spore: profile
id: caesium-iodide
spore_version: 0.2
family: optical-crystals
name: Caesium iodide (CsI), with thallium and sodium doped scintillator grades
abbreviation: CsI
role: IR optical crystal, and X-ray and gamma scintillator (CsI(Tl) and CsI(Na))
category: glasses-optics
type: optical-crystals
summary: |
  Caesium iodide is the cubic CsCl-structure crystal CsI, grown by the sealed ampoule Stockbarger method in ingots of about 70 mm diameter. Crystran gives a transmission range of 0.25 to 55 µm, a refractive index of 1.73916 at 10 µm and a density of 4.51 g/cc, and it calls CsI the material with the deepest known IR transmission. The crystal is very soft and hard to polish. Doped with thallium it becomes CsI(Tl), which emits at 550 nm and matches silicon photodiodes and SiPM arrays (Berkeley Nucleonics). Doped with sodium it becomes CsI(Na), which emits at 420 nm and matches bialkali photomultipliers (Shalom EO). The only flight record read is the Fermi LAT calorimeter, which uses CsI(Tl) in low Earth orbit and sees its light yield fall by about 1 percent a year.
best_for: IR windows that must reach far into the infrared, and X-ray and gamma scintillators read by silicon photodiodes or SiPMs (CsI(Tl))
weakest_at: 'Softness and moisture: the crystal is extremely soft and hard to polish, and it is slightly hygroscopic and soluble in water'
identity:
  formula: CsI
  cas: 7789-17-5
  crystal_system: cubic
  space_group: Pm3m, CsCl structure
  source: crystran-csi-page
  where: Product Data table, Class/Structure row (Cubic CsCl, Pm3m, no cleavage, deforms); molecular weight 259.83 on the Crystran table and 259.8099 for CID 24601 on PubChem; CAS 7789-17-5 from PubChem CID 24601
tags:
  environments:
    - low-earth-orbit
  stresses: []
  applications:
    - optics-and-cameras
properties:
  - benchmark: refractive_index
    label: Refractive index at 10 µm
    value: 1.73916
    unit: '-'
    scope: material
    conditions:
      wavelength: 10 µm
      dopant: none (undoped crystal)
    method: Printed on the Crystran product data table, with references to the Palik handbook and a 1955 paper
    status: ground
    source: crystran-csi-page
    where: Product Data table, Refractive Index row (1.73916 at 10 µm)
  - benchmark: refractive_index_temperature_coefficient
    label: Refractive index change with temperature (dn/dT)
    value: -99.3
    unit: ppm/K
    scope: material
    conditions:
      unit_in_source: 10^-6 K^-1
      wavelength: not stated on the page
      dopant: none (undoped crystal)
    method: Printed as -99.3 x 10^-6 K^-1 with reference 2 (Rodney 1955); the number is not converted; not a canonical property name
    status: ground
    source: crystran-csi-page
    where: Product Data table, dn/dT row
  - benchmark: refractive_index
    label: Refractive index at the emission maximum, CsI(Tl)
    value: 1.79
    unit: '-'
    scope: material
    conditions:
      wavelength: 550 nm (emission maximum)
      dopant: thallium, 0.10 to 0.12 mole%
    method: Printed on the Berkeley Nucleonics CsI(Tl) datasheet (2016), specifications table
    status: ground
    source: bnc-csitl-datasheet
    where: Specifications table, Refractive index at emission maximum row
  - benchmark: refractive_index
    label: Refractive index at the emission peak, CsI(Na)
    value: 1.84
    unit: '-'
    scope: material
    conditions:
      wavelength: 420 nm (emission peak)
      dopant: sodium
    method: Printed on the Shalom EO CsI(Na) product page, specifications table and comparison chart
    status: ground
    source: shalomeo-csi-na
    where: Specifications table, Refractive Index at Emission Peak row; comparison chart, Refractive Index at Emission Maximum row
  - benchmark: density
    label: Density
    value: 4.51
    unit: g/cc
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Value printed on the Crystran product data table; the same 4.51 g/cm3 is printed for CsI(Tl) and CsI(Na) on the BNC and Shalom EO pages
    status: ground
    source: crystran-csi-page
    where: Product Data table, Density row
  - benchmark: melting_point
    label: Melting point
    value: 621
    unit: °C
    scope: material
    conditions:
      pressure: not stated on the page
      dopant: none (undoped crystal)
    method: Value printed on the Crystran product data table; the Shalom EO comparison chart prints 621 °C for CsI, CsI(Tl) and CsI(Na)
    status: ground
    source: crystran-csi-page
    where: Product Data table, Melting Point row
  - benchmark: max_service_temp
    label: Maximum useful temperature, CsI(Tl) crystal
    value: 125
    unit: °C
    scope: material
    conditions:
      dopant: thallium, 0.10 to 0.12 mole%
      basis: bare crystal, useful temperature range
    method: Printed on the BNC CsI(Tl) datasheet as the upper end of a useful temperature range; the lower end is left out (see gaps)
    status: ground
    source: bnc-csitl-datasheet
    where: Specifications table, Useful temperature range row
