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   Organic molecular single crystals

Due to low effective atomic number organic materials in contrast to nonorganic ones are characterized by negligible back scattering effect for the process of charged particles absorbtion. Due to high concentration of hydrogen atoms they are used in fast neutron spectroscopy. Scintillators on the base of organic crystals are non-hydroscopic and fast.

Anthracene (C14H10)
Traditional organic scintillator finds use in a number of special applications.

Used for selective registration of fast neutrons (n) on the γ-radiation background in scintillation pulse-shape discrimination techniques, as well as for α/β-, n/β, proton/β-selection. It makes possible to perform neutron field spectrometry. The largest dimension is Ø120x100 mm.

Doped crystals of p-terphenyl (C18H14)
Undoped p-terphenyl crystals are of limited because of their low light output. Introduction of special dopants ensures the light output to be increased by 4-5 times. Doped p-terphenyl crystals retain all basic advantages of organic scintillators, i.e. short decay time (t = 3-4 ns), temperature-independent light output values in a wide temperature range (-80... + 150 °C).
The p-terphenyl single crystals are translucent, knotty over the whole volume, plastic, water-insoluble, and poorly soluble in organic solvents. Scintillators based on doped p-terphenyl crystals are used in β-radiation spectrometry, where they are the most effective organic scintillators, α-radiation one, as well as for fast neutrons detection on the γ-radiation background in schemes for radiation discrimination by the scintillation pulse shape. Due to short decay time, p-terphenyl can be used in the spectrometry of high-intensity radiation.

Selector guide for organic single crystal scintillators
MaterialImportant propertiesApplication comments
AnthraceneCrystal anisotropy (high light output anisotropy)β-radiation detection and spectrometry
StilbeneHigh structural perfection and performance stability. Selection of fast neutron on the γ-radiation backgroundFast neutron spectroscopy n/γ, α/β-discrimination
Doped p-terphenilHigh light yield. The lowermost light output anisotropyDetection and spectroscopy of α-, β-radiations. Fast neutron spectroscopy n/γ

 Anthracene (C14H10)Stilbene (C14H12)p-terphenyl (C18H14)
Molecular weight178180230
Density, g cm31.251.221.23
H/C - ratio0.7140.8570.778
Melting point, °C216124214
Wavelength of emission maximum λm, nm445390420
Refractive index at λm1.621.641.65
Light output, 104 photons/MeV2.01.42.7
Decay time, ns303.53.7
A ratio of ranges in the crystal and anthracene for:
   a) heavy charged particles
   b) electrons
Radiation degradation (Mrad)24.04.5

Fig. 1. Pu-Be spectrum for stilbene detector Ø80x50 mm.

Fig. 2. 137Cs spectrum for p-terphenyl detector Ø80x5 mm; 137Cs radionuclide source irradiates 125 conversion electrons per second through a collimator and 1.5·105 γ-photons per second in 2- π geometry.

Fig. 3. 241Am spectrum for p-terphenyl detector Ø20x10 mm.

Scintillation Detectors
Detectors based on organic crystals (doped p-terphenyl, stilbene, anthracene) are used for the detection and spectrometry of β-, γ-radiation, discriminative n/γ -measurements, and in phoswich detectors for discriminative measurements of low-energy β-radiation.
Since stilbene crystals have the most stable properties, detectors with these crystals are used as the reference ones in light output measurements.
Detectors and assemblies with organic crystals are similar in design to those with alkali halide crystals.
In the detectors (assemblies) intended for β-radiation measurement, the input window is made of aluminum-coated polyethylene-terephtalate film having a thickness less than 2 mg cm-1.

Detectors with organic crystals
Detector type & Nomenclature Scintillation material Scintillator sizes, mm Working temperature, °C Relative light output, % Energy resolution*, %
doped p-terphenyl10 to 801 to 100-60 to 80200<10
SDO.04stilbene10 to 1001 to 100-40 to 60100<10
anthracene10 to 405 to 3015 to 35150<10

*137Cs conversion electrons.

To special orders, mosaic-type detectors with doped p-terphenyl can be made up to 200 mm in diameter.
By hot pressing, polycrystalline large-area detectors are produced with a thickness of 0.2 to 3 mm.

Organic X-Ray Detectors

It is airequivalent scintillator, which is based on p- terphenyl crystal and contains a compensating addition agent. Such type of scintillators can be used for β-, γ-detection of the concentrated radionuclides, as well as for determination of their concentration. The main application is X-ray detection in a regime of count rate measurements.

Light output about 104 photons/MeV, wavelength of emission maximum λmax = 420 nm, radiation degradation 4.5 Mrad. For the energies of radiation 0.02...0.1 MeV deviation from linear dependence light output vs. energy of radiation is not more then 20%.

These scintillators are non- hydroscopic, fast, poorly soluble in organic solvents, and have temperature-independent light output in the temperature range -40...60 °C.

Possible area: scintillation technique, environment, medical and biological investigations.