NPL2611A Ion Chamber

The NPL2611A is a Secondary Standard Therapy Dosimetry Chamber produced by the National Physics Laboratory.

 Construction of the NPL2611A chamber consists of a thin walled high purity graphite thimble, amber insulator and a hollow pure aluminum electrode and is supported by a thin walled aluminum stem. The stem terminates in a robust cable entry, by which the chamber can be mounted.  A length of low noise triaxial cable connects to a TNC triaxial plug.  Since the chamber is vented, air density corrections must be applied.

Please note: This chamber is manufactured at the National Physical Laboratory (NPL), in the UK, and support, both in terms of maintenance and calibration, are available from NPL directly.


Active Dimensions Sensitive Volume—325mm3
Length of volukme—9.22mm
Thimble wall thickness– 0.5mm
Diameter—7.35 mm
Exterior Dimensions Length — 185mm

Diameter — 12.5 mm

Materials Thimble—high density (1.80b/cm3high purity extruded graphite
Collecting electrode – aluminium
99.99% pure
Main insulator — amber stem insulators P.C.T.F.E.
HV insulator – Delrin
The chamber stem is guarded to a point 15mm from the measuring Volume
Reference Point The effective measurement point for positioning the chamber should be taken to be the axis of the chamber 5mm from the tip of the graphite thimble
Venting To ensure that the chamber is equilibrated with the atmosphere the measuring volume is vented giving pressure equilibration times for just a few seconds
Build up Cap Delrin, with external diameter of 17.0 mm and wall thickness of 600mg/cm2
Cable Suhner GO 233OHT connections
Centre core—signal at HV
Inner screen—guard at HV
Outer screen—earth potential
Length 10m