CsI(Tl) Scintillation Material

Cesium Iodide

  • High density, High γ-ray stopping power
  • Emission wavelength at 550 nm
  • Compatible with photodiode readout

CsI(Tl) scintillator has a light output of 54 photons/keV and is one of the brightest scintillators known. 

The maximum of the broad emission is situated at 550nm and the emission is, therefore, not well matched to a bialkali photocathode photomultiplier tube. However, since most of its emission in the long-wavelength part of the spectrum (>500nm), it is well-suited for photodiode readout.

CsI(Tl) has very good stopping power, is slightly hygroscopic and with good plastic mechanical properties. Combined with relatively good radiation hardness properties, CsI(Tl) is also well suited for High Energy Physics. It is used in Phoswiches for whole-body counting and X-ray astronomy as the "guard crystal" with typically a thin NaI(Tl) as the primary detector.

The decay time of CsI(Tl) consists of more than one component. The fastest component has a value of about 0.6µs, the slowest 3.5µs. For excitation with highly ionizing particles, such as α-particles or protons, the ratio between the intensity of these two decay components varies as a function of the ionizing power of the absorbed particle. CsI(Tl) scintillation crystals can therefore be used for particle discrimination using pulse shape analysis. It has been demonstrated that nuclei up through Li can be identified this way.

CsI is relatively soft and plastic and does not cleave. Because it has no cleavage plane, it is quite rugged – which makes it well-suited for well logging, space research or other applications where severe shock conditions are encountered.

Recent advances in CsI(Tl) array manufacturing have resulted in afterglow reduction, improved light output, and afterglow uniformity, which make an excellent choice for security inspection and nuclear imaging. Read more details,

Density [g/cm3] 4.51
Melting point [K] 894
Thermal expansion coefficient [C-1] 54 x 10-6
Cleavage plane none
Hardness (Mho) 2
Hygroscopic Slightly
Wavelength of emission max. [nm] 550
Lower wavelength cutoff [nm]  320
Refractive index @ emission max 1.79
Primary decay time [ns] 1000
Light yield [photons/keVγ 54
Photoelectron yield [% of NaI(Tl)] (for γ-rays) 45
Packaging Options
  • Linear and two-dimensional arrays with or without readout devices compact package with photodiode
  • CsI is slightly hygroscopic. Contact with water and high humidity should be avoided

CsI(Tl) is a rugged, malleable material that can be easily fabricated into a variety of geometries. It is slightly hygroscopic but is packaged in a manner to minimize exposure to moisture. CsI(Tl) is regularly fabricated into both linear and 2-dimensional (2D) arrays with pixel sizes as small as 500 microns square.

CsI is slightly hygroscopic. Contact with water and high humidity should be avoided.

Photodiode Readout

Since CsI(Tl) has most of its emission in the long-wavelength part of the spectrum (>500nm), it is well-suited for photodiode readout. Photodiodes are available in a variety of sizes. The size of the photodiode should be such that a maximum amount of scintillation light can be detected. We have standardized detectors using 10x10mm2 and 18x18mm2 photodiodes, the smaller ones allowing lower noise levels. Compact detectors with built-in photodiodes are available.

Typical Sizes
  • Arrays - minimum 0.3mm pixel 
  • 10x10x10mm mounted on Photodiode
  • 3" diameter x 6" long capability
CsI Variations

Cesium Iodide is a material with high γ-ray stopping power due to its relative high density and atomic number. For scintillation counting, it is used either in its un-doped (Pure) form or doped with sodium (Na) or thallium (Tl). 

CsI(Na) has a wavelength of emission peak at 420nm and has a light output of 85% of NaI(Tl).  The material is attractive where severe environmental conditions are encountered. It is used in Oil and gas exploration – well logging, Space research and other harsh environment applications.

CsI(Pure) has an emission maximum at 315 nm with an intensity much smaller than either of the activated types of this material. But, this emission is characterized by a relatively short decay time of 16 ns, thus the material can be used for fast timing applications. Undoped CsI is mainly used in physics experiments because of its combination of fast timing and relatively high density.

These material variations are not standard production but can be considered for large projects.

Data Sheets

CsI Scintillation Material Data Sheet

PDF | 168.44 KB
Technical papers

Low Afterglow Linear CsI(Tl) Segmented Arrays

PDF | 662.69 KB
Technical papers
Low afterglow co-doped CsI(Tl)

Very low afterglow CsI(Tl) scintillator using antimony and other metal cations IEEE Industrial presentation 2021 - Fang Meng, Peter Menge

PDF | 2.39 MB
Data Sheets

Efficiency Calculations for Selected Scintillators

PDF | 1.36 MB