  - benchmark: thermal_conductivity
    label: Thermal conductivity
    value: 1.1
    unit: W/(m·K)
    scope: material
    conditions:
      temperature: 298 K
      dopant: none (undoped crystal)
    method: Printed on the Crystran product data table with reference 3 (Combes et al. 1951); the BNC datasheet prints 0.13 for CsI(Tl), which is not reconciled here (see gaps)
    status: ground
    source: crystran-csi-page
    where: Product Data table, Thermal Conductivity row
  - benchmark: cte
    label: Coefficient of thermal expansion
    value: 48.3
    unit: ppm/K
    scope: material
    conditions:
      unit_in_source: 10^-6 K^-1
      temperature: 293 K
      dopant: none (undoped crystal)
    method: Printed as 48.3 x 10^-6 K^-1 at 293 K; the number is not converted
    status: ground
    source: crystran-csi-page
    where: Product Data table, Thermal Expansion row
  - benchmark: cte
    label: Coefficient of thermal expansion, CsI(Tl)
    value: 51
    unit: ppm/K
    scope: material
    conditions:
      unit_in_source: 10^-6 K^-1
      temperature: not stated on the page
      dopant: thallium, 0.10 to 0.12 mole%
    method: Printed on the BNC CsI(Tl) datasheet as 51 x 10^-6 K^-1; the number is not converted; the Shalom EO chart gives 54 for CsI(Tl) (see gaps)
    status: ground
    source: bnc-csitl-datasheet
    where: Mechanical and Thermal Properties table, Thermal expansion coefficient row
  - benchmark: cte
    label: Coefficient of thermal expansion, CsI(Na)
    value: 49
    unit: ppm/K
    scope: material
    conditions:
      unit_in_source: 10^-6 K^-1
      temperature: not stated on the page
      dopant: sodium
    method: Printed on the Shalom EO CsI(Na) page as 49x10^-6 K^-1; the number is not converted
    status: ground
    source: shalomeo-csi-na
    where: Specifications table, Thermal Expansion Coefficient row
  - benchmark: specific_heat
    label: Specific heat capacity
    value: 201
    unit: J/(kg·K)
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Printed on the Crystran product data table with reference 4 (Kelly 1934); the BNC datasheet prints the same 201 J/kg K
    status: ground
    source: crystran-csi-page
    where: Product Data table, Specific Heat Capacity row
  - benchmark: dielectric_constant
    label: Dielectric constant at 1 MHz
    value: 5.65
    unit: '-'
    scope: material
    conditions:
      frequency: 1 MHz
      dopant: none (undoped crystal)
    method: Value printed on the Crystran product data table
    status: ground
    source: crystran-csi-page
    where: Product Data table, Dielectric Constant row
  - benchmark: elastic_modulus
    label: Young's modulus
    value: 5.3
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Value printed on the Crystran product data table; the BNC datasheet prints 5.3 GPa for CsI(Tl)
    status: ground
    source: crystran-csi-page
    where: Product Data table, Youngs Modulus (E) row
  - benchmark: shear_modulus
    label: Shear modulus
    value: 6.24
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Value printed on the Crystran product data table
    status: ground
    source: crystran-csi-page
    where: Product Data table, Shear Modulus (G) row
  - benchmark: bulk_modulus
    label: Bulk modulus
    value: 12.67
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Value printed on the Crystran product data table
    status: ground
    source: crystran-csi-page
    where: Product Data table, Bulk Modulus (K) row
  - benchmark: poissons_ratio
    label: Poisson ratio
    value: 0.214
    unit: '-'
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Value printed on the Crystran product data table
    status: ground
    source: crystran-csi-page
    where: Product Data table, Poisson Ratio row
  - benchmark: apparent_elastic_limit
    label: Apparent elastic limit
    value: 5.6
    unit: MPa
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Printed on the Crystran product data table as 5.6 MPa (810 psi); not a canonical property name
    status: ground
    source: crystran-csi-page
    where: Product Data table, Apparent Elastic Limit row
  - benchmark: reststrahlen_peak
    label: Reststrahlen peak
    value: 145.8
    unit: µm
    scope: material
    conditions:
      temperature: not stated on the page
      dopant: none (undoped crystal)
    method: Printed on the Crystran product data table; not a canonical property name
    status: ground
    source: crystran-csi-page
    where: Product Data table, Reststrahlen Peak row
  - benchmark: transmission_range
    label: Transmission range
    value: 0.25
    value_high: 55
    bound: range
    unit: µm
    scope: material
    conditions:
      thickness: not stated on the page
      dopant: none (undoped crystal)
    method: Range printed on the Crystran product data table; not a canonical property name
    status: ground
    source: crystran-csi-page
    where: Product Data table, Transmission Range row
  - benchmark: water_solubility
