Manufacturing of polystyrene based scintillators
Design
Crystal Polystyrene (PS) is used as base scintillator (emition around 300 nm) mainly because its low cost, simple manufacturing process and availability of raw materials. PS is doped with a two stage phosphor wavelength shifter composed of 0.5% PPO (absorbs at 300 and emits at 350 nm) and 0.02% POPOP (absorbs at 350 and emits at 410 nm) or dm-POPOP (absorbs at 370 and emits at 430 nm). Regular 12-stages PMTs are highly efficient at 400 nm (i.e. Photonis XP2020Q). This is a widely accepted scintillating technique today [Ref MINOS/AMIGA].
PS is obtained by radical initiated bulk (mass) polymerization of styrene monomer (with dissolved phosphors) in glass molds. This technique allows almost any shape to be obtained. Large volumes must be handled with care.
More advanced base materials, as polyvinyltoluene (PVT) or Q-dm-POPOP, will be considered in the future.
[Ref] With added wavelength shifters one obtains a considerably increased and well-defined attenuation length in plastic scintillators. Light from large and/or large-surface scintillator plates can be collected into rods or plates of wavelength shifter material, a technique particularly important in large sampling calorimeters. Wavelength-shifted light also matches better the frequency sensitivity of the receiver (photomultiplier, vacuum diode). On the other hand the decay time of the signal increases to about 10 to 20 ns. In the case of a common light guide for several wavelength shifters each coupled to several scintillators, only about one sixth of the light is emitted into the cone retained by total refection.
Ingredients for 10 ml scintillator
- 10 ml styrene monomer
- 100 mg Benzoyl Peroxide (BPO) paste (40%) (initiator)
- 50 mg PPO (2,5-Diphenyloxazole)
- 2 mg POPOP (1,4-bis(5-phenyloxazol-2-yl) benzene) or Dimethyl-POPOP.
Temperature control
Polymerization must be carried out for at least 60-72 hours at 70 ºC controlled temperature. Once polymerized temperature must be gradually reduced to RT.
Polymerization is a highly exothermic process (165 cal/gr). Temperature control is mandatory when dealing with large volumes. Last phase of polymerization reduces volume and releases a lot of heat that can develop a hazardous runaway problem.
Rising polymerization temperature may produce bubbles and cracks. The same applies to fast cooling.
BPO Initiator
The kinetics of styrene polymerization, initiated by benzoyl peroxide, is consistent with a free-radical polymerization mechanism and shows the expected half-order dependence on initiator concentration [Ref].
Benzoyl peroxide powder (75%) would be better than paste (40%) but it is not available in La Plata. Benzoyl peroxide paste is a white, viscous paste that muddies the styrene if disolved directly. It must be dried in a glass plate for one day before dissolving. The glass container that will be used for mixture can be used. Spread paste inside glass walls and store for one day in a dry place. Do not heat as it can burn. Dry peroxide sticks hardly on the glass walls, but dissolves well in styrene. It may be necessary to slightly heat the mixture for good dissolution of peroxide and phosphors. Stir with a glass rod.
Unmolding and covering
10 ml reduce to approximately 8 ml during last phase of polymerization. Upper surface twists and results unusable. Surfaces in contact with glass result quite smooth.
Unmolding is problemmatic. Carefully cracking the glass mold is probably the best option. Slowly cooling in a freezer can help unmolding, but be very careful as this will probably crack the mass if volume is large.
Don´t expose unmolded surfaces to air and don´t touch them with naked hands as skin acids attack it.
Immediately covering the surfaces with final white reflector is highly advisable (white TiO2 paint or PVC adhesive film, see next).
Experiments
12 experiments that lead to the actual manufacturing process. Information still in the workbook... Will be here soon.
This are the (unsorted) pictures we took in the process.
Plastic scintillator manufacturers
Saint-Gobain (formerly Nuclear Enterprises, Bicron et al.)
Rexon
Eljen Technology
Kuraray
Notes on styrene polimerization
Styrene will polymerise spontaneously on heating in an oxygen-free atmosphere but catalysts are added to ensure complete polymerisation at lower temperatures. Processes have been designed to aid heat transfer from the exothermic reaction, which can lead to low molecular weight polymers being formed if not controlled.
The main advantages of the mass process are the clarity and excellent colour of the resins produced.
Polystyrene depolymerises when heated above 300ºC and burns with a smoky flame.
