Read Microsoft Word - weos71.doc text version

HP Maharaj and R Tremblay Department of Health, Consumer and Clinical Radiation Protection Bureau, Ottawa, ON, CANADA Abstract

EVALUATION OF THE VICTOREEN 450/451P SURVEY METER RESPONSE TO LOW ENERGY X RAYS FOR RADIATION PROTECTION PURPOSES

This paper aims to provide knowledge and insight on the response of the Victoreen 450/451P survey meters to low energy x rays. Four survey meters were evaluated under broad and narrow beam geometries at 27, 61 and 73 keVs. These energies were produced by an x-ray machine equipped with appropriate Cu and Al attenuators. Under broad beam conditions the meters demonstrate significant under response thus necessitating the need for energy calibration factors. The meter manuals lack explicit guidance information on how to translate meter readings into meaningful quantitative radiation levels. For beams smaller than the detector size, there exists about 50% under-response along the detectors axial axis. The survey meters appear unsuitable for quantifying leakage radiation from small fields that might exist on baggage x-ray machines. Key words: energy response, exposure, low energy x rays, survey meter

Introduction

Acute or chronic exposure to ionizing radiations can cause adverse health effects in humans. For this reason, mitigative actions are often necessary to reduce the incidents of such effects in persons working with or in close proximity of ionizing radiation sources. Most often quantitative assessments of pre-existing radiation fields associated with x-and -ray sources in the workplace rely on measurements taken with photon-sensitive ionization-type survey meters. For reliable results, survey meters should be calibrated to cover the range of photon energies that would be encountered in field situations, shall have calibration traceability to a national standard, and provide accuracies between ±10-20% for broad beam geometries (NCRP 1991). Broad beam conditions are realised when radiation field dimensions exceed the finite size of the radiation detector. Manufacturers of survey meters generally provide sufficient response data for broad beam geometries but, very little, if any, for beams smaller than the detector size. Faced with the latter situation and to get some perspective of the radiation level or hazard presented, a meter user might make a quick conservative estimate of the radiation level by taking the product of the meter reading and the ratio of the detector and radiation beam cross-sections. Such an approach tacitly pre-supposes a linear relationship between the survey meter reading and the sensitive volume of the detector irradiated; however, it has been observed experimentally that survey meters exhibit significant nonlinear responses to radiation beams of size less than the detector dimensions (Maharaj 1990) and, unless such affects are properly taken into account, under-estimates of the radiation levels could occur with the likely consequence of compromising worker or public safety. This therefore gives rise to concerns should the Victoreen 450/451P survey meter (Inovision Radiation Corporation, Cleveland, Ohio, USA) be used to quantify leakage radiation from baggage inspection x-ray machines. The purpose of this report is to provide knowledge and insights on the response of the instruments to low energy x rays for the application perceived.

Materials and methods

The Victoreen 450P and 451P survey rate meters are designed with a 200-mg cm-2 thick plastic casing and employs a 300cm3 and 230-cm3 air-filled 6-atmospheres pressurized cylindrical ionization chamber, respectively (Inovision Radiation Measurements,1991 and 2001). The detector length or diameter is not identified on either meter's plastic casing, but removal of the casing has revealed and confirmed a sealed, opaque and cylindrical ionization chamber having dimensions of about 14 cm long and 6.8 cm diameter. The survey meters provide photon detection capability above 25 keV thereby making them potentially attractive for use in quantifying the radiation fields produced by a wide variety of x-and -ray sources utilized in industry. Except for differences in ion chamber volume, readout units and cosmetic features, the meters are basically the same. Irradiations were carried out with a 140-kVp York x-ray machine (Model 3255). Beam monitoring was achieved with an ionization chamber (MDH Model 2025 Pencil Ionization chamber, RadCal Corporation, Monrovia, California, USA) positioned 24 cm from the x-ray tube target, but off the central axis, in a special jig affixed to the x-ray machine collimator; beam variations were within ±2.0%. Combinations of copper and aluminum attenuators (with the latter behind the former) were positioned in the jig at a distance of 43 cm from the x-ray tube target to produce the test energies (Table 1). A source-todetector-center distance of100 cm was maintained, and the center of each ionization chamber was identified by intersecting the dots on the lateral sides, the front face and end surface of the meter. The radiation beams were normally incident and defined on the meter surface opposite the readout display. The x-ray machine collimator was adjusted to provide the

1

rectangular beams, and a secondary lead collimator introduced at 52.5 cm from the source was used to define the circular beams.

