摘要:

In ionography the x‐ray photons are absorbed in a high‐pressure gas between the parallel plates of an ion chamber and the ions produced are collected on an insulating foil covering one of the plates. The charge pattern is rendered visible using powder‐cloud development. The effects of plate separation and the nature of the gas on the images produced by x rays in the range 50 to 105 kVp have been investigated both experimentally and theoretically. Three potentially useful gases (freon 13B1, krypton, and xenon) have been studied. The energy spectrum of electrons set in motion in these gases have been calculated and the absorption efficiency determined. For a 2‐cm gap with a pressure of 10 atm using x rays at 45 kVp all three gases give an efficiency of about 80% while at 105 kVp the efficiencies are 62%, 28%, and 26% for xenon, krypton, and freon, respectively. Higher efficiencies can be obtained using larger gaps. The effects of gas, gas pressure, plate separation, and x‐ray spectrum on the resolving capability were calculated. Freon gives about twice the resolution of either krypton or xenon. The resolution was measured and found to be greater than 10 line pairs per mm in freon at 10 atm. Diffusion does not blur the image appreciably when positive or negative ions are collected but is important when electrons are used to form the image. Fields of the order of 25 kV/cm are required to collect 90% of the ions at exposure rates of 25 mR/sec. Our results suggest that with further technical development ionography may offer some real advantages in diagnostic radiology.

ISSN:0094-2405    

DOI:10.1118/1.1637271

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

A brief overview of the radiological imaging problem is presented, emphasizing the need for parametrizing components for the user, medical physicist, or system designer. Against this background a method is proposed for reducing MTF data. The single parameter obtained is analogous to criteria of “sharpness” used in other optical applications. The example of x‐ray tube focal spots is examined in detail; results are presented and compared with other criteria. Finally, a prescription for cascading elements into a system is given and applied to the case of magnification radiography.

ISSN:0094-2405    

DOI:10.1118/1.1637272

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

The average cross sections for52Fe production from the bombardment of 588‐MeV protons on manganese, cobalt, nickel, and copper targets were measured as 0.066±0.013, 0.152±0.012, 1.35±0.30, and 0.148±0.013 mb, respectively. A simple chemical procedure for the dissolution of the target in mineral acid and the extraction of52Fe into di‐isopropyl ether is described. The long‐lived radiocontaminants in an52Fe product from a single extraction of a nickel target irradiated for 0.5 h were44Sc (0.02%),44mSc (0.11%),47Sc (0.01%),48V (0.14%),55Fe (3.3%), and59Fe (0.16%).

ISSN:0094-2405    

DOI:10.1118/1.1637273

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

A change of source skin distance results in a significant change in percentage depth dose. The factorsP(f2) / P(f1)by which percentage depth dose at SSDf1is to be multiplied to find new percentage depth dose at SSDf2are tabulated and presented in a graphical form, forf1 = 100 cmandf2 = 110–140 cmfor 8 MV x rays and forf1 = 80 cmandf2 = 90–140 cmfor60Co gamma rays. The use of quadratic and linear equations to calculate this factor have been suggested. The ratios calculated using a quadratic equation agree very well with values from experimental data.

ISSN:0094-2405    

DOI:10.1118/1.1637274

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

Expressions provided by various authors for the weighted roentgens‐to‐rads conversion factorf¯are examined and compared. It is shown thatf‐values should be weighted by the relative exposure at each energy interval;f¯arrived at in this way is equivalent to thef¯obtained from the ratio of mass energy absorption coefficients weighted by energy fluence as described in ICRU Report 17. It is also equivalent tof¯as calculated directly as the ratio of relative total absorbed dose and relative total exposure.

ISSN:0094-2405    

DOI:10.1118/1.1637275

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

摘要:

On three separate occasions, calibrations of radiotherapy orthovoltage units disclosed considerably reduced useful beam intensities without any apparent alteration of the tube current or tube voltage. These reductions would have been undetected by routine daily operation of the equipment unless a useful beam intensity monitoring unit were installed at the tube port or daily measurements were made of the useful beam intensity with calibrating instruments. Either or both monitoring methods are recommended for all radiotherapy units.

ISSN:0094-2405    

DOI:10.1118/1.1637276

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.1637277

出版商:American Association of Physicists in Medicine    

年代:2003

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1002/j.2473-4209.1974.tb36046.x

出版商:Wiley    

年代:2003

数据来源: WILEY