Characterization of Neutron Irradiator as a Source of Reference Radiation Field for Calibrating Dosemeters and Doserate Meters at PSI Laboratory

A new neutron calibration laboratory has been installed at the Paul Scherrer Institute in Switzerland. The calibration laboratory has been equipped with a newly developed neutron irradiator with two calibration benches, remote control instruments and related ancillary equipment for calibrating dosemeters and doserate meters. The actual dimensions of the calibration laboratory are 2.0 m (H) X 6.8 m (W) X 12.6 m (L). On the steel grid at a height of 2 m from the basement floor, two calibration benches are installed. A transport channel is installed in the middle of the benches for the transfer of the radiation sources. The radiation sources are moved from the safe position of the irradiator to the reference irradiation position using an air pressure unit. The reference position is at a height of 1.2 m from the grid floor. The neutron irradiator is equipped with two Am-Be sources. Characterization of neutron radiation field has been calculated in detail by the MCNPX neutron transport code as well as energy distribution of neutrons and contribution of scattered neutrons. Experimentally, it was measured with a neutron/gamma digital spectrometric system equipped with a stilbene detector. The quality of the neutron radiation field has been characterized by ambient dose equivalent rate and the shape of the Am-Be spectra has been verified with the ISO-8529 standard.

Introduction

Neutron radiation protection devices and neutron personal dosimeters require testing and calibration response in a well-defined neutron radiation field. The neutron radiation laboratory has been developed especially for calibration measurements. The requirements of the ISO 8529 standards have been taken into consideration during the development of the neutron irradiator and design of the neutron laboratory. This mainly concerns the issue of choosing the right source of neutron radiation, characteristics of neutron reference radiation, shielding of the irradiator, influence of scattered neutrons, calibration techniques, etc. All of these components affect the quality of the neutron radiation field and the results of the calibration measurements.

Design of the Neutron Calibration Laboratory

The secondary standard of neutron radiation laboratory has internal dimensions of 2.0 × 6.8 × 12.6 m. The laboratory floor is made of steel grates. The neutron irradiator is placed under the floor of the laboratory, see Fig. 1. Two calibration benches are installed in the laboratory. A shorter path of the first calibration bench is 2450 mm long and the second calibration bench path is 3735 mm long, see Fig. 1. The aluminium transport channel is placed in the middle of the calibration benches. The reference position of the radioactive sources is at a height of 120 cm from the steel grate at the end of the aluminium transport channel. The radioactive sources are transported from the neutron irradiator to the working position by air pressure. The radiation field is omnidirectional without collimation.

Fig. 1. The model of the neutron calibration laboratory.

The radiation sources are stored in the aluminium housings of the rotating carousel. The carousel has a total of 7 positions. Two positions are occupied by Am-Be sources, see Table I.

Table I. The Neutron Source Parameters

Radionuclide	Neutron emission (n/s)	External dimensions of the source (mm)	Dimensions of the active part of the source (mm)
Am-Be	5.2E + 07	(Ø25.4 × 75.95) mm	(Ø17.7 × 56.67) mm
Am-Be	6.8E + 06	(Ø25.4 × 75.95) mm	(Ø17.7 × 56.67) mm

Other positions are free for the possibility of installing other sources. In the middle of the carousel is a lead shield with a diameter of 15 cm. This shielding reduces the effect of parasitic radiation on the transport pipeline. The irradiator carousel is further shielded from the surface to centre by a layer of polyethylene with borotron (30 cm), polyethylene (20 cm), and a layer of lead (10 cm).

The carousel can be rotated to get the selected neutron source into the axis of the transport channel. The air pressure moves the source to the working (reference) position. The geometric centre of the source is 109 mm below the top edge of the transport channel. In the closed position, the neutron sources are in the rotating shielding of the carousel, so that the total shielding thickness is 60 cm in all directions. The shielding is designed for a maximum neutron emission of 6.0E+07 n/s (~1 TBq of Am-Be).

Reference Neutron Radiation

The main parameter of the neutron calibration radiation laboratory is the range of realisable dose rates, determined by the choice of one of the sources and the choice of the appropriate distance.

The emission of neutron sources and distances have been chosen so that the dose rates overlap each other, see Fig. 2. The closest adjustable distance of the calibration bench from the center of the neutron radiation source is 75 cm.

Fig. 2. The overlap of realizable dose rate ranges.