    label: Solubility in water at 0 °C
    value: 44
    unit: g/100 g water
    scope: material
    conditions:
      temperature: 0 °C
      dopant: none (undoped crystal)
    method: Printed on the Crystran product data table; not a canonical property name
    status: ground
    source: crystran-csi-page
    where: Product Data table, Solubility row
  - benchmark: emission_peak_wavelength
    label: Emission maximum, CsI(Tl)
    value: 550
    unit: nm
    scope: material
    conditions:
      dopant: thallium, 0.10 to 0.12 mole%
      temperature: not stated on the page
    method: Printed as the maximum emission wavelength on the BNC CsI(Tl) datasheet; the Shalom EO comparison chart also prints 550; not a canonical property name
    status: ground
    source: bnc-csitl-datasheet
    where: Specifications table, Maximum emission wavelength row
  - benchmark: emission_peak_wavelength
    label: Emission peak, CsI(Na)
    value: 420
    unit: nm
    scope: material
    conditions:
      dopant: sodium
      temperature: not stated on the page
    method: Printed on the Shalom EO CsI(Na) page as the emission peak wavelength; not a canonical property name
    status: ground
    source: shalomeo-csi-na
    where: Specifications table, Emission Peak Wavelength row
  - benchmark: decay_time
    label: Decay time, fast component, CsI(Tl)
    value: 600
    unit: ns
    scope: material
    conditions:
      dopant: thallium, 0.10 to 0.12 mole%
      temperature: not stated on the page
    method: Printed on the BNC CsI(Tl) datasheet as 600 ns and 3.4 µs (two components); not a canonical property name
    status: ground
    source: bnc-csitl-datasheet
    where: Specifications table, Decay time row
  - benchmark: decay_time_slow_component
    label: Decay time, slow component, CsI(Tl)
    value: 3.4
    unit: µs
    scope: material
    conditions:
      dopant: thallium, 0.10 to 0.12 mole%
      temperature: not stated on the page
    method: Printed on the BNC CsI(Tl) datasheet in the same decay time row as the 600 ns component; not a canonical property name
    status: ground
    source: bnc-csitl-datasheet
    where: Specifications table, Decay time row
  - benchmark: decay_time
    label: Decay time, CsI(Na)
    value: 630
    unit: ns
    scope: material
    conditions:
      dopant: sodium
      temperature: not stated on the page
    method: Printed on the Shalom EO CsI(Na) page as 630 ns; not a canonical property name
    status: ground
    source: shalomeo-csi-na
    where: Specifications table, Decay Time row
  - benchmark: photoelectron_yield_relative_nai_tl
    label: Photoelectron yield with a bialkali PMT, CsI(Tl)
    value: 45
    unit: '% of NaI(Tl)'
    scope: material
    conditions:
      readout: bialkali photomultiplier, gamma rays
      dopant: thallium, 0.10 to 0.12 mole%
    method: Printed on the BNC CsI(Tl) datasheet as 45% relative to NaI(Tl); not a canonical property name
    status: ground
    source: bnc-csitl-datasheet
    where: Specifications table, Photoelectron yield row
  - benchmark: photoelectron_yield_relative_nai_tl
    label: Photoelectron yield, CsI(Na)
    value: 85
    unit: '% of NaI(Tl)'
    scope: material
    conditions:
      readout: gamma rays
      dopant: sodium
    method: Printed on the Shalom EO CsI(Na) page as 85% of NaI(Tl) for gamma rays; not a canonical property name
    status: ground
    source: shalomeo-csi-na
    where: Specifications table, Photoelectron Yield row
  - benchmark: lower_wavelength_cutoff
    label: Lower wavelength cutoff, CsI(Tl)
    value: 320
    unit: nm
    scope: material
    conditions:
      dopant: thallium, 0.10 to 0.12 mole%
    method: Printed on the Shalom EO comparison chart; not a canonical property name
    status: ground
    source: shalomeo-csi-na
    where: Comparison chart, Lower Wavelength Cutoff row, CsI(Tl) column
  - benchmark: refractive_index
    label: Refractive index at the emission maximum, undoped CsI
    value: 1.95
    unit: '-'
    scope: material
    conditions:
      wavelength: 315 nm (emission maximum of undoped CsI)
      dopant: none (undoped crystal)
    method: Printed on the Shalom EO comparison chart; the undoped value is at a different wavelength from the Crystran value at 10 µm, so the two are not compared as one property
    status: ground
    source: shalomeo-csi-na
    where: Comparison chart, Refractive Index at Emission Maximum row, CsI (Undoped) column
  - benchmark: dopant_concentration
    label: Thallium doping concentration, CsI(Tl)
    value: 0.1
    value_high: 0.12
    bound: range
    unit: mole%
    scope: material
    conditions:
      dopant: thallium
    method: Printed on the BNC CsI(Tl) datasheet as 0.10 to 0.12 mole%; not a canonical property name
    status: ground
    source: bnc-csitl-datasheet
    where: Specifications table, Thallium doping concentration row
corrosion:
  - environment: Moist air
    headline: CsI is slightly hygroscopic, so bare crystals need a dry store.
