Experimental design

For neutron transmission measurement, we used a 241Am/Be neutron source and a Canberra portable neutron detector equipments. 241Am/Be source emits 4.5 MeV neutron particles. Physical form of 241Am/Be neutron source is compacted mixture of americium oxide with beryllium metal. Fast neutrons are produced by following nuclear reaction,

|Ве(«, n)126C

5.486 keV maximum energy alpha particles emitting from 241Am. Neutron energy value produced by this nuclear reaction is 4.5 MeV. Radiation characteristics of 241Am/Be neutron source are shown in Table.2 (Dose rate values have been obtained from The Health Physics and Radiological Health Handbook, Scintra _Inc., Revised Edition, 1992.).

The NP-100B detector provides us to detect slow and fast neutrons. Tissue equivalent dose rates of the neutron field can be measured by it. The detectors contain a proportional counter which produces pulses resulting from neutron interactions within it. The probes contain components to moderate and attenuate neutrons. So that the net incident flux at the proportional counter is a thermal and low epithermal flux representative of the tissue equivalent dose rate and the neutron field. Because of neutrons have no charge; they can only be detected indirectly through nuclear reactions that create charged particles. The NP100B detector uses 10B as the conversion target. The charged particle — alpha or proton (respectively) created in the nuclear reaction ionizes the gas. Typical detector properties are shown in Table. 3. Equivalent dose rate measurement results have read on RAD ACS program in system PC. Experimental design is shown in Fig.4.


Fig. 4. Experimental Setup

Physical Half-Life: 432.2 years

Specific Activity: 127 GBq/g

Principle Emissions




Dose Rate (p. Sv/h/GBq at 1m)



13.9 (42.7%) 59.5 (35.9%) 5.443 (12.8%) 5.486 (85.2%)



4.5 MeV


http://www. stuarthunt. com/pdfs/Americium_241Beryllium. pdf Table 2. Radiation characteristics of 241Am-Be neutron source*

Specifications of Canberra NP100B Neutron Detector

Detector Type

BF3 Proportional Counter

Detector Sensitivities

0-100 mSv/h (0-10 Rem/h)

Energy Range

0.025 eV — 15 MeV

Operating Temperature Range

-10 °C to +50 °C (+14 °F to +122 °F)

Size (mm.)

244 x 292 mm

(Dia. x inch)

(9.6 x 11.5 in.)

Weight kg (lb)

10 kg (22 lb)


Moisture Proof Aluminum

Operating Humidity

0-100% non-condensing

Detector Linearity




High Voltage Supply (internally generated)

1750-1950 V

http://www. canberra. com/pdf/Products/RMS_pdf/NPSeries. pdf

Table 3. Typical Properties of Detector*

We determined dose transmission values of vermiculite loaded samples. Firstly, we counted equivalent dose rate by fast neutrons while there is no sample between source and detector
system. And then we measured for each sample neutron equivalent dose rate while there is our sample between 241Am-Be source box and detector probe. The ratio of two values is called dose transmission.