A new extended-length parallel-plate ionization chamber

Quality assurance of six cylindrical and two parallel plate chambers by radioactive check device: Influence of chamber age to its performance

PURPOSE

To evaluate the performances of eight ionization chambers, (two types) manufactured in a time span of 14 years, with radioactive source devices and photon beams.

METHODS

The measurements were performed with six Farmer and two parallel plate ionization chambers with appropriate radioactive check devices and accompanying inserts. Some tests were performed with photon beams at linear accelerator. Physical visibility check, stability, linearity of response, directional dependence, determination of leakage current, ion recombination and polarity effect and influence of background radiation tests were conducted and analyzed.

RESULTS

The performance of all Farmer ionization chambers was in agreement to IEC standard as well as for parallel plate type chambers. Long-term stability of older planparallel chamber was somewhat worse than expected.

CONCLUSION

Radioactive check devices have proven to be easy to use in clinical environment. According to results of this work Farmer type ionization chambers showed good quality regardless of the time of production, which make them long-term suitable for clinical dosimetry in radiotherapy. Also, for parallel plate type of ionization chambers the results showed fine agreement with the IEC standard, except long-term stability for older chamber, which needs further investigation.

Characterization of a homemade ionization chamber for radiotherapy beams

A homemade cylindrical ionization chamber was studied for routine use in therapy beams of (60)Co and X-rays. Several characterization tests were performed: leakage current, saturation, ion collection efficiency, polarity effect, stability, stabilization time, chamber orientation and energy dependence. All results obtained were within international recommendations. Therefore the homemade ionization chamber presents usefulness for routine dosimetric procedures in radiotherapy beams.

Characteristics of a tandem system of ionization chambers in X-ray beams, mammography level

Two parallel plate ionization chambers (inserted in slab phantoms) recently assembled at IPEN were studied in relation to their operational characteristics for use in quality control of X-ray beams, mammography level. The chambers present only one difference: one has an inner collecting electrode made of graphite and the other, of aluminum. These chambers make up a tandem system, which may be employed to verify X-ray beams energy constancy, by the confirmation of half-value layers and effective energies, and to determinate air kerma rates. The chambers presented good results for the operational tests, as recommended internationally.

Câmara de ionização de placas paralelas para radiação-X de radiografia convencional e mamografia

OBJETIVO: Desenvolver uma câmara de ionização de placas paralelas de dupla face para determinação de kerma no ar e taxa de kerma no ar em campos de radiação-X utilizados em radiografia convencional e mamografia. MATERIAIS E MÉTODOS: A câmara desenvolvida tem janelas de entrada de poliéster aluminizado, elétrodos internos e anéis de guarda de alumínio em uma face (face A) e de grafite na outra (face G). Neste trabalho foram testadas as características operacionais de linearidade, dependência angular e energética de resposta. RESULTADOS: A linearidade de resposta foi de 0,86% para a face A e de 0,92% para a face G. Para ângulos de incidência da radiação de 0° a ± 5°, a variação da resposta relativa foi menor que 0,8% para ambas as faces da câmara. A dependência energética de resposta foi de 0,8% para a face A nas qualidades de raios-X para diagnóstico convencional e de 2,4% para a face G da câmara nas qualidades de raios-X para mamografia. CONCLUSÃO: Esta câmara pode ser utilizada rotineiramente na determinação de kerma no ar e taxa de kerma no ar em feixes de raios-X utilizados em radiografia convencional e mamografia.