A. Response under broad beam conditions at low energies

Four survey meters were evaluated at effective energies of 27, 61 and 73 keVs (Table 1) in a 15 cm x 10 cm field, which is considered representative of broad beam conditions. Meter A served as a secondary transfer standard and, therefore, was calibrated at several beam qualities (kVp and filtration combinations) at the Canadian national standards laboratory (National Research Council, Ottawa) to provide the exposure calibration factors needed at the test energies. Instrument A (450P) expresses outputs in mR h-1; and meters B, C and D (451Ps) present outputs in mSv h-1. Instruments B, C and D were supplied with an operation manual and calibration information relevant to a 137Cs source (photon energy 662 keV) at the time of instrument acquisition by clients. The manual contains plots of energy response representative of broad beams and energy dependent dose-equivalent-to-exposure-conversion factors (cSv R-1). The latter document shows percentage variations (error measurement) between meter readings and exposure rates from a 137Cs calibration source.

B. Linearity test

Linearity testing was carried out to simulate broad beam conditions at 61 keV. This energy was chosen for two reasons: (1) previous studies have concluded that baggage inspection x-ray machines generate radiation beams for which the effective energies were between 53-61 keV (Maharaj 1995); and (2) current generation baggage inspection x-ray machines employ similar operational parameters (kVp, filtration) and imaging characteristics, and not much deviation is expected in their corresponding beam qualities.

C. Response to small beams

The four meters were investigated for their response to circular beams of diameter 0.7, 1.4 and 3.7 cm at 61 keV. The1.4-cm diameter beam was normally incident at various positions along the detector axial axis.

Results and discussion A. Response under broad beam conditions at low energies

Table 2 summarizes the broad beam test results in mR h-1. Column 3 shows averages of three readings. Columns 4 and 5 list the energy response factor and the dose-equivalent to exposure factor, respectively, taken from the appropriate plots in the survey meter operational manuals corresponding to the test energies. Column 6 lists the ICRU deep dose-equivalent-toexposure conversion factors for monoenergetic beams equivalent to the test energies (ICRU 1992); the differences with those of column 5 are to be noted. Ideally, low-energy photons undergo significant photoelectric absorption in the thick plastic casing and ionization chamber-wall materials, hence, the meters response would be low; an effect that is clearly demonstrated (by the data in Table 2, column 3 for 27 keV). To correct for such attenuation effects, the transfer standard (meter A) readings were multiplied by the appropriate energy- dependent exposure calibration factor provided by the standards laboratory (column 7) to yield corrected exposures (column 8). In a case where an exposure calibration factor is not available, a reasonable approximation of the estimated exposure may be obtained by taking the product of the meter readings and the inverse of the monoenergetic energy response factor provided in the operation manual (column 4). Similarly, for instruments B, C and D, an inverse of the dose-equivalent-to-exposure-conversion factor (column 5) would also apply. Based on these considerations, the product of the meter readings (column 3) and correction factors (column 7) translates into corrected exposures (column 8). It is to be noted that the operation manuals do not provide explicit guidance on how to use the information provided in the energy response and dose-equivalent plots, and this seriously hinders user education and knowledge regarding the survey meters. At 27 keV, meters B and C show significant under-response relative to the transfer standard (meter A) even when the appropriate energy response and dose-equivalent-to-exposure correction factors are taken into account (column 9). This finding reaffirms the need for proper meter calibration at low-energy x rays in order to achieve meaningful results (within ±20%). Relative to the transfer standard exposure rates at 61 and 73 keVs, meters B and C show elevated exposure rates (Table 2, column 3) by about 50% (on the assumption that 1 Sv = 100 rem, and 1 rem 1 R). Hence, users of meters B and C may accept the meter readings at face value and exercise stern mitigative measures in the workplace. However, upon closer examination of meters B and C response, it is apparent that application of the energy-dependent dose-equivalent-to-exposure

