Malaysian participation in the IAEA/WHO postal TLD and postal ionisation chamber intercomparison programmes: analysis of results obtained during 1985-2008.

During the years 1985-2008, the Secondary Standards Dosimetry Laboratory of Malaysia (SSDL Malaysia) has participated 37 times in the IAEA/WHO intercomparison programmes. This paper reports an analysis of the intercomparison data and demonstrates that the quality of the SSDL calibration service is well within the limits required by IAEA.

The 40K activity of one gram of potassium.

The activity of 40K in natural potassium as derived from the recent literature varies between 27.33 to 31.31 Bq g-1 of potassium. This paper reports measurements by gamma-ray spectrometry and compares these with independent calculations, yielding values of 30.18 +/- 0.72 Bq g-1 and 31.00 +/- 0.33 Bq g-1 respectively.

Accurate measurement of exposure rate from a 60Co teletherapy source: deviations from the inverse-square law.

A cylindrical gamma-ray 60Co source of activity alpha is predicted to produce an exposure rate X at a distance d in vacuum, given by X = gamma(T)(alpha/d2), where gamma(T) is the specific gamma-ray constant. It has been documented that this formula may be used to approximate X with an accuracy of 1% from a source of length l, provided that d/l > or = 5. It is shown that the formula is accurate to 0.1% under these conditions, provided that the distance is measured from the centre of the source. When absorption in the source and scattering in the collimator are considered, the position of the origin d = 0 can shift by a distance of the order of centimetres. Absorption in air between the source and the ionization chamber adds an exponential factor to the formula. It is shown that even when these modifications are included the discrepancy in the results, although generally less than 1%, is still large compared with the measurement errors. Some suggestions are made for the origin of this discrepancy.

Radiation dose and in vitro precision in paediatric bone mineral density measurement using dual X-ray absorptiometry.

Dual X-ray absorptiometry (DXA) is one of the most widely used techniques for non-invasive assessment of bone integrity. There is a growing demand for measurement of paediatric bone status. In DXA the principal radiation risks to patients are the carcinogenic and genetic effects. Radiation dosimetry is well established for DXA in adults, but there are limited paediatric data available. We report on a study to estimate the effective doses (EDs) received by typical 5- and 10-year-old children using the paediatric scan mode on the Lunar DPX-L bone mineral density scanner. Entrance surface doses (ESDs) and percentage depth doses for the total body and PA spine scan modes were measured using lithium borate thermoluminescent dosemeters (TLDs) located at the surface and distributed at various organ locations in anthropomorphic child phantoms. The EDs were calculated from the percentage depth doses, amount of each organ irradiated and tissue weighting factors. The ESDs were measured to be 6.0 and 0.12 microGy for the posteroanterior (PA) spine and total body, respectively. PA spine EDs were calculated as 0.28 and 0.20 microSv for the 5- and 10-year-old, respectively. Total body EDs were 0.03 and 0.02 microSv for the 5- and 10-year-old children, respectively. These results compare with an adult ED of 0.21 microSv for the PA spine. They are also more than two orders of magnitude lower than reported ESDs and EDs for paediatric chest X-rays. Bone mineral density (BMD) short-term in vitro precision was 0.5% and 1% in the 5- and 10-year-old phantoms, respectively. In conclusion, the Lunar DPX-L in the paediatric mode has a high precision and very low radiation doses, similar to those reported for the adult mode.

Body fat prediction from skinfold anthropometry referenced to a new gold standard: in vivo neutron activation analysis and tritium dilution.

Skinfold anthropometry is a widely practiced technique often with little appreciation of its limitations. The large residual error appearing in any regressions of body density versus sums of skinfolds is primarily due to biological causes, in particular the non-constancy of the ratio of subcutaneous to total body fat. Nevertheless this preliminary study shows that the residual error can be reduced by referencing to a gold standard other than body density. Using a difference technique involving in vivo neutron activation analysis and tritiated water dilution, this paper shows that at least in 20-29 year old normal subjects the residual error (expressed as a percentage of mean total body fat in the respective groups) can be reduced from 22% to 16% (p < 0.001) in males and from 17% to 9% in females (p < 0.0001). It is suggested that a large scale study could be initiated with this new gold standard to obtain accurate predictor relationships throughout the whole age range for both sexes.