The maximum distance of the calibration benches from the center of the neutron source axis is 3735 mm. To achieve the lowest possible proportion of scattered neutrons (<40%), distances greater than 1.8 m should not be used because neutron beam quality is affected by the interior of the laboratory and the construction materials, see Fig. 3.

Fig. 3. Proportion of direct neutrons from the radiation source to scattered neutrons.

Shielding Capability of the Neutron Irradiator

The effect of neutron sources located in the shielding of neutron irradiator has been carried out by MCNPX simulations [6]. The shielding has been optimized for a dose rate <1 sv h at a distance of 1 m from the irradiator in all directions due to the presence of both neutron and gamma radiations from the am-be sources the shielding must be composed of lead 10 cm polyethylene 20 cm and polyethylene with borotron 30 cm the transport channel has been shielded with 12 cm thick polyethylene due to the penetrations from the neutron irradiator see Fig. 4. The height of this shielding is measured from the neutron irradiator surface and is 936 mm.11

Fig. 4. The shielding of the neutron irradiator.

Methodology of Neutron Measurement

A two-parameter digital spectrometer has been used for measurements in neutron radiation fields. The neutron response spectra have been identified employing the PSD method. The ambient dose equivalent rate has been calculated using G-function method.

Two-Parametric Digital Spectrometer

The digital neutron spectrometer is built as a modular system allowing the use of different types of scintillation detectors. The preamplifier splits the signal from the detector into two branches. Each branch is differently amplified and digitized by separate ADC. Different amplification increases the dynamic range of particles that the spectrometer is able to process.

The input analog signal is digitized with fast 12-bit analog to digital converter with a sampling frequency of 1 GHz. Digital signal processing is implemented into FPGA. FPGA is able to process all data flowing from ADC (12 Gbits per second). The spectrometer is connected with a computer via an optical ethernet of 10 Gbit, see Fig. 5.

Fig. 5. Scheme of the digital spectrometer [1].

The digital spectrometer has incorporated the integration method [2], [3] for recognition of neutron and photon pulses. Integration method is based on the principle of pulse charge comparison. The PSD parameter is calculated to recognize neutron and photon events:

(1) $$\begin{array}{r}PSD=\frac{\underset{T_{tail}}{\overset{T_{end}}{\int }}U\left(t\right)dt}{\underset{T_0}{\overset{T_{end}}{\int }}U\left(t\right)dt},\end{array}$$

where $T_{tail}$ is an optimized beginning of the tail part of the pulse and $T_{end}$ is an optimized end point of the pulse, see Fig 6.

Fig. 6. The Mechanism of pulse shape discrimination.

The Ambient Neutron Dose Equivalent Rate

A G-function method has been used for the calculations of the ambient dose equivalent rate in real time [3], [4]. Using this function, we can convert measured spectra to the ambient dose equivalent rate. We can obtain measured neutron/gamma spectra $SP\left(E_{sp}\right)$ from the following formula: where $E_{sp}$ is deposited energy, $R(E,E_{sp})$ is a response function, $\mathrm{\Phi }\left(E\right)$ spectral fluence in the energy range ($E_{min}$, $E_{max}$). The integral ambient dose equivalent can be expressed using fluence and ambient dose equivalent for given energy:

(2) $$\begin{array}{r}SP\left(E_{sp}\right)=\underset{E_{min}}{\overset{E_{max}}{\int }}R(E,E_{sp})\cdot \mathrm{\Phi }\left(E\right)dE,\end{array}$$

(3) $$\begin{array}{r}{H}^{\ast }\left(10\right)=\underset{E_{min}}{\overset{E_{max}}{\int }}\mathrm{\Phi }\left(E\right)h\left(E\right)dE.\end{array}$$

The G-function $G\left(E_{sp}\right)$ is defined by the integral equation [4] and [5]:

(4) $$\begin{array}{r}h\left(E\right)=\underset{E_{spmin}}{\overset{E_{spmax}}{\int }}R(E,E_{sp})G\left(E_{sp}\right)d{E}_{sp}.\end{array}$$

The following formula allows us to calculate ambient dose equivalent using measured apparatus spectra and G-function:

(5) $$\begin{array}{r}{H}^{\ast }\left(10\right)=\underset{E_{spmin}}{\overset{E_{spmax}}{\int }}SP\left(E_{sp}\right)G\left(E_{sp}\right)d{E}_{sp}.\end{array}$$

Shadow-Cone Method

The shadow-cone method has been used for the neutron response measurement. This method employs measurement with and without the shadow cone placed between the source and the detector. Generally, measurements are made at a distance greater than twice the shadow-cone length. The difference between the measurements with and without the shadow cone expresses the number of scattered neutrons. The accuracy of this method depends strongly upon the design of the shadow cone and upon its position relative to the source-detector geometry. According to the ISO 8529-2, the maximum number of scattered neutrons is 40%.

