Available online at Procedia Engineering 7 (2010) Procedia Engineering 00 (2010)

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1 Available online a Procedia Engineering 7 (2010) Procedia Engineering 00 (2010) Procedia Engineering Design of High Reliabiliy Nuclear Logging Probe Yingchun Xiao a, Shihong Xiang b, Zhengyu Zhao a, Zuping Qian c * a Universiy of Wuhan, Wuhan Ciy, , China b Shanghai Insiue of Technical Physics of he Chinese Academy of Sciences, Shanghai Ciy, , China c Nanjing PLA Universiy of Science and Technology, Nanjing Ciy, , China Absrac Nuclear Logging Probe may measure he naurally occurring gamma ray radiaion level and deec gamma rays from a radioacive source wihin a well bore. I is used for deph correlaion, lihology, mixure densiy measuremen, idenificaion of radioacive scale. Bu because of he harsh working environmen, high emperaure above 100, high pressure, shock, i is difficul o ge accurae measuremen resul. In his paper several possible cases ha affec he measuremen resul were discussed; and hen provide appropriae soluions. The applicaion of phoomuliplier ube (PMT), NaI crysal, design of phoon couning uni, divider of PMT were also discussed; A deailed circui wih high reliabiliy is presened. Some measured resuls of he acual circui are shown Published by Elsevier Ld. Open access under CC BY-NC-ND license. Keywords: nuclear well logging; PMT; preamplifier; phoon couning; gamma ray; 1. Inroducion Gamma-rays logging insrumens have been used commercially for decades o evaluae he formaion in oil, gas and mineral well drilling. There are mainly hree ypes of gamma-rays logging echniques. 1) Naural gamma-rays logging. Differen ypes of rock emi differen amouns and differen specrum of naural gamma radiaion. The measuremen of oal inensiy of radioaciviy of 238 U, 232 Th, 40 K, is ofen called by he naural gamma-ray logging. Since he concenraion of hese naurally occurring radio elemens varies beween differen rock ypes, naural gamma-ray logging provides an imporan ool for lihologic mapping and deph correlaion and idenificaion of radioacive scale [1]. For example, shales usually emi more gamma-rays han oher sedimenary rocks, such as sandsone, clean sand, coal, dolomie, or limesone. 2) Naurally specral gamma-ray logging. Naural gamma-rays are emied in a decay of subsurface maerials such as horium ( 232 Th), uranium ( 238 U) and poassium ( 40 K), each of which emis a characerisic specrum resuling from an emission of gamma-rays a various energies, shown in figure 1. These radioacive isoopes emi hree clear gamma rays ha have characerisic energy levels, 2.62 MeV (Th), 1.76 MeV (U), 1.46 MeV ( 40 K). Accordingly we can divide he gamma-rays coun based on heir energy specrum, and hen may disinguish volume of radioacive maerials Th, U, K, evaluae he lihology. 3) Scaering gamma-rays logging. When gamma-rays irradiae scinillaor, i happen hree effecs: Compon scaering, phooelecric effec and pair producion. In general, for a given scinillaor, he phooelecric effec predominaes a * Yingchun Xiao. Tel.: address: yc_xiao@yeah.ne c 2010 Published by Elsevier Ld. doi: /j.proeng Open access under CC BY-NC-ND license.

2 224 Y. Xiao e al. / Procedia Engineering 7 (2010) low quanum energies, he Compon scaering a medium energies, and pair producion a energies above 1.02 MeV [2]. For example, if a radioacive source 137 Cs of gamma-ray MeV irradiaes he formaion of a well, we can record he inensiy of scaering gamma-rays, which is relaed o he densiy of rock. So we can analyze he densiy of rock by recording scaering gamma-rays. Figure 1. Specral Gamma-ray log (from Schlumberger) In he above menioned logging mehods, i is necessary o uilize a high reliabiliy probe recording he gamma-rays because he performance of inner elecronics or componens is sensiive o he environmen. Nuclear logging probe mainly consiss of scinillaor, phoomuliplier ube (PMT), high volage power supply, preamplifier, discriminaor, couner, emperaure sensor, Dewar flask, radioacive source, shield and borehole cable. In his paper, a nuclear logging probe was designed o record he couns of oal gamma-rays or separaely specral gamma-rays. Because of he harsh environmen, how o design a reliable nuclear logging probe is always a challenge o he designers. I includes appropriae selecion of PMT, selecion of scinillaor, divider design of PMT, preamplifier design, discriminaor design, couner design and so on. We also show some es resuls performed on he prooype. 2. Probe design The nuclear probe was consruced of crysal scinillaor, PMT deecor, high volage power supply, preamplifier, discriminaor, couner, emperaure sensor, Dewar flask, and well cable. The schemaic diagram is shown in Figure 2. Radioacive Source 137 Cs PMT Preamplifier Discriminaor High volage Power Digial Poeniomeer MCU Corex-M3 Temp sensors +5V +12V Dewar flask Well cable Figure 2. Schemaic diagram

