Related Experiment Video
Updated: Jun 13, 2026

Preparing an Isotopically Pure 229Th Ion Beam for Studies of 229mTh
Published on: May 3, 2019
Zero-shift thimble ionization chamber
Purpose:
Verify experimentally the theoretical prediction of F. Tessier and I. Kawrakow [Med. Phys. 37, 96-107 (2010)] that it is possible to design a thimble ionization chamber with no shift in its effective point of measurement (EPOM), i.e., a chamber that provides a measure of the dose to the medium at the location of its central axis.
Methods:
Measure dose from a 25 MV photon beam incident on water with an Exradin A1SL ionization chamber inside a thin sleeve (as a means of effectively increasing the thimble wall thickness). The depth-dose curve is compared to that obtained using a well-characterized PTW Roos parallel-plate chamber.
Results:
With an appropriate increase in thimble wall thickness, the EPOM shift of the Exradin A1SL vanishes. Further increase of the wall thickness yields a chamber with a positive (downstream) shift in its point of measurement.
Conclusions:
It is possible to design a thimble ionization chamber with a zero EPOM shift by adjusting the wall thickness.
Related Concept Videos
Mass Analyzers: Common Types
Thomson's e/m Experiment
A particle with charge q, speed v, and mass m enters an area from the top, where the magnetic and electric fields are perpendicular both to the particle's motion and to one another. The magnetic...
Ionization Energy
Inductively Coupled Plasma–Mass Spectrometry (ICP–MS): Overview
Inductively Coupled Plasma-Mass Spectrometry (ICP-MS): Interferences
π Electron Effects on Chemical Shift: Overview

