by Toshinobu Itoh**

A Temperature Compensated Scintillation Gamma

Ray Detector for Well Logging*

by Toshinobu Itoh**

Summary: In the logging methods which make the energy spectral analysis of gamma ray radiated from a formation, the accuracy of logging device is strongly affected by a gain stability of down hole scintillation gamma ray detector. The gain of the detector changes in a wide range in accordance with the formation temperature as the nature of the scintillation gamma ray detector. Therefore, a gain drift compensation system for such gain drift of the detector should be equipped in the logging device. The method of gain drift compensation described in this paper is achieved by the combi-

nation of a reference isotope which is attached to a high-temperature scintillation gamma ray detector

(120℃) and a gain drift indicator equipped at the surface control panel.

1 Introduction

Many kinds of logging methods which make

energy spectral analysis of gamma ray radiated

from the radio isotope in a formation or radiated

at the time of nuclear reaction have been proposed

recently. These are classified as spectral natural

gamma ray loggings1), induced nuclear reaction

gamma ray loggings2)～4), and neutron activation

loggings5)～7). The accuracy of the logging dcvice

of these methods is strongly affected by a gain

stability of the scientillation gamma ray detector.

Therefore, the gain stabilizing system of the

detector is absolutely necessary for precise pulse

height analysis.

The down hole probe of logging device is nor-

mally operated under the following severe con-

ditions.

(1) The bottom hole temperature of a drilledhole of 4,500 meters deep reaches 150℃.

(2) The maximum hydraulic pressure sur-rounding the probe reaches 1,000kg/cm2.

(3) The probe is normally run down through a hole size of 81/2 inch at the bore hole and 5 inch at the cased hole.

(4) The maximum voltage supplied to the down hole probe through a logging cable is 300 volts because of the voltage limitation of cable insulators. The condition (1) indicates that a gain of down

hole scintillation gamma ray detector will change

in a wide range in accordance with the formation

temperature and the probe should be covered

with a thermal insulator when it is operated

under the temperature as high as 150℃. The

condition (3) indicates that a diameter of the

probe should be less than 41/2 inch in the largest case and a large scaled thermal insulator covering the detector can not be used in the down hole device. The condition (4) indicates that a high voltage (2,000 volts) for the photomultiplier should be generated at the down hole device.

The temperature increasing rate of the probe

depends not only on the ability of the thermal

insulator and the temperature increasing gradient

of the underground but also on the probe going

down and coming up speed. Because the large

scaled thermal insulator covering the detector

can not be used in the logging device as men-

tioned before, the probe may frequently be

operated under the high temperature of more

than 100℃ when it is run down over 3,000 meters.

Under such high temperature circumstances,

the gain of detector sharply falls down from the

normal operating value as the nature of the scintil-

lation detector. Therefore, the surface control

panel should be equipped with a gain drift com-

pensation system for such gain drift of the detector.

2 Description of Equipment

Fig. 1 is a block diagram of the temperature

compensated scintillation gamma ray logging

device. The down hole probe consists of a refer-

ence isotope -2inφ ×2in l CsI(Na) scintillation

crystal assembly, a high-temperature photo-multiplier (EMI 9607B), a pre-amplifier and a

* Received November 25, 1970. ** Central Laboratory, Japan Petroleum Exploration

Co., Ltd. (Kichijoji-Kitamachi 3-7-20, Musashino, Tokyo, Japan)