    effect: The BNC datasheet calls CsI(Tl) slightly hygroscopic and gives a storage limit of less than 50 percent relative humidity for a bare crystal. The Shalom EO chart calls CsI(Na) hygroscopic, which is stronger.
    rating: moderate
    conditions:
      basis: qualitative statement and storage limit; no uptake rate printed
      agent: water vapour
    source: bnc-csitl-datasheet
    where: Specifications table, Hygroscopicity and Storage condition rows; Shalom EO comparison chart, Hygroscopic row
  - environment: Liquid water at 0 °C
    headline: CsI dissolves in water, at 44 g per 100 g of water at 0 °C.
    effect: The Crystran table gives a solubility of 44 g per 100 g of water at 0 °C, which is far higher than the calcium fluoride figure. This is a solubility figure, not a corrosion rate, and no dissolution test is printed.
    rating: severe
    conditions:
      temperature: 0 °C
      note: solubility figure, not a corrosion rate
    source: crystran-csi-page
    where: Product Data table, Solubility row
  - environment: Low Earth orbit, trapped radiation
    headline: The light yield of the Fermi LAT CsI(Tl) calorimeter falls about 1 percent a year.
    effect: The LAT paper reports a slow decrease of about 1 percent a year in the scintillation light yield of the crystals from radiation damage in low Earth orbit. It fits a time constant of about 2 years and predicts an energy-scale shift of about 4.5 percent after 10 years, which the team plans to correct.
    rating: low
    conditions:
      basis: flight data over three years, and a fit with a time constant; not a ground test
      agent: trapped charged particles in the South Atlantic Anomaly
    source: arxiv-1206-1896
    where: Calorimeter section, text on radiation damage in the first three years; section 7.3.2 fit and prediction
  - environment: Thermal cycling
    headline: Large temperature ramps can crack CsI(Tl), because it expands more than most crystals.
    effect: BNC says CsI(Tl) has a relatively high thermal expansion coefficient and recommends slow ramp rates, and it gives a maximum thermal gradient of 10 °C per minute for small crystals. No cycle count is printed.
    rating: moderate
    conditions:
      basis: vendor guidance; no fatigue test printed
      agent: temperature ramp
    source: bnc-csitl-datasheet
    where: Mechanical and Thermal Properties table, Maximum thermal gradient rows; thermal note below the specifications
risks:
  hazards:
    - text: PubChem's aggregated GHS record for CsI gives H315 (causes skin irritation), H317 (may cause an allergic skin reaction), H319 (causes serious eye irritation) and H335 (may cause respiratory irritation). The percentages PubChem prints are shares of supplier notifications, not a classification by a regulator.
      ghs:
        - H315
        - H317
        - H319
        - H335
      source: pubchem-cid-24601
      where: GHS Hazard Statements list, CID 24601 record
  failure_risks:
    - mode: Plastic deformation under light load
      trigger: Mechanical contact, or polishing a window
      consequence: The Crystran table lists CsI as having no cleavage and as deforming, and it gives an apparent elastic limit of 5.6 MPa. The crystal therefore bends or scratches under light load, and Crystran says it is extremely difficult to polish.
      severity: moderate
      source: crystran-csi-page
      where: Class/Structure row; Apparent Elastic Limit row; Crystran product description
    - mode: Thermally induced stress fracture
      trigger: Fast temperature ramp, especially in larger crystals
      consequence: BNC warns that a high thermal expansion can cause stress fractures during temperature cycling and gives a maximum ramp of 10 °C per minute for small crystals.
