Cesium-137

The main components in this measurement were the detector, radioactive point source, lead collimator and the glass samples or an absorber needed for measuring. The schematic diagram of these components is illustrated in Figure 1. The High Pure Germanium (HPGe) Detector of model: GC1520 (Manufacturer of radiation detection and analysis instrumentation, Meriden, USA) was used. The relative efficiency of the detector was 15% and the resolution (FWHM) at 1.33 MeV was 2 keV. Three point sources were used in the measurement to cover a wide range of energies. The Am-241 point source is a very important source that emits a line in low energy (59.54 keV), and the initial activity of this source was 259 kBq. Cs-137 point source emits two lines (32, 661.6 keV) but the line due to X-ray emission (32 keV) was totally absorbed and therefore the most probable line and higher emission probability is (661.6 keV), the initial activity of this source is 385 kBq. Eu-152 is a multi-line source and covered more energies from low to high energy. The lines were chosen according to the higher emission probability (121.78, 244.69, 344.28, 964.13 and 1408.1 keV), the initial activity of this source is 290 kBq. The reference date of all three point sources was 1 June 2009 [27 (link),28 (link)].
The narrow beam method was used in measurements by the lead collimator. The detector first was calibrated and the background was measured using Genei 2000 software (V3.3, Mirion Technologies (Canberra), Inc., Canberra, Australia) [29 (link)]. The measurement occurred within and outside the glass sample with different sources to obtain the net peak area or the count rate of the line which represent the intensity of this line. So, the intensity of the line outside the absorber (I₀) and within the absorber (I) can be calculated. By knowing the thickness of the glass absorber (x), the linear attenuation coefficient (LAC) easily estimated via the relation [30 (link)].
$LAC=-\frac{\ln (\raisebox{1ex}{$I$}\!\left/ \!\raisebox{-1ex}{$I_0$}\right.)}{x}$
The MAC can be calculated experimentally by dividing the LAC on the density of an absorber (ρ). The MAC examined theoretically by the XCOM program for all present glass samples using the chemical composition of each sample. Other related parameters were investigated such as HVL, TVL, and MFP, as well as RPE. The HVL and TVL are the thickness layers of an absorber needed to reduce the count rate of a line a half and a tenth of its original value, respectively, and are given by the following equations [31 (link)].
$\mathrm{HVL}=\frac{Ln2}{LAC}$
$\mathrm{TVL}=\frac{Ln10 }{LAC}$
The mean-free path can be estimated by Equation (4) [32 (link)].
$\mathrm{MFP}=\frac{1}{LAC}$
The shielding efficiency of an absorber sample can be investigated using a parameter called the radiation protection efficiency (RPE) and given by the next equation [33 (link)].
$\mathrm{RPE}=(1-\frac{I}{I_0})\times 100$

A ready-made RTV2 silicone rubber in liquid form was purchased from a local store in Egypt, with its stiffener made in China, in addition to purchasing micro-magnesium oxide from El-Gomhouria Chemicals Company in Cairo, Egypt, with a purity of 97.8% and an average particle size of 60 ± 4 μm. Nano-magnesium oxide was purchased from Nano Tech Company, Egypt, with a purity of 99.8% and an average particle size of 20 ± 5 nm. The nanoparticles were prepared chemically and their purity was confirmed using EDX analysis. Samples were photographed using a transmission electron microscope (TEM) to confirm their size.
The mixing process was manually carried out until it became a single mixture that had no lumps or voids, after which it was poured into molds until dried. The samples were mixed in the same proportions as listed in Table 1, with 2 grams of hardener added to every 50 grams of silicone rubber, then adding either micro- or nano-MgO and mixing well until homogeneous. Then they were poured into molds and left for 24 h.
First, the density was measured using the law of mass per volume; the sample was weighed for mass and the volume was measured by the thickness and the sample radius. Then, a system was designed, as shown in Figure 1, to measure the attenuation coefficient of the existing samples using three radioactive sources (Co-60, Cs-137, and Am-241) and a HPGe detector at the Environmental and Radiation Measurements Laboratory, Institute of Graduate Studies and Research, Alexandria University, Alexandria, Egypt.
The measurement was made in the presence and absence of the absorbed sample to determine the intensity of gamma ray photons in both cases. Genie 2000 software was used to analyze the resulting spectrum and determine the intensity of the photons in the presence of the silicon rubber sample (N) and in the absence of the silicon rubber sample (N₀). The linear attenuation coefficient (LAC) was experimentally determined using Equation (1) [18 (link),19 (link),20 (link),21 (link),22 (link)]: $$LAC=\frac{1}{d} ln\frac{N_0}{N}$$
where d is the thickness of the sample. From LAC, we can determine the half-value layer (HVL) by the following Equation (2) [23 (link),24 (link),25 (link),26 (link),27 (link),28 (link),29 (link),30 (link)]: $$HVL=\frac{LN \left(2\right)}{LAC}$$
The mean free path (MFP) is calculated using Equation (3): $$MFP=\frac{1}{LAC}$$
The radiation absorption ratio (RAR) is an useful quantity for estimating the efficacy of shielding materials and given by Equation (4) [31 (link),32 (link),33 (link),34 (link)].
$$RAR\left(\%\right)=\left(1-\frac{N}{N_0}\right)\times 100$$

The sources analyzed, in analogy with what was found, are two gamma emitters, Co-60 and Cesium 137, with an activity of 10 GBq, and then respectively increased to 10⁴ and 10³ factors. Table 1 reports the main characteristic of the sources.
For the calculation of the quantities of radionuclides deposited on the ground, the conversion tables provided by Eckerman and Leggett [8 ], reported in Table 2, were used, while for the calculation of the effective dose from direct exposure from soil and cloud and from inhalation, calculated for only adults, the tables taken from the CEVaD manual [9 ] were used.
The presence of a seal is not included in the modeling of the source term, assuming it is destroyed or melted. In the real case, the sources were protected by an Ir-192 (Iridium-192) seal.

In order to evaluate spectroscopic performances, the detectors were exposed to uncollimated gamma ray sources irradiating from the cathode side. Radioisotopes were chosen in order to cover a wide energy range, from a few tens of keV to more than 1 MeV. Tested sources were: Am-241, Ba-133, Co-57, Co-60, Cs-137, Na-22, U (10% U-235), Pu-239. During gamma-ray measurement, the detectors were biased by means of a customised High-Voltage generator, already used for current–voltage measurements. A digital readout electronics chain realised by Due2Lab s.r.l. was used to readout detector signals. The readout electronic chain is composed by a customised charge sensitive preamplifier (sensitivity 36 mV/MeV, about 100 electrons RMS, decay time: 20 µs), coupled with a fast ADC (100 Msample/s, 14 bit) and a FPGA unit, which converts the digitalized signals into the energy spectrum. No pulse shape analysis or correction was performed in order to better compare the real detectors performances.