Method and apparatus for measuring radiation in a borehole

Abstract

Apparatus and methods for measuring radiation in a borehole environment using a YAlO 3 :Ce (YAP) scintillation crystal. Borehole instruments are disclosed which employ a gamma ray detector comprising a YAP scintillator coupled to a light sensing means such as a photomultiplier tube. One instrument embodiment combines a YAP scintillation detector and a source of pulsed neutrons. Borehole environs are irradiated with neutrons, and induced gamma radiation is measured using a YAP scintillation detector. Response of the detector is used to determine characteristics of the borehole environs. Mechanical and physical properties of YAP are utilized to obtain improved measurements. The relatively short light decay constant of YAP minimized pulse pile-up in the detector when measurements require that the detector be operated during a neutron pulse.

What is claimed is:  
     
         1 . A borehole instrument responsive to radiation from borehole environs, the instrument comprising a YAP scintillator.  
     
     
         2 . The borehole instrument of  claim 1  further comprising a light responsive means optically coupled to said YAP scintillator, wherein output of said light responsive means is a function of scintillation intensity within said YAP scintillator.  
     
     
         3 . The borehole instrument of  claim 2  further comprising means for transforming said output from said light responsive means into a characteristic of radiation reacting with said YAP scintillator.  
     
     
         4 . The borehole instrument of  claim 2  wherein said light responsive means comprises a photomultiplier tube.  
     
     
         5 . The borehole instrument of  claim 3  wherein said characteristic is intensity of radiation reacting with said YAP scintillator.  
     
     
         6 . The borehole instrument of  claim 3  wherein said characteristic is energy of radiation reacting with said YAP scintillator.  
     
     
         7 . The borehole instrument of  claim 1  wherein said radiation is gamma radiation.  
     
     
         8 . The borehole instrument of  claim 7  wherein said gamma radiation is inelastic scatter gamma radiation.  
     
     
         9 . The borehole instrument of  claim 3  further comprising means for transforming said characteristic of radiation into a property of borehole environs in which said borehole instrument is disposed.  
     
     
         10 . The borehole instrument of  claim 9  wherein said property is a property of material penetrated by said borehole.  
     
     
         11 . The borehole instrument of  claim 9  wherein said property is a property of materials within and in the immediate vicinity of said borehole.  
     
     
         12 . The borehole instrument of  claim 1  wherein said instrument is conveyed along a borehole by a wireline.  
     
     
         13 . The borehole instrument of  claim 1  wherein said instrument is conveyed along a borehole by a drill string.  
     
     
         14 . A method for measuring radiation in a borehole environment, comprising the step of analyzing scintillations generated by interaction of said radiation from borehole environs with a YAP scintillator.  
     
     
         15 . The method of  claim 14  wherein said analysis of scintillations comprises the steps of: 
 (a) optically coupling a light responsive means to said YAP scintillator;  
 (b) measuring output of said light responsive means; and  
 (c) relating said output to a characteristic of said radiation.  
 
     
     
         16 . The method of  claim 15  wherein said characteristic is energy of said radiation.  
     
     
         17 . The method of  claim 15  wherein said characteristic is intensity of said radiation.  
     
     
         18 . The method of  claim 14  wherein said radiation is gamma radiation.  
     
     
         19 . The method of  claim 14  wherein said YAP scintillator is conveyed along a borehole by means of a wireline.  
     
     
         20 . The method of  claim 14  wherein said YAP scintillator is conveyed along a borehole by means of a drill string.  
     
     
         21 . A borehole logging system comprising: 
 (a) a radiation source; and    (b) a radiation detector responsive to radiation induced in borehole environs by said radiation source and comprising a YAP scintillator, a light responsive means coupled to said scintillator, and circuitry connected to said light responsive means for processing output from said scintillator, wherein    (c) said radiation detector processes detected scintillations with minimal light decay time from said scintillator to minimize pulse pile-up in said radiation detector.    
     
     
         22 . The system of  claim 21  wherein said radiation source is a neutron source.  
     
     
         23 . The system of  claim 22  wherein said neutron source is a pulsed neutron source.  
     
     
         24 . The system of  claim 23  wherein said radiation detector is operated during pulses from said pulsed neutron source.  
     
     
         25 . The system of  claim 23  wherein said radiation detector is operated during quiescent periods between pulses from said pulsed neutron source.  
     
     
         26 . The system of  claim 21  wherein said radiation detector is a gamma ray detector.  
     
     
         27 . The system of  claim 21  further comprising: 
 (a) a housing in which said radiation source and said radiation detector are disposed; and  
 (b) a wireline for conveying said housing along a borehole.  
 
     
     
         28 . The system of  claim 21  further comprising: 
 (a) a housing in which said radiation source and said radiation detector are disposed; and  
 (b) a borehole drilling string for conveying said housing along a borehole.  
 
     
     
         29 . The system of  claim 21  further comprising means for transforming response of said radiation detector into a borehole environs parameter of interest.  
     
     
         30 . A method for measuring a characteristic of environs penetrated by a borehole comprising the steps of: 
 (a) irradiating said environs with a source of radiation thereby inducing radiation within said environs; and    (b) measuring said induced radiation with a radiation detector comprising a YAP scintillator, a light responsive means coupled to said scintillator, and circuitry connected to said light responsive means for processing output from said scintillator, wherein    (c) said radiation detector processes detected scintillations with minimal light decay time from said scintillator to minimize pulse pile-up in said radiation detector.    
     
     
         31 . The method of  claim 30  wherein said source of radiation is a neutron source.  
     
     
         32 . The method of  claim 31  comprising the additional step of pulsing said neutron source.  
     
     
         33 . The method of  claim 32  comprising the additional step of operating said radiation detector during pulses from said pulsing neutron source.  
     
     
         34 . The method of  claim 32  comprising the additional step of operating said radiation detector during quiescent periods between pulses from said pulsing neutron source.  
     
     
         35 . The method of  claim 30  wherein said radiation detector is a gamma ray detector.  
     
     
         36 . The method of  claim 30  comprising the additional steps of: 
 (a) disposing said source of radiation and said radiation detector in a housing; and  
 (b) conveying said housing along a borehole by means of a wireline.  
 
     
     
         37 . The method of  claim 30  comprising the additional steps of: 
 (a) disposing said source of radiation and said radiation detector in a housing; and  
 (b) conveying said housing along a borehole by means of a drill string.  
 
     
     
         38 . The method of  claim 30  further comprising the step of transforming response of said radiation detector into said characteristic of environs.