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The objective of this work is to check the dosimetric performances of the TLD-100 as stated by the manufacturer as well as the technical standards of radiation protection. The purpose of the performance audit is to assess the inhomogeneity of TLD sensitivity, repeatability and reproducibility, linearity, energy dependence, angular dependence, and fading. All tests were performed under the conditions of ambient temperature and relative humidity recommended by the manufacturer. We began the study by calibrating the Harshaw 6600 Plus, and checking its performance. The TLD-100 performance verification results were all acceptable and in accordance with the manufacturer’s advertised values and the radiation protection technical standards. However the performance of the TLD-100 that we have evaluated may have some limitations; these limits, which are sources of uncertainty, have been taken into account in this work by evaluating the overall uncertainty of the Hp (10) dose in the uncertainty range 9.45% to 15.80% by simple formulas. The TLD-100 personal dosimeters and the 6600 Plus reader system indicate that the calculated values of the overall uncertainty Hp (10) are well below the allowable values of 21% to 42% suggested for personal dosimetry services. The obtained data encourage the use of the system for the routine evaluation of the external exposure of workers under ionizing radiation in our laboratory.

Individual radiological monitoring of workers under ionizing radiation is a regulatory requirement of a radiological protection program [

The Harshaw 6600 Plus dosimetry system developed and produced by Thermo Fisher Scientific (TFS) is composed of thermoluminescent dosimeters (TLDs) in LiF: Mg, Ti, and a reader.

The dosimeter consists of four chips LiF: Mg, Ti mounted in Teflon on an aluminum card and placed in a plastic holder. The carrier contains a unique filter for each copper, acrylonitrile-butadiene-styrene, Mylar and tin chip. These chips have the property of storing energy received during irradiation and returning it after heating in the form of light. Two of these pellets of size 3.2 * 3.2 * 0.38 mm^{3} and 3.2 * 3.2 * 0.15 mm^{3} in the plate make it possible to evaluate respectively equivalent doses at the level of the skin Hp (0.07) and at the level of the body Hp (10). Each card has a separate number associated with a barcode allowing for faster backup during playback.

The reader of

The reader is calibrated in physical unit (mSv) by irradiating the dosimeters “gold” of calibrations at the Secondary Laboratory of Dosimetry Calibration (LSED) of the Nuclear Research Center of Algiers (CRNA, Algeria), compared to Hp (10) and Hp (0.07). The reader calibration factor (RCF) and the factors that correct the different sensitivities of the dosimeters (ECC) have been determined. We collected data from the CCT database. Then, in

We found a normal distribution shifted to the left of which 5% of the cards for the chip in Hp (10) and 4.5% for the pellet in Hp (0.07) have after the calibration of the reader a recurring value of 0, Mean values and standard deviations of the shifted normal distribution are 0.94 ± 0.09 for Hp (10) and 0.96 ± 0.1 for hp (0.07), respectively. From the analysis we have chosen the cards whose ECC were outside the average value and the recurrent value of the distribution that is to say out of range (0.85 - 1.03). These are the chosen maps that have been experimentally characterized. With the exception of the inhomogeneity of batches of

dosimeters, Repeatability and Reproducibility carried out using strontium source internal to the reader; all the other irradiations were carried out in different SSDLs during training courses financed by the IAEA. After all the irradiations, the dosimeters with similar fadings [

1) Linearity

We exposed to Cs-137 12 batches of dosimeters; each batch is composed of 4 TLD dosimeters at doses ranging from 0.03 to 15 mSv. Exposure time ranges from 1.69 minutes to 20.34 minutes.

2) Inhomogeneity of Batches of Dosimeters

For these measurements, we have, as far as possible, used TLDs from the same batch of our choice. We have irradiated 100 TLD dosimeters of the same batch at an identical dose of 1 mSv from a beam strontium 90 Sr/Y.

3) Repeatability and Reproducibility

Ten (10) exposures of ten (10) 90 Sr/Y TLD-100 dosimeters were performed under the same conditions. The reference dose is of the order of 1000 gU or 10.83 mSv for an exposure time of 101.7 s per card. The mean value, standard deviation, and coefficients of variation and responses were determined in

4) Angular Dependence

The irradiations were carried out in accordance with the reference standards of the national center for radiological protection with a calibrated beam of Cs-137 (OB6 irradiator). We exposed to 2.56 mSv which corresponds to the exposure time of 16 min a series of eleven batches of dosimeter each consisting of two TL cards irradiated in the same reference position fixed by the lasers. Each lot is exposed in a clearly defined direction. TLD positions for all measures were identical. After each rotation, the geometric center of the detectors is returned. The rotation of the set (phantom + dosimeter) in the clockwise direction assumed to be the positive values of the angles (0˚, 15˚, 30˚, 45˚, 60˚, 75˚) and counterclockwise

Irradiation (Réproducibility) | Average ( x ¯ ) | Standard déviation (σ) % | Coefficient of variation (CV)% | Response % |
---|---|---|---|---|

