400000
600000
800000
1000000
1200000
1400000
0.01 %PPO 0.03 %PPO 0.05 %PPO 0.07 %PPO
Wavelength (nm)
Figure 4.5 Fluorescence spectra of PPO doped naphthalene crystals at
270 nm excitation
300 350 400 450 500 550 600-500000
0
500000
1000000
1500000
2000000
2500000
3000000 0.03 %PPO 0.05 %PPO pure naphthalene
Wavelength (nm)
Figure 4.6 Fluorescence spectra of PPO doped naphthalene crystals at
290 nm excitation
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4.3 TIME RESOLUTION STUDIES
The timing spectra were carried out by combining with BaF2
scintillator. The laboratory source 22Na (511 keV -rays) was used for this
study by means of time-to-amplitude converter (TAC) setup. The detail of the
experimental setup has been given in section 2.14 (Figure 2.18). The time
resolution spectrum was recorded without delay and with delay time of 20 ns.
The shift in the counts for 20 ns delay was measured and time/channel
(0.025 ns/channel) was calculated.
The timing spectra of naphthalene, anthracene and doped
naphthalene crystals are given in Figure 4.7�Figure 4.10. The timing
resolutions (FWHMs) are given in Table 4.2. The values are better than the
time resolution of trans-stilbene crystal which was reported as 8.5 ns for 22Na
gamma source while the time resolution was done with BGO
(Arulchakkaavarthi et al 2002c).
0 250 500 750 1000 1250 1500 1750 2000 2250 2500
20
40
60
80
100
120
Channel Number
Pure naphthalene
Figure 4.7 Timing spectrum of naphthalene crystal
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500 750 10 00 1250 150 0 1 7500
200
400
600
800
1000
1200
Channel N um ber
Anthracene
Figure 4.8 Timing spectrum of anthracene crystal
500 750 1000 1250 1500 17500
100
200
300
400
500
600
700
Naphthalene:Anthracene
Channel Number
Figure 4.9 Timing spectrum of anthracene doped naphthalene crystal
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500 75 0 1000 1250 1500 175 00
100
200
300
400
500
600
Channe l Num ber
Naphtha lene:0.03% PPO Naphtha lene:0.05% PPO
Figure 4.10 Timing spectrum of PPO doped naphthalene crystal
Table 4.2 Time resolution of grown scintillation crystals
Scintillators Time resolution (FWHM) (ns)
Pure naphthalene 2.9
Anthracene 1.6
Anthracene doped naphthalene 1.8
0.03%PPO doped naphthalene 1.5
0.05%PPO doped naphthalene 1.6
In our measurement the better time resolution was obtained for
0.03%PPO doped naphthalene crystal.
4.4 DECAY TIME ANALYSIS
One important property which distinguishes organic scintillators
from the alkali halide crystals is their fast decay time of a few nanoseconds
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(Kapoor and Ramamurthy 1993). The details of the experimental description
for decay time analysis are given in the section 2.15. Since the shaping time is
depending on the decay time constant of the light pulse, the shaping time must
be larger than the decay time to collect photons enough (Sato et al 2006).
Figure 4.11 represents the decay curve of pure naphthalene. The
analysis of the decay curve for pure naphthalene shows the fits with
biexponential decay function (Equation 2.1) with lifetime of 18.7( 1) ns and
53( 2) ns. The decay time observed for the naphthalene crystal is the lowest
observed ever in the literature when compared to the values reported
(68±10 ns (Sangster and Irvine 1956), 80 ns (Muller et al 1988), 82 ns
(Kohler et al 1976), 85 ns (Richard Powell 1971), 103 ns (Swank and Buck
1955), 106±3 ns (Mansour and Weinreb 1968), 144 ns (Birks 1964)).
0 200 400 600 800 1000
10
100
1000Pure naphthalene
Channel Number
Figure 4.11 Decay curve of naphthalene crystal
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The decay curve for a NA crystal is shown in Figure 4.11 which
was excited via energy transfer from naphthalene (practically all the
excitation energy is absorbed by naphthalene only). The decay (~93%) of the
optical emission from NA is in 27.8 ns. The similar graphical observations for
anthracene doped naphthalene crystals are reported by Mansour and Weinreb
(1968). However clear value was not mentioned in their report. If the
concentration of anthracene is >10-3 mole in naphthalene, the decay time of
the anthracene doped naphthalene depends only on the anthracene.
In a binary system the decay time of the main scintillation emission
is similar to the fluorescence decay time of the emitting solute (Birks 1964).
The scintillation decay in Figure 4.12, measured at RT in the NA crystal, is
governed by the 27.8 ns decay time, which is close to the anthracene
photoluminescence lifetime in NA crystal (Sean Lawrence Prunty 1978).
The decay profile for PPO doped naphthalene crystal at room
temperature is shown in Figure 4.13. Analysis of the decay curve shows that
biexponential decay function with a fast lifetime 1 = 7.5 ns and a second
lifetime 2 =50.2 ns has the lowest 2 fit. The decay time is independent of the
PPO concentration. The second lifetime is attributed to naphthalene decay
which has long decay (80 ns) compared to PPO. The fast scintillation decay
is governed by the 7.2 ns decay time, which is close to the PPO fluorescence
lifetime (Birks 1964).