Polimerización por radicales libres: Los radicales de estireno se forman espontáneamente, a mayor velocidad cuanto mayor sea la temperatura. Por ello el estireno es almacenado en tanques refrigerados y estabilizado con inhibidores, que consumen los radicales libres. La velocidad de reacción se vuelve significativa a partir de una temperatura superior a los 100ºC. Se puede acelerar añadiendo iniciadores como por ejemplo peróxidos, que generan radicales libres adicionales.
Los reactores son en esencia recipientes en los que se fija una temperatura (típicamente entre 100 y 200ºC) y se asegura la homogeneidad mediante agitación. Para acelerar la reacción se pueden añadir también peróxidos, que actúan como iniciadores de polimerización. Existen muchos diseños diferentes de reactor que se diferencian principalmente por la forma de evacuar el calor (por tubos internos o condensador externo), por la distribución de tiempos de residencia (tanque agitado o flujo pistón) y por el tipo de agitación.
Mediante el uso de catalizadores se puede controlar de forma precisa la tacticidad del polímero formado. Un catalizador es una sustancia (compuesto o elemento) capaz de acelerar (catalizador positivo) o retardar (catalizador negativo o inhibidor) una reacción química, permaneciendo éste mismo inalterado (no se consume durante la reacción). A este proceso se le llama catálisis. Los catalizadores no alteran el balance energético final de la reacción química, sino que sólo permiten que se alcance el equilibrio con mayor o menor velocidad.
El proceso más utilizado en la actualidad para el poliestireno se basa en la polimerización radical en masa (mass/bulk). "Radical" significa que la reacción es iniciada por radicales libres, generados bien térmicamente bien mediante moléculas específicas denominadas iniciadores. "En masa" significa que el medio de reacción está formado esencialmente por estireno y poliestireno, añadiéndose a veces otro hidrocarburo inerte perfectamente miscible con el estireno, a menudo etilbenceno, que sirve para moderar la velocidad de reacción. Las líneas basadas en procesos en emulsión y en solución han quedado anticuadas hoy día, siendo reservadas a la producción de grados de especialidad [Wikipedia].
http://es.wikipedia.org/wiki/Poliestireno
Plásticos: Polimerización y Estructura (FI-UBA)
Polystyrene Report
Phosphores
Wavelength shifting mechanism.
[REF]
Fluorescence Lifetime Standards Excitation [nm] Emission [nm] PPD 240-340 310-440 PPO 280-350 330-480 POPOP 280-390 370-540 Dimethyl-POPOP 300-400 390-560 PPD = 1.5-diphenyl-1,3,4-oxadiazole PPO = 2.5-diphenyl-oxazole POPOP = 1, 4-bis(5-phenyloxazole-2-yl)benzene REF: J.R. Lakowicz, Principles of Fluorescence Spectroscopy, 2nd Ed., Kluwer Academic/Plenum Publishers, New York, London, Moscow, Dordrecht, 1999.
See also:
Q-POPOP
Acta Histochem. 1997 Jun;99(2):195-205.
New cationic fluorochromes from diaryloxazole scintillators: fluorescence of chromatin DNA induced by N-quaternary POPOP derivatives.
Stockert JC, Pelling C, Espada J.
N-quaternary derivatives of the diaryloxazole scintillators POPOP and dimethyl-POPOP (dmPOPOP) in chloroform solution were obtained by methylation with dimethylsulfate. After drying, aqueous solutions of the corresponding oxazolium compounds (Q-POPOP and Q-dmPOPOP) revealed strong fluorescence (peaks at 485 and 493 nm, respectively). Under 365 nm excitation, both N-quaternary derivatives induced a bright greenish blue fluorescence in nuclei of chicken erythrocytes and human buccal cells, as well as in the kinetoplast DNA of Trypanosoma cruzi epimastigotes; mouse mast cell granules showed a green-yellow metachromatic emission. Chromatin fluorescence was dependent on the presence of DNA; it was abolished by washing with a 10 mM solution of the bisguanidine compound Phenformin, whereas 1 M NaCl or MgCl2 had no effect. The oxazolium derivatives were hydrophilic (log P: -6.409 and -5.373 for Q-POPOP and Q-dmPOPOP, respectively). Molecular modelling studies revealed that these cationic and non-rigid (cis) scintillator derivatives are well suited to locate along the convex floor of the narrow DNA minor groove from adenine-thymine regions.
Reflector
White TiO2 paint or PVC adhesive film...