1 | 10.853 | 11 | 1.07 | 100 |

2 | 10.856 | 11 | 1.04 | 100 |

3 | 10.743 | 11 | 1.04 | 99 |

4 | 10.826 | 17 | 1.65 | 99 |

5 | 10.67 | 12 | 1.13 | 98 |

6 | 10.823 | 13 | 1.25 | 99 |

7 | 10.743 | 12 | 1.09 | 99 |

8 | 10.853 | 12 | 1.15 | 100 |

9 | 10.751 | 12 | 1.12 | 99 |

10 | 10.865 | 13 | 1.17 | 100 |

Dosimeter (Repetability) | Average ( x ¯ ). | Standard deviation (σ)% | Coefficient of variation (CV)% | Response % |
---|---|---|---|---|

1 | 10.795 | 9 | 0.9 | 99 |

2 | 10.983 | 7 | 0.6 | 100 |

3 | 10.679 | 9 | 0.8 | 98 |

4 | 10.911 | 8 | 0.7 | 99 |

5 | 10.547 | 6 | 0.6 | 99 |

6 | 10.827 | 7 | 0.6 | 99 |

7 | 10.861 | 6 | 0.6 | 98 |

8 | 10.856 | 7 | 0.6 | 100 |

9 | 10.757 | 7 | 0.6 | 99 |

10 | 10.767 | 5 | 0.5 | 100 |

the negative values (0˚, −15˚, −30˚, −45˚, −60˚, −75˚).

5) Energy Dependence

The irradiations were carried out in accordance with the reference standards of the Secondary Laboratory of Dosimetry Calibration (LSED) of the Nuclear Research Center of Algiers (CRNA, Algeria) using three types of beams. An OB6 type emitter emitting a 137Cs gamma beam, a philips radiography apparatus for the X ray beams, an ELDORADO 78 therapy unit for a 60 Co beam.

6) Thermal Fading

The irradiations were carried out in accordance with the reference standards of the CNESTEN laboratory (Morocco). To highlight the phenomenon of thermal fading, or loss of signal, twenty-four (24) dosimeters were positioned in groups of six (06) on a standardized ghost. They were irradiated with a source of cs-137, which emits γ-rays of 662 keV at a dose of 5 mSv for 32 min. After irradiation, TLDs were stored under ambient temperature conditions (25˚C). Batch readings of four (04) dosimeters were performed at variable times ranging from 24 hours immediately after irradiation to 180 days.

Two methods of formulating and calculating the overall uncertainty of the TLD system have been applied.

1) Global Method

The first method, called global, is a posteriori estimation of the total uncertainty of the system. It defines two types of uncertainty: The type “A” and the type “B” [

Type A, called random involves uncertainties that can in fact be reduced by increasing the number of measurements.

Type B, called systematic is made up of uncertainties that cannot be reduced in number of repeated measures.

It is assumed that the variables to be taken into account follow a uniform or normal statistical distribution. In case the distribution is normal, the Type B uncertainty can be in the form of a standard deviation by dividing the half

maximum difference measured by σ i = maximum measured half difference 3 [

where the maximum measured half difference of an amount X is calculated as follows: [

Maximum measured half difference = M a x ( X ) − M i n ( X ) 2

The overall uncertainty is then:

U totalsyst � me = ∑ U A 2 + ∑ U B 2

U totalsyst � me = ∑ U A 2 + 1 3 ∑ M a x ( X ) − M i n ( X ) 2 (1)

Or U_{A} and U_{B} are the uncertainties of type A and type B.

For the calculation of this formula, sources of type A uncertainties are:

o The Variation of Sensitivity Factors of the Dosimeters (ECC)

o Dosimetric variability (Repeatability and reproducibility of the response)

Sources of uncertainty type B are:

o The irradiation source 90 Sr/Y obtained by experience

o The calibration factor of the reader provided by laboratory of Algiers

o The electronic parameters of the reader provided by the specification sheet of the manufacturer [

o The nonlinearity obtained provides the specification sheet of the manufacturer or experimentally.

2) Quadratic Summation Method of Each Source of Uncertainty

From formula (2), the dose received by a dosimeter j is evaluated.

H p ( 10 ) j = Q j ∗ E C C j R C F (2)

Or RCF is the calibration factor of the reader, ECC_{j} are the sensitivity factors relative to each dosimeter, Q_{j} is the apparent dose, Hp(10) are the actual dose in positions (ii) TLD cards

For the TLD dosimeter, the uncertainty was estimated from the quadratic propagation law of the uncertainties of equation [

σ y = ∑ i n ( ∂ f ∂ x i ) ( σ x i ) 2 (3)

This formula does not take into account the correlations between the different sources of uncertainty. Variables are assumed to be independent.

From Equation (2), it is possible to identify the different sources of uncertainty of the TLD measure. This gives an expression of the total uncertainty on the TLD measure presented in Equation (4).

σ D D = ∑ i 5 ( σ E C C E C C i ) 2 + ( σ R C F R C F ) 2 + ( σ Q Q ) 2 (4)

Uncertainty on reading σ_{Q}

The term σ_{Q} corresponds to the uncertainty on the reading. The estimation of this term is mainly based on the results of the study of the characterization of the parameters of performance Equation III summarizes all the uncertainties in reading: It is mainly the most significant influencing factors that have been used to estimate the uncertainty in reading.