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0 200 400 600 800
1
10
100
Channel Number
Anthracene doped naphthalene
Figure 4.12 Decay curve of NA crystal
0 200 400 600 800 1000 1200
10
100
1000
Channe l N um ber
pure naphthalene 0.03 % PPO 0.05% PPO 0.07% PPO
Figure 4.13 Decay curves of pure and PPO doped naphthalene crystals
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(4.1)
4.5 ENERGY RESOLUTION STUDIES
Energy resolution setup is shown in Figure 2.16. Because of their
low density and the low atomic number (Z) of the constituent elements (H= 1,
C= 6, N= 7, O= 8) organic scintillators have much lower -ray absorption
coefficients than inorganic scintillators. Because of the low Z, the
photoelectric absorption is small when the -ray energy E >30 keV, and
Compton scattering is the main gamma-ray absorption process upto
E ~20 MeV. The following relation gives the Compton edge due to the
energy transfer to the Compton electron by the -ray energy;
which is only the significant peak which normally appears in the scintillation
pulse height spectrum. Where m0c2 = 0.51 MeV is the rest mass energy of the
electron. Using the above relation, the Compton edge was calculated for
gamma rays from 137Cs as 478.7 keV. The energy resolution is defined as the
full width of the peak in the pulse height spectrum at half the maximum
intensity (FWHM) divided by its energy. In general the energy resolution of
organic scintillator is poorer than inorganic scintillator. The energy resolution
spectra are shown in Figures 4.14-4.16.
The energy resolution for pure naphthalene is 35%. The energy
resolution of the NA scintillator was measured to be 18% at room
temperature. Thus, the energy resolution of NA is ~2 times better than that for
naphthalene. Higher light output is responsible for high energy resolution
obtained with those crystals. The energy resolution of the pure anthracene for 137Cs was reported as 13 % (Van Hise et al 1967). But in our result the pure
anthracene gives 15% resolution for the same gamma energy source. Energy
resolution of PPO doped naphthalene crystal is 32 %.
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500 1000 1500 2000
500
1000
1500
2000
2500
3000
Channel Number
NA crystal
50
100
150
200
250
300
pure naphthalene
Compton edge
Compton edge
0 500 1000 1500 2000 25000
200
400
600
800
1000
1200
1400
1600
Channel Number
Anthracene
Figure 4.14 Energy spectra of 137Cs source for pure and anthracene
doped naphthalene crystal
Figure 4.15 Energy spectra of 137Cs source for anthracene crystal
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Compton edge
0 500 1000 1500 2000 2500
100
200
300
400
500
600
700
800
900
Channel Number
Naphthalene:0.03%PPO
Figure 4.16 Energy spectra of 137Cs source for pure and PPO doped
naphthalene crystal
4.6 PULSE SHAPE DISCRIMINATION (PSD) STUDIES
It is well known that some scintillators when excited by different
ionizing particles produce pulses of different shapes. PSD methods have been
extensively used to separate neutrons from gamma rays. These techniques are
based on the fact that neutron and gamma rays give different pulse shapes
when interacting with the neutron sensitive organic scintillators. The neutron
interaction results in a slower timing signal than the gamma ray interaction.
This means that gamma ray pulse rises and decays faster from and to the
baseline, respectively, than a neutron pulse generated by the recoiled protons.
A neutron creates a large ionization density by producing a recoil proton
resulting in a long tail. A gamma ray produces a scattered electron with a very
small ionization density, and as a result decays much faster.
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There are two different methods of PSD: �charge integration� and
�zero crossing�. In the charge integration method, two charge-sensitive
analog-to-digital converters (ADCs) are used to differentiate between the two
pulses. In zero-crossing method, the detector signal is sent to shaping
amplifier for integration and differentiation. This causes the zero crossing of
the pulse, in which a gamma ray pulse crosses the zero crossing baseline
much earlier than a neutron pulse. In this method, a time-to-amplitude
converter is used to measure the zero crossing point.
The zero crossing method of pulse shape discrimination (PSD) has
become increasingly popular as a means of suppressing gamma ray
background in neutron detection systems which utilize organic scintillators.
The main advantage of this method of PSD is its suitability for use over a
large dynamic range of pulse amplitudes (variations greater than 100:1). It is
not generally realized, however, that zero crossing systems are equally
suitable for use with inorganic scintillators where the scintillation decay times
are comparatively long (~ 200ns). In addition zero crossing PSD systems are
sufficiently versatile for the determination of the PSD properties of most
scintillators (organic and inorganic) to be demonstrated rapidly with the
minimum of adjustment to the system parameters (Winyard et al 1971).
Figure-of-merit (FOM) is calculated by using the following relation:
Ma, b = T/(t a + tb), (4.2)
where T is the separation between the time peaks and ta and tb are the
respective FWHM of the peaks �a� (corresponding to gamma ray) and �b�
(corresponding to neutron).
