ORIGINAL ARTICLE Year : 2013  Volume : 36  Issue : 2  Page : 9094 Effective center measurement of long counter and calibration 241Am Be neutron source Sahar Rastegar Moghadam, Feridon Abbasi Davani, Mohammad Danaei Department of Radiation Application, Shahid Beheshti University, Tehran, Iran Correspondence Address: Long counter (LC) is one of the standard instruments in neutron source flux measurements. Its counting efficiency is independent on the neutron energy and can be useful device in many neutron physics applications. In this paper, LC effective center for a ^{241} AmBe neutron source has been measured in two cases one with source inside a collimator and other with bare neutron source. The effective center for the neutron source inside collimator and under bare conditions has been measured to be 13.5 cm and 9.5 cm, respectively. Using effective center and flux at various distances, the emission rate of the source has been estimated. Neutron emission rate was found to be (4.78 ± 0.164) Χ 10 ^{7} and (4.92 ± 0.37) Χ 10 ^{7} n/s for the source inside the collimator and bare source, respectively. This technique is one of the useful methods to determine neutron emission rate and for neutron fields«SQ» calibration.
Introduction Fast neutron flux measurement by thermal neutron detector requires moderating materials such as paraffin and polyethylene. [1] When a cylindrical thermal neutron detector is in moderator, effective center of this detector is changed. Effective center is a differential volume of detector that is assumed point detector for different distances from radioactive (neutron) source. Effective center is dependent on neutron energy. If neutron field is generated with energetic neutron, effective center is deeper into front surface of the detector. [2] On the other hand, penetration length of energetic neutron is greater compared to low energy neutron. [3] Measurement of neutron flux in laboratory requires a detector with characteristics properties such as independent response on wide range of neutron Energy, directional response and insensitive to gamma ray response. Such detectors are generally called long counter (LC) and sensitive minimum neutron energy is cut off of cadmium. [4] In neutron laboratory, due to neutron scattering from environment (such as walls and the air in the exposure environment), elimination of scattered neutrons is one of the important requirements for neutron flux measurements. [5] Furthermore, in mixed fields of neutron and γray, neutron separation from γray will be helpful for net neutron flux measurement in the laboratory. [6] This kind of counter can be applicable in neutron flux monitoring for neutron generators, calibration of neutron sources and neutron fields in neutron secondary standard dosimeter laboratory. [7] In this paper, the LC has been used as calibration reference detector for neutron flux measurement (neutron sources calibration). Also the effective center of the counter has been measured. Materials and Methods Effective center and efficiency While making a fluence measurement with a LC, under conditions of efficiency for a point detector and a point source in a vacuum, the following equation can be assumed. [INLINE:1] Where C is the detector count rate and r is the source to detector distance. Allowing for scatter and geometry effects, a corresponding expression for a LC has been suggested by Huntand is given by: [INLINE:2] Where Ct (r) and Ci (r) are total and inscatter measured count rates at a source to detector distance r measured to the front face of the counter moderator, Fa (r) is the air outscatter correction factor (derived from the oxygen and nitrogen cross sections), r0 is the effective center distance relative to the detector front face (i.e., a positive value means that the effective center is within the counter and further from the source than the moderator front face which is effectively used as a reference surface for the sourcetocounterdistance) and Kisthesourcedetector characteristicconstant. Rearranging the above expression gives: [INLINE:3] And it follows that r0 and K can be obtained by a least squares fitting of [INLINE:4] With the gradient equal to and the intercept equal to. Dividing the intercept to the slope, the effective center obtained. Subsequently, the efficiency ε can be evaluated by: [INLINE:5] Where Q is the source strength, i.e., the total emission rate into 4π steradians and FI is the anisotropy factor of the radionuclide source. [8] LC, neutron sources, exposure area, collimator LC in this paper has been constructed by a BF 3 stainless steel proportional counter with active length and diameter of 311.5 mm and 38 mm respectively. LC has been surrounded by three parts of the cylindrical moderator; inner, outer and rear moderator (made of polyethylene of 0.93 g/cm 3 density. Inner and rear moderators have the diameter of 190 mm and 340 mm and