Sealed tube neutron generator incorporating an internal...

Details Sealed tube neutron generator incorporating an internal... Sealed tube neutron generator incorporating an internal...

1999-01-22

2001-10-02

Evans, F L. (Department: 2877)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C250S269100

Reexamination Certificate

active

06297507

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a device for generating neutrons for well logging and more particularly to a sealed tube neutron generator incorporating an internal associated &agr; particle detector(&agr;-STNG). The &agr;-STNG can be employed within a carbon/oxygen well logging apparatus which adopts the Associated-Particle and Time of Flight Approach for collecting data to satisfy the special requirement of well logging.
BACKGROUND OF THE INVENTION
In a nuclear carbon/oxygen ratio (C/O) well logging tool, a sealed tube neutron generator (STNG) is employed to provide 14 Mev fast neutrons(n) by the nuclear interaction d+t=&agr;+n. The existing conventional STNG, for example, disclosed by CN patent No. 2052573u, consists of gas-tight insulator electrodes, a gas-tight envelope, Penning ion source, gas supply, accelerating electrode and target. Bombarding the formation with 14 Mev neutrons, prompt nelastic gamma rays of carbon (4.43 Mev) and of oxygen (6.13 Mev) are induced. By measuring the gamma ray spectrum, the atomic carbon/oxygen ratio and consequently oil saturation in the formation should be obtained under ideal conditions.
However, the C/O values logged by logging apparatus that employs a conventional STNG have large errors. The error is induced mainly by the cased-hole materials (includes cased hole fluid) that contain a large amount of the elements C, O, S and Ca. The &ggr; rays emitted from them constitute a very high background for the gamma spectrum logged. Moreover, the &ggr; rays from thermal neutron capture(capture-&ggr;) also constitute the background for the inelastic &ggr;-spectrum. These backgrounds adversely interfere with the C/O ratio logged. Although the method and apparatus that employs conventional STNG have been improved recently, the problem remains unsolved.
CN patent application No.93109244 disclosed a novel Carbon/Oxygen Well Logging Apparatus which adopts an Associated-Particle and Time of Flight Approach for collecting data. The apparatus will be called APTF-C/O-WLA hereinafter. Referring to
FIG. 1A
, the APTF-C/O-WLA comprises a STNG with a &agr;-signal acquiring device
9
(&agr;-STNG), fast timing electronics
25
, BaF
2
&ggr; detector
17
and other conventional members that are used in conventional well logging apparatus. The &agr;-signal acquiring device
9
comprises a plurality of small &agr;-detectors that surround the incident ion beam; each small &agr;-detector consists of a scintillator, a light guide, and a photo multiplier tube. It has been tested and verified in well logging that the APTF-C/O-WLA is able to resolve the above-mentioned problem of using conventional well logging apparatus, and to improve the precision of the C/O logged significantly. The working principle is introduced as follows:
Referring to FIG.
1
A and
FIG. 1B
, by the T(d,n)He nuclear reaction caused by the bombardment of the deuterium ions on the target, a 3.6 Mev &agr;-particle and an associated 14 Mev neutron are produced simultaneously and emitted in opposite directions. The direction of the &agr;-particle specifies the subsequent trajectory of the individual neutron. When the neutron emitted forward is scattered inelastically with the materials (both cased-hole material
18
and formation
16
) surrounding &ggr;-scintillator
17
, a gamma ray is emitted. The cross sections of the cased-hole material
18
and detectable formation
16
are two adjacent annuluses. Thus mounting an annular &agr;-detector
9
before the target, and by using the Associated Particle and Time of Flight Approach, the background of the &ggr; spectrum can be eliminated. This is explained as follows: Assume that Tt represents the total time of flight of a neutron and the &ggr; induced by the neutron. Because the speed of &ggr; is much faster than the speed of the neutron, then the Tt may represent the time of flight of neutron Tn approximately, and subsequently represents the distance from the target to position P where the &ggr; is induced by the neutron. The equal-value surface of Tn may be convened to a sphere approximately. In
FIG. 1A
Tl and Tr represent two equal-value surfaces of Tn for predetermining the formation region to be logged. In
FIG. 1B
, the output signals of &agr; detector
9
are shaped by constant-fraction discriminator (CFD) and routed through Delay to the start-input of a Time to Amplitude Convener (TAC); the &ggr;-rays generated by inelastic neutron interactions are converted to an energy spectra with &ggr;-detectors
17
and
19
, the fast timing signals are shaped by CFD and routed through Delay to the stop-input of the TAC to provide a range-proportional, Time-of-Flight Spectrum (TOF) at TAC output (Tt). The TOF spectrum includes the contributions from the time-of-flight of the &agr; particle and the electronics; but if the contributions are approximately constants, the TOF spectra represent approximately the relative time-of-flight spectra of the neutrons. Using SCA to select the Tl and Tr at the TOF spectrum, the signals from the portion between Tl and Tr correspond to the &ggr; rays that occur within the formation regions of interest. From viewing their longitudinal section, the regions will be that surrounded by curved lines Tl, Tr and ray lines T
D
′ and T
C
′, and that surrounded by curved lines Tl, Tr and ray lines T
A
′ and T
B
′. The time signals mentioned above are routed to the gate input of multiple channel analyzer (MCA) to turn on the time gate, the energy signals in the &ggr; energy spectra are routed to the E-input of MCA, so that only the energy signals of &ggr; rays that occur within the predetermined formation region are recorded by MCA. The &ggr; rays that occur within the cased-hole materials are not recorded at all. The reason is that a part of them has no associated-&agr; particle (occurring within the region between rays T
B
′ and T
D
′), and another part of them occurs out of the region predetermined by Tl and Tr (within the region to the left of Tl). On the other hand, the procedure relative to capture-&ggr; is a slow procedure being of the order of 10 &mgr;s; the procedure relative to inelastic prompt &ggr; is a fast one being of the order of ns; the main point of the associated particle and time of flight approach used here is fast coincidence method; its time resolution is equal to the difference of Tr-Tl, which usually is about 8 to 10 ns. So the capture-&ggr; induced by a thermal neutron can not be counted in MCA inevitably. It just can be counted occasionally.
Obviously, the key element of the APFT-C/O-WLA is the &agr;-STNG, but the above CN patent application has not disclosed its specific structure. There will be described in this specification that a &agr;-STNG suitable for using in APTF-C/O-WLA for well logging has particular structure rather different from that of a conventional STNG.
Particularly, the above mentioned CN Patent application has not disclosed the integral structure of the &agr;-STNG that can be used with the above well logging apparatus which adopts Associated-Particle and Time of Flight Approach, the structure of the parts relative to ions beam, and the specific standards for designing and manufacturing the &agr;-signal acquiring device and the relatively small &agr;-detectors associated therewith. However, in order to manufacture a qualified &agr;-signal acquiring device, the designers have to obey some particular and specific standards, which are given as follows:
(1) The associated-&agr; detector should have the following features:
1. Possessing satisfactory geometric characteristics: that is, having wide enough sterad for accepting associated &agr; particles; capable of distinguishing the cased-hole materials from the formation region to be logged; the optimized and predetermined formation region to be logged being located nearby and surrounding the &ggr;-scintillator; its whole body being suitable to be used in the limited space of a cased hole well.
2. Possessing satisfactory signal characteristics: that is, output signals hav

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