2

conversion factors (column 5), as well as the broad beam energy response data (column 4) provided in the operational manual yields exposure rates that are in good agreement with the transfer standard (column 8). The operation manual does not provide explicit guidance on how to use the energy response information presented in the plots, a shortcoming that has certain ramifications: (1) Misleading end users of the meter, especially those who might have limited experience or knowledge in the use of survey meters or in the interpretation of their results, into a false sense of security in that the meter's readings are to be taken at face value and need no further corrections. (2) Creating an atmosphere of worker distrust or of radiation phobia in a facility where meters of different outputs are utilized, one of which provides outputs in mR h-1 (e.g., meters A or D) and another which displays outputs in mSv h-1 (meters B or C); the user is unable to establish conclusively which meter yields reliable results, inevitably, resulting in improper decisions and credibility. The 137Cs calibration information provided with the survey meters is simply a single-energy (662 keV) check point on meter performance (i.e., percentage differences of the meter readings and true exposure rates produced by the calibration source). The individual reports for meters B, C and D show variations were within ±5%; however, larger variations (column 8) are apparent for low-energy x rays. Clearly, it would seem incorrect to assume that the calibration factor at the137Cs energy check point or, the level of accuracy determined at that energy check point also applies when the instrument is used for estimating the radiation levels produced by lower-energy photon sources. For this reason, specific energy calibration factors need to be determined at a standards calibration laboratory.

B. Linearity test

All of the detectors responded linearly under broad beam conditions (Figure 1). In addition, as discussed previously, appropriate calibration factors need to be applied to the survey meter readings in order to yield reasonable estimates of the radiation levels.

C. Response to small beams

In the case of response to small beams, results are normalized at the detector center (Table 3). Meters B and C relative to meter A significantly under-estimate exposure rates by about 47% for a 0.7-cm diameter beam at 61 keV; this result does not support the perceived notion that such meters, by virtue of their pressurized ionization chamber, respond as "geiger" instruments. As the beam size increases, differences in exposure rates appear smaller. Figure 2 shows significant non-linearity in response along the detector axial axis. Although the detector length is unmarked on the plastic casing, it is apparent that within about 5.0 cm of the detector center a variation of±20% could be achieved. However, in practice, to satisfy this ±20% criterion is difficult: a meter user would not know a priori what is the size of the radiation beam that intercepts the detector, and where on the detector that beam actually intercepts it. Amid such uncertainties, including an under-response of the meter to small beams, a user is less likely to make any reasonable estimates of the leakage radiation that might emanate from small hot spots on the external surfaces of the baggage x-ray machines, thereby, casting serious doubts on the suitability or reliability of these survey meters for quantifying leakage radiation levels under such conditions.

Conclusions

There is much uncertainty and significant under-response of the Victoreen 450/451P survey meters to low-energy x rays for beams smaller than the detector size, and this suggests that the meters would seem unsuitable for use in quantifying leakage radiation levels from small fields that might exist on baggage x-ray machines. Survey meters need to be calibrated over several energies to cover the photon energy range for which they would be used; a single point energy calibration, though helpful, is insufficient. The survey meter manuals do not provide explicit guidance information to users on how to translate instrument readings and reasonably quantify radiation levels. This deficiency can mislead the meter users and potentially compromise worker or public safety.

Table 1: Characteristics of the test energies.

kVp Added filters (a) 3 HVL Effective energy (b)

29.5 76 86

(a)

0.3 mm Cu + 1.0 mm Al 3.3 mm Cu + 3.1 mm Al 4.5 mm Cu + 4.1 mm Al

0.0053 mm Cu 0.0500 mm Cu 0.0785 mm Cu

(keV) 27 61 73

The aluminum filters were placed behind the copper filters to absorb the K radiation emitted from copper.

(b) Effective energy means the monoenergetic photon that has the same attenuation coefficient as the heterogeneous radiation beam. It was obtained by

determining the linear attenuation coefficient from the measured HVL in copper and consulting published attenuation data.

Table 2: Summary of broad beam test results for the Victoreen 450/451P survey meters at various effective energies. Survey meters Operation manual exposure response 0.6 0.6 0.6 1.15 1.15 1.15 1.17 1.17 1.17 Correction factor Operation manual dose ICRU dose equivalent equivalent response response (a) (cSv R-1) (cSv R-1) 0.9 0.9 1.18 1.18 1.16 1.16 0.9 0.9 1.52 1.52 1.51 1.51 -

Effective energy (keV) 27

ID A B C D A B C D A B C D

Average readings 3.5 (mR h-1) 9.2 (µSv h-1) 8.0 (µSv h-1) 3.9 (mR h-1) 38.6 (mR h-1) 0.80 (mSv h-1) 0.78 (mSv h-1) 49.2 (mR h-1) 51.2 (mR h-1) 1.01 (mSv h-1) 0.99 (mSv h-1) 65.2 (mR h-1)

Final 2.1(b) 1.85(c) 1.85(c) 1.67(d) 1.38(b) 0.74(c) 0.74(c) 0.87(d) 1.32(b) 0.74(c) 0.74(c) 0.84(d)

61

73

(a) (b) (c) (d)

Corrected exposure (mR h-1) 7.2 1.7 1.5 6.5 53.2 59.2 57.7 46.2 67.6 74.7 73.3 55.7

Difference (%) 0 -76.0 -79.0 -10.0 0 11.3 8.5 -19.5 0 10.5 8.4 -18.6

Values are taken from ICRU Report 47, Table A.2 (ICRU 1992); note the differences between the values in the preceding column. Exposure calibration factor of transfer instrument as determined by the standards laboratory. Inverse of the product of the exposure response and the dose-equivalent response factors (x102 R Sv -1). Inverse of the exposure response factor.