      severity: moderate
      source: bnc-csitl-datasheet
      where: Thermal note below the specifications; Maximum thermal gradient rows
    - mode: Slow loss of light yield in orbit
      trigger: Trapped radiation in low Earth orbit
      consequence: The LAT calorimeter loses about 1 percent a year of light yield, which is corrected by a calibration applied channel by channel. The loss is slow and does not stop the instrument.
      severity: low
      source: arxiv-1206-1896
      where: Calorimeter section, text on radiation damage; section 7.3.2
    - mode: Dissolution in water
      trigger: Liquid water or condensation on an uncoated crystal
      consequence: The Crystran table gives a solubility of 44 g per 100 g of water at 0 °C, so a wet surface can dissolve the crystal.
      severity: high
      source: crystran-csi-page
      where: Product Data table, Solubility row
  side_effects: []
  handling:
    - text: Store a bare crystal below 50 percent relative humidity, as the BNC datasheet advises.
      source: bnc-csitl-datasheet
      where: Specifications table, Storage condition row
    - text: Use gentle handling and avoid polishing or cleaning with water, because the crystal is extremely soft and dissolves in water.
      source: crystran-csi-page
      where: Product Data table, Solubility row; product description
  mitigations:
    - text: Use a sealed or lightly protected housing for CsI(Tl). BNC says that a sealed or lightly protected housing lets the crystal tolerate normal laboratory humidity, and that hermetic encapsulation is available.
      source: bnc-csitl-datasheet
      where: Introduction text on hygroscopicity; Wrapping row of the configurations table
    - text: Use an encapsulated crystal. Shalom EO says polished and encapsulated CsI(Na) crystals are available.
      source: shalomeo-csi-na
      where: Product description text
uses:
  - application: optics-and-cameras
    device: IR spectrophotometer components
    example: Crystran says CsI is sometimes used for components in the widest range spectrophotometers, because it has the deepest known IR transmission.
    source: crystran-csi-page
    where: Product description, first paragraph
  - application: optics-and-cameras
    device: X-ray image intensifier input phosphor and X-ray panels
    example: USGS says cesium iodide is used primarily in X-ray panel production, and in fluoroscopy equipment as the input phosphor of X-ray image intensifier tubes.
    source: usgs-mcs2026-cesium
    where: Domestic Production and Use paragraph
  - application: optics-and-cameras
    device: Gamma-ray telescope calorimeter (Fermi LAT)
    example: The Fermi LAT calorimeter has 8.6 radiation lengths of CsI(Tl) crystals stacked in 8 layers. Its crystal logs measure 326 mm by 26.7 mm by 19.9 mm, and the paper reports the light yield loss in orbit.
    source: arxiv-1206-1896
    where: Introduction, paragraph describing the CAL; calorimeter module description
  - application: optics-and-cameras
    device: Silicon photodiode and SiPM readout of CsI(Tl) detectors
    example: BNC says the 550 nm CsI(Tl) emission matches standard silicon photodiodes and SiPM arrays, so it can be read without a photomultiplier.
    source: bnc-csitl-datasheet
    where: Introduction text; Typical Applications list
supply:
  - subject: Caesium, the metal in the feedstock (USGS 2026 summary, data for 2025)
    producers: []
    critical_lists: []
    recycling: 'USGS: cesium iodide is recycled from radiography panels and used in the production of new panels.'
    substitutes:
      - 'USGS: many applications have used mineral substitutes, because primary cesium is not available globally.'
    source: usgs-mcs2026-cesium
    where: Domestic Production and Use paragraph; Recycling paragraph. The USGS summary gives no official world production figure and no country shares. It names Canada, China, Germany and Russia as the primary global producers, estimated without shares, in the Salient Statistics paragraph.
  - subject: Iodine, the iodide part of CsI (world mine production, 2025)
    producers:
      - country: Chile
        share_percent: 67.6
        year: 2025
      - country: Japan
        share_percent: 26.5
        year: 2025
    critical_lists: []
    recycling: 'USGS: small amounts of iodine were recycled.'
    substitutes: []
    source: usgs-mcs2026-iodine
    where: 'World Mine Production and Reserves table, 2025 column: Chile 23,000 and Japan 9,000, of a world total of 34,000 in metric tons of elemental iodine. Shares calculated by us as country value divided by the printed world total. The United States is shown as W, withheld.'