With j = {linearity, inhomogenety, Repeatability and reproducibility, linearity, enegy, angle}

Or,

σ Q Q = ∑ σ j 2 (5)

Uncertainty on Ecc sensitivity of TLD σ_{ecc}:

σ_{ecc}: corresponds to the uncertainty about the sensitivity of the TLD. It is given by the builder or experimentation by determining the standard deviation of the Ecc distribution.

Uncertainty about calibration:

σ_{RCF} corresponds to the uncertainty on the calibration. This uncertainty was provided by Algeria’s secondary calibration laboratory. It takes into account the intrinsic uncertainty and the standard deviation of the measurement performed on cesium-137.

uncertainty ( % ) | Type | Distribution |
---|---|---|

σ linearity | B | Normal |

σ inhomogenety | A | Expérimental standard déviation of the mean |

σ Repetabilityetr � productibility | A | Expérimental standard déviation of the highest average enters on repeatability and reproducibility |

σ energy | B | Normal |

σ angle | B | Normal |

σ fading | B | Normal |

Overall uncertainty about the dose measurement of the TLD.

Finally, the total uncertainty on the TLD measure was calculated from the quadratic summation method on the over the low dose range.

we found a value of 22% which leads us to conclude that the batches of dosimeters used have a good homogeneity and a good stability for evaluation of the doses of routines of the workers exposed to the ionizing radiations.

The repeatability or coefficient of variation is calculated by realizing the ratio of the standard deviation to the average of the measurements. The reproducibility of each irradiation (different dosimeters) is between 98% - 100%. These high values are considered good because the system manages to distinguish between dosimeters. The repeatability of each dosimeter is similar only by reasoning on the coefficient of variation for each measurement repeated on the same map it could be said that sometimes happens or the source 90 Sr/Y still does not put the same dose in the sensitive volume of the tablet when it always carries out the same measurement process because we observe a slight variation of 0.5% to 0.9% or a margin of error of 0.4%.

For each of the 10 irradiations the values obtained are ranged from 1.71% - 2.07%.

For each of the 10 dosimeters the values obtained are ranked from 1.02% - 1.52%.

The coefficients of variation found at each irradiation and each map are below 2% for all exposures. These values obtained are in line with that of the manufacturer and that of the standard [

appropriate dose algorithm incorporating the X-ray beam qualities for the evaluation of Hp (10) [

The signal of the first reading, 24 hours after irradiation, taken as reference is designated 100% of the luminescence. It is found that the greatest variation

occurs during the first 30 days after storage. After 30 days, the fading changes become less progressive, and can reach a correction factor so the average is 10% - 12% within 90 days to 150 days. What is very small compared to the characteristics of the manufacturer’s material TLD-100 [

With regard to

The highest type A uncertainty is the variation in ECCs. This can be reduced by increasing the number of cards to be calibrated. The main source of type B uncertainty is the energy and angle dependency. The overall uncertainty calculated by the two approaches is in the range of 9, 45% to 15%, 80%. These values found in our current work are less than 21% and 42%, values recommended respectively by the ICRP [

This work has assessed several parameters, the inhomogeneity of TLD sensitivity, repeatability and reproducibility, linearity, energy dependence, angular dependence and fading as part of our technical quality assurance approach proposed by the laboratory from the ARSN. This assessment made it possible to check the dosimetric performances of the TLD-100 as announced by the manufacturer and the technical standards of radiation protection. We then showed

Source of uncertainty | Type A | Type B | |
---|---|---|---|

Variation of ECC | 9% | ||

Irradiation source | 0.4% | ||

Reader | Reference light | 1.0% | |

High tension | 0.005% | ||

Heating temperature | _ | ||

PMT electronic noise | _ | ||

linearity | 1% | ||

Reader Calibration Factor | 2.5% | ||

Total type | 9% | ||

Total system | 9.45% |

Source of uncertainty | Type of distribution | Number of TLDs | Uncertainty value Hp (10) (%) |
---|---|---|---|

Reader Calibration Factor | - | 24 | 2.5 |

Inhomogeneity of cards | Gaussienne (A) | 100 | 6,7 |

Repeatability and reproducibility | Gaussienne (A) | 10 | 4 |

Repeatability of ECC | Gaussienne (A) | 187 | 9 |

Linearity | Gaussienne (B) | 48 | 4.5 |

Angular dependence | Gaussienne (B) | 24 | 10 |

Energy dependence | Gaussienne (B) | 52 | 8 |

Fading | Gaussienne B) | 24 | 2 |

Total system | 15.80 |

that the energy and angle dependency is the main source of uncertainty [

The authors declare no conflicts of interest regarding the publication of this paper.

Kouakou, O., Monnehan, G.A. and Huberson, G.B.D.L. (2019) Evaluation of Dosimetric Performance and Global Uncertainty of the Harshaw 6600 Plus System Used to Staff Monitoring in Côte d’Ivoire. World Journal of Nuclear Science and Technology, 9, 159-173. https://doi.org/10.4236/wjnst.2019.94012