Measurement and prediction of total body fat.

Though it has been established that skinfold anthropometry has severe limitations as a method of deriving total body fat (TBF), the possibility that the problem might be related more to the assumptions implicit in densitometry has to be addressed. This paper suggests that smaller residual errors might be obtained if sums of skinfolds are regressed against TBF measured by a difference technique (IVNAA + 3H2O dilution), suggesting that the latter may perhaps be a better standard than densitometry.

Multielement analysis of human hair and kidney stones by instrumental neutron activation analysis with the k0-standardization method.

This paper focuses on the evaluation of the k0 method of instrumental neutron activation analysis in biological materials. The method has been applied in multielement analysis of human hair standard reference materials from IAEA, No. 085, No. 086 and from NIES (National Institute for Environmental Sciences) No. 5. Hair samples from people resident in different parts of Malaysia, in addition to a sample from Japan, were analyzed. In addition, human kidney stones from members of the Malaysian population have been analyzed for minor and trace elements. More than 25 elements have been determined. The samples were irradiated in the rotary rack (Lazy Susan) at the TRIGA Mark II reactor of the Malaysian Institute for Nuclear Technology and Research (MINT). The accuracy of the method was ascertained by analysis of other reference materials, including 1573 tomato leaves and 1572 citrus leaves. In this method the deviation of the 1/E1+ alpha epithermal neutron flux distribution from the 1/E law (P/T ratio) for true coincidence effects of the gamma-ray cascade and the HPGe detector efficiency were determined and corrected for.

The specific absorbed dose constant: comparison of values published for 60Co photons.

For the specific absorbed dose constant for 60Co photons, three values quoted directly in the literature and two derived indirectly from published information are reported. The three publications giving the direct values mentioned no medium of absorption, whereas the other two specify tissue. A database of the specific absorbed dose constant is generated for each of 14 media namely air, water, bone and 11 types of soft tissue. These values are consistent with the three directly quoted values plus one of the indirectly obtained values. Air is found to be unlikely as the medium for the first three; and appropriate media for these are suggested. For the other two values, the generated database suggests that one is too small to be accurate; while the other is correct for tissue (as stated in the publication). An apparent error of 10(3) is identified in one of the values directly quoted.

Absorbed Dose to Water Determination Using IAEA, HPA, NACP, AAPM, NCRP and ICRU Protocols for 1.25 MeV Gamma Ray 6 MV and 10 MV X-Rays: An Intercomparison of Results when IAEA was taken as a Standard Protocol.

Absorbed dose to water was measured with ionisation chambers NE 2561 (#267), NE 2581 (#334), NE 2571 (#1028), using the IAEA standard water phantom. The ionisation chamber was inserted in the water phantom at a reference depth dependent on the type of the radiation quality used. Three radiation qualities were used namely 1.25 MeV gamma ray, 6 MV x-rays and 10 MV x-rays. The values of the absorbed dose to water were determined by the N(K)- and N(X)- based methods, i.e with the use of IAEA, HPA, NACP, AAPM, NCRP and ICRU protocols. The aim of this study was to make an intercomparison of the results, by taking the IAEA protocol as a standard. The largest deviation contributed by any of these protocols was recorded for each quality. It was found that AAPM, NCRP and ICRU protocols contributed 0.94% for 1.25 MeV gamma ray, NACP contributed 2.12% for the 6 MV x-rays, and NACP contributed 2.35% for 10 MV x-rays. Since the acceptable limit of deviation set by the IAEA for this absorbed dose work is ± 3%, it is clear that the overall deviations obtained were all satisfactory.