  - subject: US iodine import sources (2021 to 2024, as printed)
    producers:
      - country: Chile
        share_percent: 88
        year: 2024
      - country: Japan
        share_percent: 11
        year: 2024
    critical_lists: []
    recycling: Not repeated here; see the first supply entry.
    substitutes: []
    source: usgs-mcs2026-iodine
    where: Import Sources (2021 to 2024) line, printed as Chile 88 percent, Japan 11 and other 1. The year shown is the last year of the range.
documents:
  - title: 'Crystran: Caesium Iodide CsI optical material page, Product Data table (undated)'
    type: datasheet
    url: https://www.crystran.com/optical-materials/caesium-iodide-csi
    year: 2026
  - title: 'Berkeley Nucleonics ScintIQ: CsI(Tl) Cesium Iodide Scintillation Crystals datasheet (revision 04-01-2016)'
    type: datasheet
    url: https://www.berkeleynucleonics.com/cesium-iodide/
    year: 2016
  - title: 'Hangzhou Shalom EO: CsI(Na) Scintillator Crystals product page, with CsI, CsI(Tl) and CsI(Na) comparison chart'
    type: datasheet
    url: https://www.shalomeo.com/Scintillators/CsI-Na/product-392.html
    year: 2026
  - title: 'PubChem: Cesium iodide (CID 24601), compound record with GHS classification'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/24601
    year: 2026
  - title: 'PubChem: Caesium (CID 5354618), compound record with molecular weight'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/compound/5354618
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Cesium'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-cesium.pdf
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Iodine'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-iodine.pdf
    year: 2026
  - title: 'The Fermi Large Area Telescope On Orbit: Event Classification, Instrument Response Functions, and Calibration (arXiv:1206.1896)'
    type: paper
    url: https://arxiv.org/pdf/1206.1896
    year: 2012
gaps:
  - No ground or orbital radiation test of undoped CsI or CsI(Na) was found in a source we could open. The only in-orbit result is the Fermi LAT calorimeter light yield trend for CsI(Tl), which is an instrument measurement and not a material test.
  - No outgassing (TML/CVCM) or atomic-oxygen result for caesium iodide was found in the sources read.
  - Thermal conductivity conflicts between sources. Crystran gives 1.1 W/(m·K) at 298 K for undoped CsI; BNC gives 0.13 W/(m·K) for CsI(Tl). Both are entered with their own grade conditions and are not reconciled.
  - Thermal expansion conflicts between sources. Crystran gives 48.3 ppm/K at 293 K for undoped CsI; BNC gives 51 for CsI(Tl); the Shalom EO chart gives 54 for CsI(Tl) and 49 for CsI(Na). The 54 is not entered, to keep one value per grade.
  - Light output in photons per MeV is printed by Shalom EO as 52 to 56 thousand for CsI(Tl) and 38 to 44 thousand for CsI(Na). It is not entered as a property, because the printed form (x10^3) does not match the value check; the relative photoelectron yield is entered instead.
  - 'Decay time for CsI(Tl) conflicts: BNC gives 600 ns and 3.4 µs, and the Shalom EO chart gives 1 µs. The BNC two-component figure is entered and the 1 µs is not.'
  - The lower end of the useful temperature range for CsI(Tl) is printed as -40 °C with a typographic minus sign. It is left out of the numeric fields and only the upper end is entered.
  - The Crystran data table prints Knoop hardness as Knoop 20 with a 200 g indenter, with no unit. It is not entered.
  - The refractive index of undoped CsI at 10 µm (Crystran, 1.73916) and at 315 nm (Shalom EO, 1.95) are at different wavelengths. Both are entered with their own conditions and are not compared as one property.
  - Thallium is toxic and is the dopant in CsI(Tl). No thallium safety data sheet or thallium GHS record was read in this run, so the dopant hazard is not sourced.
  - No supplier safety data sheet for CsI was opened. The GHS entry is PubChem's aggregated notification record, which prints notification shares, not a regulator's classification. The Crystran page links a 'Caesium Bromide (CsI) SDS' under a name that does not match the grade, and it was not opened.
  - The atomic weight of iodine was not read; only the molecular weight of I2 (253.8089, CID 807) was seen. Iodine is therefore listed in the recipe without an atomic weight.
  - The USGS cesium summary gives no official world production figure and no country shares, so no caesium production shares are entered. The producer list it names is estimated, not measured.
  - Supply shares for iodine are calculated by us from the printed world total and are shares of mine output, not of CsI crystal output.
  - The Fermi LAT crystal count of 1536 appeared only in a search summary and is not entered.
  - The vendor pages give values for their own grades, not independent measurements. The Crystran page is undated and the BNC datasheet is dated 2016.
  - No media (photograph or drawing with a licence) was checked for this profile.