3 Y. Xiao e al. / Procedia Engineering 7 (2010) Scinillaor and PMT When ionizing radiaion eners a scinillaor, i produces a fluorescen flash wih a shor decay ime. In he case of gamma rays, he scinillaion occurs as a resul of exciaion of he bound elecrons by means of free elecrons inside he scinillaor. The amoun of scinillaion produced by he phooelecric effec is proporional o gamma-ray energy because all he energy of he gamma-ray is given o he orbial elecrons [3]. NaI (Tl) scinillaion crysals are used in mos sandard applicaions for deecion of Gamma radiaion because of heir unequalled high ligh oupu and he excellen mach of he emission specrum o he sensiiviy of phoomuliplier ubes, resuling in a good energy resoluion. The average decay ime of NaI is 230ns, and he emission max is 415nm. Because NaI(Tl) is hygroscopic, so i is only used in hermeically sealed meal conainers o preserve is properies. In he nuclear logging probe, he combinaion of a scinillaor and PMT is one of he mos commonly used deecors for pracical applicaions. The PMT oupus an elecrical charge in proporion o he amoun of his scinillaion; as a resul, he oupu pulse heigh from he PMT is essenially proporional o he inciden radiaion energy. Accordingly, a scinillaion couner consising of a scinillaor and a PMT provides accurae radiaion energy disribuion and is dose rae by measuring he PMT oupu pulse heigh and coun rae. In nuclear well logging applicaion, PMT should be capable of high emperaure and ani-shock. Here, we choose R1288 phoomuliplier produced by Hamamasu Inc Divider The performance of divider influences he performance of PMT. An ideal volage divider nework is one which mainains he dynodes a fixed poenials independen of he level of oupu anode curren. In fac, i is possible o approach, bu never compleely saisfy. A high coun raes, he volage dynodes may drop and he average bleeder curren should always be defined a leas 10 imes larger han he average anode curren of PMT. A sandard resisor value beween dynodes may be 330 kohm or 470 kohm or 510 kohm, exac value decided by specified applicaion and PMT ype. This is a compromise beween bleeder curren and gain sabiliy which is sufficien for couns raes up o approximaely 50,000 couns per second. Volage dividers may be linear (mos common), apered or sabilized wih Zener diodes or ransisors. The gain of a scinillaion deecor varies wih each PMT and is also srongly influenced by he exac design of he volage divider. The gain of PMT can be changed by adjusing is high volage by means of a precision poeniomeer. The schedule of divider is following: K D1 D8 D9 D10 Anode R C1 0.01uF K D1 D8 D9 D10 Anode C1 0.01uF R14 Q1 Q2 Q3 Q11 Q12 R R15 R14 D1 D2 D3 D10 D11 D12 R1 R2 R3 R4 R11 R12 R13 R1 R2 R3 R4 R11 R12 R13 C2 C3 C4 Noe: R1 =R2 =... = R13 = 510 kohm; R = 50 ohm; PMT: R1288, Hamamasu corp. HV HV Noe: R1 =R2 =... = R13 = 10 Mohm; R = 50 ohm; R14 = 1.1 Mohm; Q1 = Q2 =... = ZVP1320F, Zeex corp. D1 = D2 =... = BZV55C15 PMT: R1288, Hamamasu corp. Figure 3. The schedule of divider 2.3. Preamplifier When he gamma-ray is ranslaed by scinillaor, and hen is muliplied by PMT, here is phoocurren pulse occurring on he anode of PMT in he order of several milliamps. Provided hus curren pulse is imposed on a 50 load, he signal level is of he order of several ens of millivols o several hundreds of millivols. This signal mus

4 226 Y. Xiao e al. / Procedia Engineering 7 (2010) be amplified so as o be recognized by he following circui. The design of preamplifier should mee several requiremens such as: High inpu impedance Low inpu curren noise. Since he PMT may be considered as a curren source, i has raher high inheren impedance. Considering he noise effec, he pre-amplifier commonly make use of he JFET inpu amplifier, for example ADA4817 operaional amplifier (Analog Inc.) wih a 500 Gohm inpu impedance. In gamma-rays deecion, AD8067 of Analog Inc. is uilized as preamplifier. The reason is ha AD8067 is of super low curren noise 0.6 fa / Hz. Gain larger han 15 db. The gain is relaed o he characerisic of anode curren pulse generaed by gamma-rays. A ypical bialkali phoocahode for example R1288, produced by Hamamasu inc., generaes abou 8 phooelecrons / kev energy deposied when a good qualiy NaI (Tl) irradiaed by 137 Cs radioacive source. Therefore, he oal charge a he PMT anode corresponding o a 137 Cs gmma phoon is calculaed by he following formulas: I Q N E G Ce p (1) Where, N = Numbers of phooelecrons / kev : 8; E = Energy of Gamma phoon: 662 kev for 137 Cs; G = phoomuliplier Gain: 3.8 x 10 5 ypical; Ce = Charge per elecron: 1.6 x coulomb; : naural decay ime consan of NaI (Tl) is 230 nanoseconds. Therefore I Q N E G Ce p = 1.4 ma If he anode is conneced hrough he coupling capacior o a load in 50 ohms, hen he volage is given by: = V I R 1.4mA 50 70mV Here, a operaional amplifier wih 20 db gain in enough; If he oupu signal of PMT anode is conneced o he inpu of a charge sensiive preamplifier hrough a coupling capacior, 1 kohms resisor may be placed beween he negaive inpu erminal and he oupu erminal of operaional amplifier. C3 1k (From Anode) Ia C1 0.01uF R1 51 R2 15k 0 0 R3 +5V 70k AD R5 V- OUT U1 V+ R4 0 30k C2 0.47uF 0 +5V 6 (To comparaor) 0 Figure 4. Preamplifier for PMT

