The NA4 Noise Analyser provides a convenient way to test fission chambers that are used in the Mean Square Voltage mode (also known as Campbell mode or fluctuation mode). The Mean Square Voltage (MSV) technique uses the property that the mean of the square of the signal from a fission chamber, within a fixed bandwidth, is proportional to the neutron flux. When combined with a conventional pulse counting technique this enables a single fission chamber to cover ten or more decades of neutron flux.
A complication of the MSV technique is that the frequency spectrum from a fission chamber is not flat. Inside the chamber, electrons travel very fast, producing a flat spectrum that extends into the MHz range, but the ions travel much more slowly and only contribute to frequency components below about 10kHz. This results in two plateaus with a transition between them at around 10kHz. The transition frequency varies with applied voltage and the pressure of the gas in the chamber. It is therefore important to periodically measure the spectrum of in-service fission chambers to ensure that measurement is taking place on a stable plateau. Most MSV neutron flux measurement systems operate on the electron plateau for rapid response. Impurities in the filling gas of a fission chamber can cause the amplitude of the electron plateau to fall with respect to the electron plus ion plateau, leading to a potentially unsafe under-estimate of neutron flux. This condition can be revealed by measuring the ratio of the two plateau amplitudes. Measurement of the spectrum provides information about the condition of fission chambers and can reveal trends, enabling informed decisions to be made about their replacement.
The NA4 provides in one portable instrument, everything required to measure the spectrum from an in-service chamber. It can also be used to test newly manufactured or spare fission chambers using a research reactor or accelerator. An integral low-noise EHT supply polarises the chamber. Measurement frequencies can then be selected manually, or the process can be run automatically. In the automatic mode, the start frequency, stop frequency, number of points per decade and averaging time for each result are selected. On pressing the Run button, the instrument averages the signal at each frequency point and stores the results on a USB flash drive. Results have the units A2
/Hz and are therefore proportional to neutron flux. The file format is suitable for importing into Excel for graph plotting and further analysis. The stored file also contains the time and date of the measurement and the instrument settings.
||33, 50 or 75 ohms, user selectable
||+350V to +1000V
||0 to 5mA
||100Hz or 1kHz, user selectable
||1kHz to 60MHz
|Signal Measurement Range
||1x10-22A2/Hz to 1x10-15A2/Hz
||1, 4, 15 & 60s, user selectable
||USB flash drive
||90-264V at 47-63Hz
||365 x 192 x 347mm deep (ex. feet & handle)