Energy Calibration

The energy calibration has been performed using gamma-ray sources. Integrated digitized pulses have been linearly calibrated in keVee units or keV electron equivalent. The linear transformation coefficients were derived from the positions of the Compton edges [5] in the spectra of two gamma-ray sources ¹³⁷Cs and ⁶⁰Co. Measurement time has been determined in accordance with count rates from the detectors. The surrounding background has been subtracted from the measured spectra.

Measurement Results

The measurements of neutron spectra and ambient dose equivalent rate have been performed using the two-parametric digital spectrometer in the neutron calibration laboratory at the Paul Scherrer Institute in Switzerland.

Neutron Response Spectra

The neutron spectra have been measured at a distance 100 of 200 cm from the center of the Am-Be source with neutron emission of 5.2E+07 n/s, see Table I. Measurements have been carried out with a stilbene detector (45 × 45 mm). The shadow-cone method has been used. The measured shapes of the neutron spectra correspond to the Am-Be spectrum published in ISO 8529-1, see Fig. 7.

Fig. 7. Comparison of the measured and published spectra (ISO 8529-1).

Neutron Ambient Dose Equivalent Rate

Measurements of the ambient dose equivalent rate have been performed with a stilbene detector (25 × 25) mm at a distance of 50 to 230 cm from the center of the Am-Be source, see Table I. The shadow-cone method has been applied from a distance of 88 cm.

A comparison of MCNPX [6] simulations and measurements of the neutron ambient dose equivalent rate are shown in Tables II and III.

Table II. Verification of the Radiation Field Produced by Am-Be Source with Neutron Emission of 5.2E+07 n/s

Distance d [cm] detector-source (center-center)	Calculated (MCNPX) dose rate H*(10) [μSv/h]	Measured dose rate H*(10) [μSv/h]	Relative error [%]
50	2511.41	2800.82	11.52
60	1818.78	1990.92	9.46
88	814.29	874.91	7.45
100	631.06	672.17	6.52
115	479.93	502.42	4.69
130	374.33	379.12	1.28
150	281.63	289.59	2.83
180	195.99	201.38	2.75
200	159.12	163.45	2.72
230	120.52	125.52	4.15

Table III. Verification of the Radiation Field Produced by Am-Be Source with Neutron Emission of 6.8E+07 n/s

Distance d [cm] detector-source (center-center)	Calculated (MCNPX) dose rate H*(10) [μSv/h]	Measured dose rate H*(10) [μSv/h]	Relative error [%]
50	329.05	364.86	9.81
60	238.30	259.11	8.03
88	106.69	114.42	6.76
100	82.68	87.68	5.70
115	62.88	67.33	6.61
130	49.04	52.29	6.22
150	36.90	39.51	6.61
180	25.68	27.72	7.36
200	20.84	23.1	9.78
230	15.79	17.5	9.77

Conclusion

The neutron calibration laboratory has been installed and commissioned at PSI. The neutron irradiator has been equipped with two Am-Be sources. The shielding of neutron irradiator has been designed for a maximum neutron emission of 6.0E+07 n/s (~1 TBq of Am-Be source) with maximum loading capacity of 7 sources.

Two calibration benches, a radiation monitoring system and a system for controlling the technology of the laboratory are installed in the irradiation room. The minimum distance of the calibration bench from the source is 400 mm and the maximum is 3735 mm. In the middle of the calibration benches, the aluminium transport channel is placed. The ambient dose equivalent rates can be changed in the range from 17.8E-06 to 1.2E-03 Sv/h. At a distance greater than 1.8 m, the proportion of scattered neutrons is higher than 40%. This is the maximum allowed by the ISO 8529 standard without using the shadow-cone method.

The quality of the radiation fields has been measured with a newly developed digital spectrometer. The ambient dose equivalent transmission calibration procedure using a digital spectrometer has been verified, see Tables II andIII and Fig. 7. The digital spectrometric system was tested in the primary calibration laboratory of the Czech Metrological Institute in Prague.

The design of the neutron irradiator is made in such a way that the interaction between the neutron sources is negligible. Specifically, it is guaranteed that the neutron activation of selected irradiator components is lower than 10 kBq/year at the maximum activity of the neutron sources, i.e., 1 TBq of Am-Be source.