121

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

122

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).

123

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

124

Figure 4.1 Crystal elements used in the experiment

125

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

126

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

127

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

128

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.

129

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

130

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

131

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

133

(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

134

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).

135

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

136

(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 %.

137

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

138

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.

139

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

140

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

141

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

142

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

143

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

144

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

145

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.