The pulse shape discrimination has been done by using the Pulse
Shape Discriminator (PSD) setup. The anode output from the PMT was
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processed by PSD module (Canberra 2160A Module). In yet another way, the
same signal is processed by means of CFD and it was gated and delayed and
given to start of the TAC (ORTEC 567) and the stop pulse was the PSD
output. The circuit diagram used in the experiment is shown in Figure 4.17.
Figure 4.17 Block diagram of PSD setup
The strobe pulse was given to PSD. Careful zero-crossover
adjustment along with strobe adjustment was done to get good neutron
gamma discrimination for 252Cf and 22Na sources before the commencement
of the experiment. The californium emits neutrons along with gamma
radiation of 100 and 160 keV energy. Commonly used gamma ray source 22Na in the timing application, and 252Cf were used simultaneously in pulse
shape discrimination. The TAC range was kept as 100 ns. The TAC output
was delayed and matched with the energy signal and the noise level was
decreased in the time signal and fed to ADC 811. The neutron-gamma
CRY PMT XP2020
ANODE
CAMAC
C F D
DYNODE
TAC
PSD
START
STROBE
AMP
ADC
PC CANDLE NSC DAS 2
STOP
OUT
HV BIAS ORTEC SOURCE
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discrimination properties of the grown crystals were tested. The resulting
spectra are given in Figure 4.18-Figure 4.22. Two parameters have been
measured that are of interest for pulse shape discrimination. The separation
has been measured between the neutron and gamma peaks. The
figure-of-merit has been measured. The parameters for our crystals are given
in Table 4.3. Naphthalene crystal shows the peaks corresponding to gamma
and neutron but it has very low counts. Due to this reason, FWHM for the
peaks could not be measured.
Figure 4.18 n- pulse shape discrimination spectra of naphthalene crystal
n
500 1000 1500 2000
0
5
10
15
20
25
30
Naphthalene
Channel Number
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Figure 4.19 n- pulse shape discrimination spectra of anthracene crystal
Figure 4.20 n- pulse shape discrimination spectra of anthracene doped
naphthalene crystal
n
0 500 1000 1500 2000 25000
200
400
600
800
1000
Channel Number
Anthracene
n
400 600 800 10000
100
200
300
400
500
Channel Number
Naphthalene:Anthracene
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Figure 4.21 n- pulse shape discrimination spectra of 0.03% PPO doped
naphthalene crystal
Figure 4.22 n- pulse shape discrimination spectra of 0.05% PPO doped
naphthalene crystal
n
500 1000 1500 2000 2500
0
10
20
30
40
50
60
70
80
Channel Number
Naphthalene: 0.03%PPO
n
200 400 600 800 1000 12000
20
40
60
80
100
Channel Number
Naphthalene:0.05%PPO
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Table 4.3 Results from PSD studies for the grown crystals
Scintillator Separation
T (Channel)
Figure of merit
Ma, b=T/(t a+tb)
Resolution
T/tb
Naphthalene 287 - -
Anthracene 307 0.64 0.73
Naphthalene:Anthracene 73 0.69 1.04
Naphthalene:0.03%PPO 448 2.2 3.75
Naphthalene:0.05%PPO 205 2.9 5.85
The separation between the neutron and gamma peaks is greatest
for naphthalene:0.03%PPO. The FOM of naphthalene: 0.03% PPO is higher
than stilbene crystal which is 2.4 for the same source (252Cf-22Na)
(Arulchakkaravarthi et al 2002d). The resolution of the system for particle
identification, defined as the separation of the neutron and gamma peaks
divided by the full width at half maximum of the neutron peak (Muller et al
1988), has also been measured. Naphthalene:0.05%PPO is having better
resolution than the other crystals. The resolution for anthracene is 0.73, but
1.92 was reported for the source of 11B-12C (Muller et al 1988).
4.7 CONCLUSION
Experimental data suggest that even small amount of aromatic
compounds of impurities play an important role in determining the
scintillation properties of simple aromatic hydrocarbons. The emission
spectral analysis shows that the Stoke�s shift is increased due to the dopant
molecules. The luminescence decay time of the NA crystal is 27.8 ns. The fast
decay time of NPPO crystal is 7.2 ns. The energy response and the timing
properties of the NA scintillator were characterized for the first time. The
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energy resolution and the timing resolution of NA crystal are 18 % and 1.8 ns
respectively at room temperature. The decay time observed for the
naphthalene crystal is lower than the values reported in the literature for
naphthalene. Two decay components were observed in PPO doped
naphthalene crystal and the fast component (7.2 ns) belongs to PPO
impurities. PPO doped naphthalene scintillator crystals may be used instead
of PPO doped liquid scintillator. This research shows that it is possible to
develop good scintillating crystals by doping organic scintillating activators.
The 0.03 mole% of PPO concentration in naphthalene is interesting for
scintillation detector because in this concentration the naphthalene shows
maximum fluorescence efficiency. And also it shifts the luminescence to
higher wavelength, which matches the spectral sensitivity of most of the
phototube without much change in luminescence efficiency.