Dependence of the value of m on the lifetime of ions in parallel-plate ionization chambers

The saturation characteristics of two sizes of parallel-plate free-air ionization chambers were investigated for synchrotron radiation at bending-magnet, wiggler and undulator beamlines of SPring-8. The gaps of the electrodes were 4.2 and 85 mm. The monoenergetic photon energies ranged from 10 to 115.56 keV and the air kerma rates from 0.2 mGy s-1 to 150 kGy s-1. Ion recombination at the high dose rate was found to be smaller than that predicted by Boag's expression, which was based on volume recombination, and the difference increased with an applied electric field. In the high dose rate region, the reciprocal of the current was linear to the reciprocal of the electric field near saturation, which represented the occurrence of initial recombination and diffusion loss. At the low electric field and the low dose rate, the reciprocal of the current was linear to the reciprocal of the square of the electric field. The reduction of total ion recombination was attributed to the shift of the contribution from volume recombination to initial recombination and diffusion loss.

Section: From Volume To Initial Recombination and Diffusion Lossmentioning

confidence: 88%

Section: Introductionmentioning

confidence: 92%

Ion recombination in parallel-plate free-air ionization chambers for synchrotron radiation

Nariyama

2006

“…Bohm (1976) obtained a value of El for ,&rays using an extrapolation chamber. In a previous paper Takata and Matiullah (1991) used that value to correct errors in the physical parameter m measured for 6oCo y-rays. In the present work, we study initial recombination using 6oCo y-rays and examine the value of E1 for several different humidities.…”

Section: (7)mentioning

confidence: 99%

Ion loss due to initial recombination in a parallel-plate cavity ionization chamber

Takata¹

1994

Self Cite

Collection efficiencies for ion loss due to initial recombination and back-diffusion were measured for several humidities using a parallel-plate cavity ionization chamber irradiated with 60Co gamma-rays. It was shown, from measurements in a range of inverse electric field strengths from 0.05 to 14 mm V(-1), that initial recombination took place both in clusters and columns of ions produced along the path of the secondary electrons ejected by the y-rays. The ion loss due to recombination in clusters was found to increase with humidity, but that in columns did not. Effects of ion clustering reactions on recombination may be reduced after longer periods of ion drift, when recombination in columns takes place. Ion loss due to back-diffusion was also found to have no dependence on humidity.

“…In addition to low experimental precision, this was ascribed to the fact that no adequate corrections for initial recombination and back-scattering losses had been made (Takata and Matiullah 1991).…”

Section: Introductionmentioning

confidence: 99%

The effects of humidity on volume recombination in ionization chambers

Takata¹

1994

Self Cite

Saturation curves were measured with a parallel-plate ionization chamber for air with humidities of 0%-81%. From the curves, values of m--which is equal to (alpha/eK+ K-)1/2 where alpha is the recombination coefficient, e the charge of an ion, and K+ and K- are mobilities of positive and negative ions, respectively-were obtained. It was confirmed that humidity was the decisive cause of the increase in the value of m with the lifetime of ions in the ionization chamber. It was found that the value of m increases sharply in a range of small values of the product of the ion lifetime and the partial pressure of water vapour and increases more slowly at high values of this parameter.