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Pulsed Neutron Detectors
High energy inelastic gammaray spectroscopy presents its own set of challenges in pulsed neutron logging. There are two factors for getting high resolution inelastic data which are:
1. Neutrons generated per second by generator which we already discussed in previous article.
2.Detectors employed by the tool. In this article we will give detailed look at detectors used in pulsed neutron tool.
For good understanding of this topic and to appreciate the need to develop understanding about detectors, we highly recommend readers to go through previous articles on this topic which are cased hole saturation evaluation and pulsed neutron physics
Main challenge in pulsed neutron logging is recording of inelastic data. Hence, most of the discussion in this article will be done on detectors with focus on inelastic data recording. However it should be noted that if detector is capable of recording inelastic data with high certainty than that detector should be capable of recording capture data easily.
Main properties on which pulsed neutron detectors capability is evaluated are:
1. Energy Resolution (Dependent upon detector density and effective atomic number)
2. Decay time
3. Light output & Relative light output
4. Temperature coefficient of light output
5. If detector is hygroscopic or not
Energy resolution of detector is determined using 662 kev 137 Cs source. It’s a direct function of Detector density and detector’s effective atomic number. Higher the density and higher the effective atomic number better the energy resolution of detector. For a given energy peak of spectra,it is measured as full width at half of its maximum divided by half of peak channels than multiplied by 100 to convert it into percentage. Higher energy resolution results into sharper peak of spectra which is extremely important in inelastic gamma ray spectroscopy for carbon-oxygen mode data acquisition.
Next factor is the decay time,it is the time required for GR counts hitting the detector to decrease to 36.8% of its maximum value.Faster decay time ensures higher gamma ray counting rates resulting into better statistical precision of measurement.
Light output of detector is defined as amount of photons produced by detector per kev of energy deposited. Its unit is photons/kev. Light output of detector is one of the most important parameter as it affects detector efficiency and detector energy resolution. Here detector efficiency is ratio of number of detected gamma ray particles to total particle hitting the detector. Light output of Sodium Iodide (NaI) detector is 38 photons/kev. It is taken as standard and hence relative light output of NaI detector is taken as 100%.For all the detectors used is industry relative light output is measured as relative to that of NaI detector.
Some of the detectors in industry suffers degradation with increasing temperature specially above 120deg C. So, they need to be shielded in Dewar flask which increase tool diameter. This is highly undesirable as increasing tool diameter means logging company loses business in small pipe diameter holes. So that brings into consideration high temperature performance of various detectors. It is always desired to have detector which suffers minimum degradation with increasing temperature and hence lower temperature coefficient of light output.
A hygroscopic detector gets damaged when exposed to moisture present in atmosphere, so there is a need to take care not to expose these detectors to atmosphere.
In the table below we have listed some of the detectors and their properties along with tools in which they are used for review of reader and to understand capability of individual detectors and hence the tools.
Table: Tools and their detectors
Looking at above table we may observe that detector with highest density / highest effective atomic number, also having highest mass will provide best energy resolution so this detector could possibly be the best choice but looking at decay time ,if its decay time is high and its light output is low than it may not be the best choice. It is possible that given detector may not be best in each of the parameters; we should look for overall parameters of detectors. Temperature coefficient of light output is also very important parameter as when tool spends time down hole, it will face high temperatures and using Dewar flask to counter it increases the outer diameter of tool.
Do comment and let us know which detector you think is best among above detectors. To get in touch with author please write us at support@oilfieldknowledge.com