Background

Scintillation is the process by which a material converts radiation into light.

The use of scintillation in inorganic salts to detect radiation dates back over a century to when it was first used in the discovery and calibration of radioactivity. Today, scintillation detectors are used in a variety of remote sensing and non-invasive applications such as medical imaging, security screening for nuclear materials, astrophysical exploration and geophysical exploration in the pursuit of new energy reserves. At the heart of such detectors is a high purity material that scintillates in response to ionizing radiation. Over the decades, dozens of different scintillator materials have become commercially available and triggered further developments through continued research.
Key Factors for an Ideal Scintillator

Key factors for an ideal scintillator:

· High light output (brightness)

· High gamma ray stopping efficiency

· Fast response

· Low cost

· Good proportionality

· Minimal afterglow
Overcoming Materials Shortcomings

Although many candidate materials possess a good combination of physical properties, no single material provides the desired combination of stopping power, light output and decay time. To overcome some of these shortcomings, advanced signal processing techniques have been used; however, existing materials and signal processing technologies are approaching their physical limits, creating new material challenges.
Demands of Next Generation Scintillation Detectors

The ever more demanding imaging and exploratory applications call for the next generation of scintillation detectors to have very high response rates, be highly sensitive to low amounts of radiation and be tunable to specific radiation types.
General Types of Scintillator Materials

There are three general types of scintillator materials:

· Organic

· Inorganic

· Gas
Organic Scintillator Materials

Properties of organic scintillator materials:

· Fast response time

· Low cost

· Ease of processing

· Poor light output

· Non-linear conversion
Inorganic Scintillator Materials

Properties of inorganic scintillator materials:

· Best light output

· Most linear conversion

· Highest sensitivity

· Slow response times
Gaseous Scintillator Materials

Properties of gaseous scintillator materials:

· Fast response time

· Low scintillation efficiency
Producing the Ideal Inorganic Scintillator

Breakthroughs in materials processing, particularly techniques in making ultra-pure materials and in fabricating unique compositions for crystal growth, are enabling the pursuit of the ideal inorganic scintillator.
Sodium Iodide Doped with Thallium

The 1948 discovery by Hofstadter that sodium iodide doped with thallium exhibits extremely high light-yield and conversion efficiency launched the era of modern radiation spectrometry. More than half a century later, inorganic halide salts, particularly when doped, possess some of the best characteristics of all scintillation materials. Thallium doped sodium iodide, in fact, still exhibits the highest conversion efficiency of any known scintillation material (see Table 1). Scrap shards cut from desired crystals can be reprocessed by Sigma-Aldrich® to provide pristine materials, thus increasing the economic efficiency of scintillation crystal growth.

Table 1. Properties of some common scintillation materials.

Mat’l


Density (g/cm3)


Emission Max. (nm)


Refractive Index


Decay Constant (ns)


Con-version Efficiency*


Absolute Light-Yield (photons /MeV)


Hygroscopic

Doped Halides

NaI(Tl)


3.67


415


1.85


230


100


38,000


yes

CsI(Na)


4.51


420


1.84


630


85


39,000


slightly

CaF2(Eu)


3.18


435


1.47


840


50


24,000


no

CsI(Tl)


4.51


550


1.80


600/3,400


45


65,000


no

6LiI(Eu)


4.08


470


1.96


1,400


35


11,000


yes

Undoped Halides

BaF2


4.88


315


1.56


630


16


9,500


no

220


0.8





1,400

CsF


4.64


390





3 – 5


5-7





yes

CsI


4.51


450


1.80


2/20


4-6


2,000


no

305


several µs





varies

Cerium Doped Inorganics

YAP(Ce)


5.55


350


1.95


27


35 – 40


18,000


no

GSO(Ce)


6.71


440


1.85


30 to 60


20 – 25


9,000


no

LSO(Ce)


7.34


420


1.82


47





25,000


no

LuAP(Ce)


8.4


365


1.94


17





17,000


no

YAG(Ce)


4.56


550


1.82


88/302





17,000


no

Other Undoped Inorganics

CdWO4


7.9


470 / 540


2.30


20,000/5,000


25 – 30


15,000


no

BGO


7.13


480


2.15


300


15 – 20


8,200


no

Glasses

6Li - glass


2.4 – 2.7


390 – 430


1.55-1.58


60 – 100


4 – 6


3,300 – 5,600


no

Tb - glass


3.03


550


1.50


~3,000 – 5,000





~50,000




Plastics

Anthracene


1.25


447





30


43


16,500


no

Stilbene


1.16


410





4.5


22


8,300


no

*Scintillation signal relative to NaI(Tl) at room temperature for g-rays when coupled to a photomultiplier tube with a Bi-alkali photocathode (the absolute energy conversion of NaI(Tl) is 0.12).


Doped Lanthanide Halides – A Promising New Class of Scintillator Materials

Doped lanthanide halides are a promising new class of scintillation crystals. Recent papers show that these materials not only possess high light output, but also the proportionality necessary for high-energy resolution. In addition, doped lanthanide halides exhibit fast response times and good gamma ray stopping efficiency. High-purity, anhydrous lanthanide halide source materials are expensive and difficult to obtain in the quantities necessary for bulk crystal growth. Sigma-Aldrich holds a unique position as a supplier of ultra-dry rare-earth salts available in bulk quantities.
Inorganic Halide Crystals Represent Benchmark Scintillator Materials

Inorganic halide crystals represent the benchmark in scintillator materials. They continue to meet material challenges through active research and development. Essential to the growth of these crystals are ultra-high purity, anhydrous source materials. Proprietary Sigma-Aldrich technology is used to produce beaded materials whose reduced surface area minimize moisture absorption and allow increased crucible loading, boosting crystal yield. Precursor materials must also be free of significant amounts of trace radioactive impurities. Research and manufacturing needs for source materials are met by suppliers like Sigma-Aldrich that can provide high quality materials and the technical knowledge and commitment necessary to help advance high technology applications.