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 31
  unreachable: 0
  caveats: 5
  note: 'Written by an AI assistant on 2026-10-26 from the Crystran product page, the Berkeley Nucleonics CsI(Tl) datasheet, the Shalom EO CsI(Na) page, PubChem records, the USGS 2026 cesium and iodine summaries and the Fermi LAT paper read in the same run. Caveats: the vendors conflict on thermal conductivity, thermal expansion and decay time; no supplier SDS was opened; the thallium dopant hazard is not sourced; caesium supply has no official figures; light output in photons per MeV is not entered.'
sources:
  crystran-csi-page:
    title: 'Crystran: Caesium Iodide CsI optical material page, Product Data table'
    id: Crystran product page for caesium iodide (undated)
    url: https://www.crystran.com/optical-materials/caesium-iodide-csi
    year: 2026
    kind: datasheet
    license: supplier page, linked not reproduced
  bnc-csitl-datasheet:
    title: 'Berkeley Nucleonics ScintIQ: CsI(Tl) Cesium Iodide Scintillation Crystals datasheet'
    id: CsI(Tl)_datasheet, revision 04-01-2016
    url: https://www.berkeleynucleonics.com/cesium-iodide/
    year: 2016
    kind: datasheet
    license: vendor datasheet, linked not reproduced
  shalomeo-csi-na:
    title: 'Hangzhou Shalom EO: CsI(Na) Scintillator Crystals product page, with comparison chart for CsI, CsI(Tl) and CsI(Na)'
    id: product 392, CsI(Na) page
    url: https://www.shalomeo.com/Scintillators/CsI-Na/product-392.html
    year: 2026
    kind: datasheet
    license: vendor page, linked not reproduced
  pubchem-cid-24601:
    title: PubChem compound record, cesium iodide, CID 24601, with GHS classification
    id: CID 24601; CsI; molecular weight 259.8099
    url: https://pubchem.ncbi.nlm.nih.gov/compound/24601
    year: 2026
    kind: database
    license: public domain (NCBI)
  pubchem-cid-5354618:
    title: PubChem compound record, caesium, CID 5354618, with molecular weight
    id: CID 5354618; Cs; molecular weight 132.9054520
    url: https://pubchem.ncbi.nlm.nih.gov/compound/5354618
    year: 2026
    kind: database
    license: public domain (NCBI)
  usgs-mcs2026-cesium:
    title: Mineral Commodity Summaries 2026, Cesium (USGS, Candice C. Tuck)
    id: USGS MCS 2026, data for 2025
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-cesium.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-mcs2026-iodine:
    title: Mineral Commodity Summaries 2026, Iodine (USGS)
    id: USGS MCS 2026, data for 2025
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-iodine.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  arxiv-1206-1896:
    title: 'The Fermi Large Area Telescope On Orbit: Event Classification, Instrument Response Functions, and Calibration'
    id: arXiv:1206.1896
    url: https://arxiv.org/pdf/1206.1896
    year: 2012
    kind: paper
    license: arXiv, linked not reproduced
story:
  recipe:
    line: Caesium iodide is an ionic compound of caesium and iodine, with one caesium atom for each iodide ion. Crystran grows the crystal by the sealed ampoule Stockbarger technique in ingots of about 70 mm diameter. Thallium or sodium is added as a dopant to make the scintillator grades, CsI(Tl) at 0.10 to 0.12 mole percent thallium and CsI(Na) with sodium.
    parts:
      - name: Caesium
        symbol: Cs
        atoms: 1
        atomic_weight: 132.905452
        role: The alkali metal cation. One caesium atom per formula unit (PubChem CID 5354618 for the atomic weight).
      - name: Iodine
        symbol: I
        role: The halogen anion. One iodine atom per formula unit (formula CsI from PubChem CID 24601).
      - name: Thallium
        symbol: Tl
        role: Activator dopant in the CsI(Tl) grade, at 0.10 to 0.12 mole percent (BNC).
      - name: Sodium
        symbol: Na
        role: Activator dopant in the CsI(Na) grade (Shalom EO).
  risk_groups:
    - title: For people
      level: 1
      verdict: Irritant; may cause allergy or breathing irritation
      lead: PubChem's aggregated GHS record for CsI lists H315, H317, H319 and H335. The percentages are notification shares, and no supplier SDS was read.
      points:
        - label: Skin and eyes
          line: H315 (skin irritation), H317 (allergic skin reaction) and H319 (serious eye irritation), per PubChem's aggregated record.
        - label: Inhalation
          line: H335 (respiratory irritation), per PubChem's aggregated record. The dopant thallium is toxic and is not sourced in this run (see gaps).
    - title: If it fails
      lead: The failure results come from vendor datasheets and one flight instrument. No flight failure record was read.
      points:
        - label: Softness
          line: Crystran says the crystal is extremely soft, with an apparent elastic limit of 5.6 MPa, so it deforms and is hard to polish.
