Radioprotection No. 60-1 | Page 94

88 N. T. Le: Radioprotection 2025, 60( 1), 84 – 90

Table 2. Combined standard uncertainties of technical parameters of neutron reference fields available in Vietnam.

Neutron reference field Quantity Combined standard uncertainty, k = 1(%) 241 Am-Be f 2.1

239 Pu-Be

Notation: Value of uncertainty budget, u b(%): B b( E b): 1.5 %( stated in a certificate); l: 1.0 %( maximum estimated); F A( l): 1.0 %( based on influenced factors in calculation dependent on different total cross section uncertainty); F 1( 90): 0.3 %( maximum investigated); h *( E b): 4.0 %( stated in( ICRU, 1998)); M i: 1.0 %( targeted in measurements); R ib( E b): 3.0 %( estimated). For direct component of neutrons from a bare 241 Am-Be source( ISO-8529 criteria): Uncertainty budgets taken into account for standard uncertainty of f: f b( E b), l, F A( l), F 1( 90); Uncertainty budgets taken into account for standard uncertainty of _ H ð10Þ: f b, h *( E b); Uncertainty budgets taken into account for standard uncertainty of E: f b Uncertainty budgets taken into account for standard uncertainty of h *: f; _ H ð10Þ For total component of neutrons from other sources( ISO-12789 criteria): Uncertainty budgets taken into account for standard uncertainty of f: M i, R ib( E b) Uncertainty budgets taken into account for standard uncertainty of _ H ð10Þ: f, h *( E b); Uncertainty budgets taken into account for standard uncertainty of E: f Uncertainty budgets taken into account for standard uncertainty of h *: f; _ H ð10Þ

4 Discussion

4.1 Feasible range for calibration

From Table 1, ones can figure out that the feasible range of _ H ð10Þ, for calibrations of neutron meters, is from 10 to

150 mSv / h( for the calibration distance from 100 to 250 cm). This range is limited for calibrations of neutron meters at higher measuring range of _ H ð10Þ. However, it is acceptable for calibrations of neutron meters with preferred measuring range at radiation protection level.

In the investigated distances, the feasible delivery range of f is from 15 to 105 cm �2. s �1 while the fluence-spectrumaveraged energies( E) of the neutron fields vary from 0.83 to 4.16 MeV( corresponding to the values of h * from 134 to 391 pSv. cm 2).

For calibration of neutron meters following the ISO-8529 criteria, the contribution of scattered neutrons to the reading of meter to be calibrated must be less than 40 % that of the direct component. It was found that it corresponds to the distance less than 250 cm from the source’ s center.

For calibration of neutron meters following the ISO-12789 criteria, the total component of neutrons can be used. Therefore, the calibrations can be done since the statistical uncertainty of neutron meters whose readings have a statistical uncertainty less than 10 %. It was found that the distance from the source’ s center less than 250 cm is acceptable.

4.2 Accuracy in neutron field parameter

From Table 2, ones can see that the combined standard uncertainties of technical parameters of the neutron reference fields are acceptable in conjunction with those stated in international publications, i. e., acceptable standard uncertainties in determination of f as 5.0-10 %; in determination of h * as 15 %( ISO 12789-2, 2008). That means the acceptable combined standard uncertainty( k = 1) in determination of _ H ð10Þ can be even higher than 15 %( based on uncertainty

propagation principle of equation( 7)). Even, the acceptable standard uncertainty in determination of E is not yet mentioned in Ref.( ISO 12789-2, 2008) but it could be understood that the uncertainty of E can be acceptable if it does not cause the change in _ H ð10Þ over 15 %.

5 Conclusions

Various neutron reference fields were established in Viet Nam for calibrations of neutron meters. The technical parameters of neutron reference fields were characterized and presented in this work, such as: physical quantity( integrated neutron fluence rate, f), operational dosimetric quantity( neutron ambient dose equivalent rate, _ H ð10ÞÞ as well as radiation quantities( neutron fluence-spectrum-averaged energy, E; fluence-to-ambient dose equivalent conversion