Table 3: Response of the Victoreen 450/451P survey meters at 61 keV for small circular beams. Beam diameter (cm) 0.7 Parameters Average readings Correction factor (a) Exposure rate % difference (b) Average readings Correction factor (a) Exposure rate % difference (b) Average readings Correction factor (a) Exposure rate % difference (b) A (450P) 0.625 mR h-1 1.38 0.86 mR h-1 0 1.34 mR h-1 1.38 1.85 mR h-1 0 8.6 mR h-1 1.38 11.9 mR h-1 0 Survey meters B (451P) C (451P) 6.3 µSv h-1 0.74 0.46 mR h-1 -46.5 20.7 µSv h-1 0.74 1.53 mR h-1 -17.3 172.5 µSv h-1 0.74 12.8 mR h-1 7.6 6.3 µSv h-1 0.74 0.46 mR h-1 -46.5 20.5 µSv h-1 0.74 1.52 mR h-1 -17.8 169.5 µSv h-1 0.74 12.5 mR h-1 5.0 D (451P) 0.80 mR h-1 0.87 0.7 mR h-1 -19.6 1.59 mR h-1 0.87 1.38 mR h-1 -25.4 10.8 mR h-1 0.87 9.4 mR h-1 -21.0

1.4

3.7

(a) (b)

Correction factors are the same as those provided in column 7 of Table 2 for the appropriate meters. Exposure rate values are compared to the transfer standard (meter A) and expressed as a percentage.

Figure 1: Exposure versus time plots for various survey meters at 61 keV and broad beam conditions.

4

200 175 Exposure (µR) 150 125 100 75 50 25 0 0 5 10 Time (seconds) 15 20 + A B C D R2 = 0.9996 R2 = 0.997 R2 = 0.9969 R2 = 0.9856

Figure 2: Relative axial response of meters to a 1.4-cm diameter beam at 61 keV.

30 20 RELATIVE DIFFERENCE (%) 10 0 -10 -20 -30 -40 -50 -60 -5 -3 -2 0 2 3.5 6 BEAM POSITION (CM) B C D A

References

Inovision Radiation Measurements (2001) Instruction Manual on Ionchamber Survey Meter Model 451P. Cleveland, Inovision Radiation Corporation. Inovision Radiation Measurements (1991) Instruction Manual on Ionchamber Survey Meter Model 450 P. Cleveland, Inovision Radiation Corporation. International Commission on Radiation Units and Measurements (1992) Measurement of Dose Equivalents from External Photon and Electron Radiations, Report 47, Bethesda, ICRU. Maharaj, H.P. (1990) Response of the Victoreen 440 RF/C and the Rad-Owl RO-1 Survey Meters to Low Energy Photons (<66 keV) for Narrow Beam Geometries. Radiation Protection Dosimetry, 30: pp.129-133. Maharaj, H.P. (1995) Predicting absorbed doses and risks from some inspection x-ray machines. Journal of Radiological Protection (15), pp.303-310. National Council on Radiation Protection and Measurements (1991) Calibration of Survey Instruments used in Radiation Protection for the Assessment of Ionizing Radiation Fields and Radioactive Surface Contamination, Report 112. Bethesda, NCRP.

5

Information

Microsoft Word - weos71.doc

5 pages

Find more like this

Report File (DMCA)

Our content is added by our users. We aim to remove reported files within 1 working day. Please use this link to notify us:

Report this file as copyright or inappropriate

512312


You might also be interested in

BETA
MEASUREMENT OF THE ENERGY SPECTRUM OF GAMMA-EMITTING RADIONUCLIDES IN WORK AREAS TO DETERMINE REM PER ROENTGEN DOSE CONVERSION FACTORS FOR THE PHOSPHATE INDUSTRY
Microsoft Word - Rad-Radiation_Safety_Handbook-v1.7a.doc
Antenna Modeling & System Planning
Microsoft Word - IEEE Antenna Design Challenge v11.docx
Untitled Document