        - label: Thermal stress
          line: BNC warns that the high thermal expansion can crack a crystal during fast temperature changes.
        - label: Water
          line: Crystran gives a solubility of 44 g per 100 g of water at 0 °C, so a wet surface can dissolve the crystal.
    - title: Handling and storage
      gap: BNC gives a storage limit of less than 50 percent relative humidity for a bare crystal. No storage temperature is printed.
      points:
        - label: Humidity
          line: Keep a bare crystal below 50 percent relative humidity, and use a sealed or lightly protected housing.
        - label: Cleaning
          line: Do not clean or polish with water, because the crystal dissolves in water.
  why: Caesium iodide is a cubic crystal that transmits from 0.25 to 55 µm (Crystran), so it is used where the infrared reaches furthest. Doped with thallium it becomes a 550 nm scintillator that silicon photodiodes read (BNC). Doped with sodium it becomes a 420 nm scintillator that bialkali photomultipliers read (Shalom EO). The Fermi LAT calorimeter flew CsI(Tl) in low Earth orbit and measured a light yield loss of about 1 percent a year.
  uses:
    - where: IR spectrophotometers
      line: Crystran says CsI is sometimes used for components in the widest range spectrophotometers, because it has the deepest known IR transmission.
    - where: X-ray panels and image intensifiers
      line: USGS says CsI is used primarily in X-ray panel production, and as the input phosphor of X-ray image intensifier tubes in fluoroscopy.
    - where: Gamma-ray telescope (Fermi LAT)
      line: The LAT calorimeter has 8.6 radiation lengths of CsI(Tl) crystals in 8 layers, and its light yield is tracked in orbit.
  can_do:
    - label: Range
      value: 0.25 to 55 µm
      line: Crystran's data table gives a transmission range of 0.25 to 55 µm for undoped CsI, the widest of the windows listed on the page.
    - label: Refraction
      value: '1.73916'
      line: Refractive index 1.73916 at 10 µm, per the Crystran data table. Refractive index 1.79 at the CsI(Tl) emission maximum (BNC).
    - label: Light
      value: 550 nm
      line: CsI(Tl) emits at 550 nm (BNC) and CsI(Na) emits at 420 nm (Shalom EO), so each grade suits a different photosensor.
    - label: Heat
      value: 621 °C
      line: Melting point 621 °C on the Crystran table. BNC gives a useful temperature range up to 125 °C for CsI(Tl).
    - label: Density
      value: 4.51 g/cc
      line: Density 4.51 g/cc for the crystal, the same value printed for CsI(Tl) and CsI(Na).
  compare: Crystran's sheet and the two scintillator vendors give different figures for each grade, so the comparison is between grades and not between suppliers. Undoped CsI transmits to 55 µm, and its thermal conductivity is 1.1 W/(m·K) at 298 K on the Crystran table, a value higher than the 0.13 that BNC gives for CsI(Tl). CsI(Tl) is about 51 ppm/K in thermal expansion on the BNC sheet, and CsI(Na) is 49 on the Shalom EO chart. The sheets give no side-by-side ground or space test and no cost comparison.
  compare_rows:
    - label: Heat conduction
      benchmark: thermal_conductivity
    - label: Thermal expansion
      benchmark: cte
    - label: Stiffness
      benchmark: elastic_modulus
  handling:
    - Keep bare crystals below 50 percent relative humidity, as BNC advises.
    - Do not clean or polish with water, because CsI dissolves in water (44 g per 100 g at 0 °C).
    - Handle gently and avoid pressure on the surface, because the crystal is extremely soft.
  good_for:
    - IR windows and spectrophotometer components that must transmit to 55 µm (Crystran).
    - X-ray panels and image intensifier input phosphors (USGS).
    - CsI(Tl) scintillators read by silicon photodiodes or SiPMs (BNC).
  watch_out:
    - Humidity. CsI is slightly hygroscopic (CsI(Tl)) and hygroscopic (CsI(Na) per Shalom EO). Keep bare crystals dry.
    - Softness. The crystal deforms and is hard to polish, so the transmission quality is compromised for range.
    - Thermal ramps. The high thermal expansion means slow ramp rates are needed for larger crystals (BNC).
    - Orbit. The Fermi LAT calorimeter lost about 1 percent a year of light yield, and a correction is applied.
    - Source conflicts. Vendors disagree on thermal conductivity, thermal expansion and decay time (see gaps).
  verdict: Caesium iodide is a soft, moisture-sensitive crystal with the widest IR window of the common halides, and as CsI(Tl) it is a rugged scintillator that a silicon photodiode can read. The sources read describe its optics and its one flight use, but give no material test in space, so its orbital life is not established by this profile.