5 Y. Xiao e al. / Procedia Engineering 7 (2010) In he nuclear well logging probe, AD8067 operaional amplifier (Analog Inc.) is configured as rans-impedance amplifier, a resisor (R5) and a capacior (C3) are placed beween he negaive inpu erminal and he oupu erminal. The capacior is necessary for operaional amplifier o sably work. Resisors R3 and R4 se a 1.5 V oupu bias poin for he oupu signal o swing abou. I is criical o have adequae bypassing o provide a good ac ground for he reference volage. The bandwidh of he reference nework is seleced o be approximaely 10 Hz o reduce he noise. Resisor R1 equal 51 ohms o mach ransmission line. Resisor R2 and capacior C1 consruc a high pass filer -3 db Band widh larger han 20 MHz 2.4. Discriminaor In order o recognize he signal from he oupu of AD8067, a comparaor ADCMP602 is uilized. The signal is conneced a he negaive inpu erminal (pin No. 3) of ADCMP602. Volage reference is adjusable by digial poeniomeer AD5243, conrolled by I 2 C bus. Differen references mean differen inciden specral gamma-rays, associaed pulse heigh Couner Couner couns he numbers of anode pulse. The reason of using CPLD bu no FPGA is ha he delay ime of CPLD can be pre-esimaed, and ha makes CPLD is more reliable when used in couning and ime conrolling Dewar Flask Usually, he operaing emperaure of he insrumen used in nuclear logging is below 100, even he insrumen using high emperaure elecric pars mus work below 150. Bu he emperaure of oil well is dependen of he deph of he oil well, if he deph increases 100 meers, he elevaed emperaure is 2 3. The emperaure of downhole usually reaches , which is a beyond he operaing emperaures range, even PMT R1288 may up o 175. As emperaure rise, he noise of PMT rise a he same ime, so is he elecronics. Tha is why hea insulaion is imporan. Usually, he deecor PMT and elecronics were placed in a Dewar flask. The flask is a double-walled evacuaed housing proecing he inernal componens for approximaely 4 hrs wihin a 300 oil well wih only 4 rise. The logging probe is made up of a maerial which is of low aomic number, preferably ianium [4]. 3. Measuremens The oufield es of logging probe which is used in radiaion environmen is hard o carry ou, so he couner sysem conaining PMT and elecric sub-sysem is esed in he lab environmen, and he oufield performance of his sysem will be esed in he fuure. Figure 5 The recording couns of anode curren pulse

6 228 Y. Xiao e al. / Procedia Engineering 7 (2010) In order o observe he sabiliy of his phoon couner sysem a room emperaure, a LED conrolled by curren source was uilized as a ligh source insead of he combinaion of he 137 Cs radiaion source and he NaI scinillaor. The performance of his phoon couner sysem was analyzed hrough he recording daa. The sabiliy of couner sysem which was calculaed by he daa is in he range of 5%. 4. Conclusion This nuclear well logging probe is of he low power consumpion (acive divider reduces he power consumpion), high couning rae lineariy and sable. This low power sysem will benefi reducing he elevaed emperaure of elecric sub-sysem. Low power dissipaion will make Dewar flask work as longer as possible. The sabiliy of couning remains in he range of 5% variaion can mee he requiremen of gamma-ray logging. Reference [1] Tom Baird, Rober Drummond, Bjorn Langseh, Lisa Silipigno, High-Pressure, High-Temperaure Well Logging, Perforaing and Tesing, [2] Kur Bram, Sephane Hiron, The KTB Borehole-Germany s Superdeep Telescope ino he earh s Crus. [3] Hamamasu, Phoomuliplier Tubes Basics and Applicaions, 3 rd ediion, p136 [4] Osburn, Terry D, Well logging radioacive deecor assembly, Unied Saes Paen , 10/06/1992.