length of 515 mm and 160 mm, respectively. Furthermore, the outer moderator has the inner and outer diameter and the length of 194 mm, 311 mm and 340 mm, respectively. A channel was drilled inside the inner moderator to improve the lowenergy neutron counting efficiency (below 1 MeV) with an aperture of 25 mm and depth of 100 mm. LC schematic is shown in [Figure 1]. A cylindrical 241 AmBe isotropy neutron source has been used in the measurement, with nominal activity of 20Ci with neutron emission rate of (4.4 ± 0.2) × 10 7 n/s in 4π and the cylinder was made of aluminum. Measurements were carried out in a room with dimensions of (23 × 4 × 3.8 m). The wall and floor of the laboratory was made of normal bricks and flat concrete, respectively. Concrete cube with dimension of 75 × 75 × 75 cm was used as collimator [Figure 2] and its aperture dimension was changed into 37.5 × 15 × 15 cm. The neutron source was placed at the center of concrete and LB6411 was used for measuring the neutron dose rate. The neutron dose rate outside the concert shield was reduced to <17% of bare source. Further reduction of neutron flux to <4% was achieved by placing polyethylene blocks around the concrete shield. In all measurements, a layer of cadmium with 1 mm thickness was placed at the front face of LC to absorb thermal neutrons.{Figure 1}{Figure 2} Results and Discussions Measurement of effective center and neutron emission rate (neutron source inside collimator) Initially, the source was placed inside the collimator and neutron flux was measured at more than 15 sourcetocounter distances ranging from 125 cm to 550 cm. The LC position was varied in steps of 25 cm. The results are showing [Figure 3].{Figure 3} From [Figure 3], the exponent of " r" and constant " Q" of Equation 6 can be obtained. [INLINE:6] To measure the effective center, the inverse of the square root of the count rate has been plotted against source to LC distance. Linear leastsquares fitting parameters are showing [Figure 4]. From [Figure 4] and using Equation 3, the effective center of 13.5 cm is obtained for LC. As it has shown in [Figure 5], the exponent of r is shown by B and is equal to 1.91. Further by reducing the neutron scatter; the exponent of " r" will be closer to 2. Using this effective center and Equation 6, the neutron emission rate was found to be (4.78 ± 0.164) ×10 7 n/s. The relative difference between the neutron emission rate and measured value by LC for source inside the collimator is 8.64%.{Figure 4}{Figure 5} Measurement of effective center and neutron emission rate (bare neutron source) In this section, neutron source is bare and neutron count rate has been measured at more than15 source to counter distances ranging from 125 cm to 550 cm and the detector position has been varied in steps of 25 cm. From the figure and as in the previous section constants of formula 6 were determined. The exponent of " r" was found to be 1.45 as the count rate is not the same as in the previous one. Thus, the exponent of " r" is far from 2. In previous section, when the source had been located inside the collimator, the exponent of " r" was equal to 1.91. The scattering has resulted in the exponent " r" being equal to 1.45 [Figure 6]. The scattering was removed with shadow cone made of paraffin, steel and iron (paraffin and iron have length of 300 mm and 200 mm, respectively and the diameter of front and rare surface are 200 mm and 300 mm respectively), to obtain the exponent " r" equal to 1.89 and the emission rate equal to 2.87 ± 0.26 × 10 7 n/s [Figure 7]. In this case, the obtained emission rate in 4π is far too different from the nominal value of neutron emission rate. Neutron source flux can be measured accurately, if the LC effective center is measured after scattering removal and added to the measured distances. As it has shown in [Figure 8], (the effective center is equal to 9.5 cm [Figure 9]) neutron emission rate has been obtained (4.92 ± 0.37) ×10 7 n/s. The exponent of r is equal to 1.98 when effective center has been measured and added to the distance. The relative difference between the nominal neutron emission rate and the measured value by LC for bare source and after removal of the scattering is about 12%. The measured and calculated results of effective center and emission rates are presented in [Table 1].{Figure 6}{Figure 7}{Figure 8}{Figure 9}{Table 1} Conclusions From the measurements results, the effective center of the LC has been determined and was in good agreement with the computed value by Monte Carlo NParticle. The neutron emission rate in 4π by nominal activity of 20Ci has been measured outside and inside of the collimator, 4.78 ± 0.16 × 10 7 and 4.92 ± 0.37 × 10 7 n/s, respectively. It can be improved further by the removal scattered neutrons and collimator geometry optimization. This method can be used for calibration of neutron sources. References