Volume 13, No. 1, May 1971

98 Itoh: A Temperature Compensated Scintillation

Fig. 1 Block Diagram of the Temperature Compensated Scintillation Gamma Ray Logging Device

high voltage source for photomultiplier. The gamma ray signal of the detector is sent

into the surface of the ground as a negative pulse having the pulse width of about 30 microseconds through an emitter follower pre-amplifier whose out put impeadance is matched with a cable impeadance (Zo=50 ohms). Fig. 2 shows

gamma ray pulse which is passed through the logging cable of 4,000 meters. Since an electro-magnetic shielding of the logging cable (USS 6Hl, armoured cable, carbon coated shiedling) is extremely poor in comparison with a normal coaxial shielding cable, a direct current having voltage of 150 volts is supplied in a power line to minimize the electro-magnetic induction be-tween the signal line and the power line. There-

fore, a dc high voltage for the photomultiplier

is regenerated at the down hole high voltage

source which is composed of a SCR dc-ac con-

vertor, a step-up transformer and high voltage

rectifier unit. The SCR is driven by an astable-

multivibrator with a frequency of 1,000Hz,

then, the direct current supplied from the surface

of the ground is formed into the square wave

having the same frequency of multivibrator.

This square wave is stepped up to 2,000 volts

and then this ac high voltage is rectified into the

direct current again.

Fig. 2 Photograph of Gamma Ray Pulses which Passed Through the Logging Cable of 4,000

Meters

The surface control panel is composed of a

dc power supply for the down hole probe, a linear

amplifier which allows manual gain control and

a probe gain drift monitor. The gamma ray

signal is amplified at the linear amplifier at first

and then they are routed towards the multi-

channel pulse height analyzer and the probe

gain drift monitor simultaneously. The gain monitor of the detector used in this logging device

is composed of a single-channel pulse height

discriminator and count rate meter. The present

level of the discriminator is set in such a manner

that the count rate meter counts only the gamma

ray having energy between the photopeak and

the upper edge of photopeak of the reference

gamma ray at the calibrated stage. Therefore, the deviation of the detector gain from the cali-

brated value can be checked by the monitor

readings because the decrease of count rate means

the decrease of detector gain and the increase of

count rate means the increase of detector gain

on the contrary. The gain drift monitoring

and compensation system will be described in

the section of "Gain Drift Compensation" in

detail.

Fig. 3 shows the scintillation gamma ray log-

ging device which consists of the surface control

panel and the down hole probe having a diame-ter of 6cm and the length of 142cm.

Fig. 3 Photograph of the Temperature Compensated Gamma Ray Logging Device

Bulletin of The Japan Petroleum Institute

Gamma Ray Detector for Well Logging 99

3 Temperature Characteristics of Down

Hole Scintillation Detector

3.1 dc High Voltage Source

A gain of photomultiplier is strongly affected by the voltage regulation of high voltage source. An experimental data using the photomultiplier EMI 9607B shows that the decrease of 10% at the anode supplying voltage causes the decrease of 50% on the out put pulse height. Therefore, it is absolutely necessary to use a highly regulated high voltage source to stabilize the detector gain. But the high voltage source set in the down hole

probe is normally operated under the varioustemperature circumstances when the gamma ray measurement is made in the hole. Therefore,

the out put voltage of the source will vary in

accordance with the surrounding temperature,

because many of elements constituting source

change their operating characteristics with the

surrounding temperature. The factors con-

troling the temperature characteristics of the high voltage source are as follows.

-reverse current of the high voltage rectifier.

-forward resistance of the SCR .

-oscillation frequency of the astable-multi-

vibrator.

-power loss on the step-up transformer.

Fig. 4(A) is the temperature characteristics of the reverse current of various kinds of high vol-tage rectifier. Those curves show the reverse current of rectifiers increases rapidly when the surrounding temperature passes the threshold value shown in the operating data of individual rectifier. Therefore, the dc high voltage de-creases according to the increase of such reverse current because electric charges stored in the capacitor of the filter discharge to the earth through the rectifier and this phenomena are shown in Fig. 4(B). The forward resistance of SCR decreases with

the increase of temperature and then this causes

the SCR forward current increasing when the

supplying voltage of SCR is kept at constant.

The effect of frequency change of the multivib-

rator on the out put voltage of high voltage source

is negligible small when the time constant of

filter is much higher than the period of square

wave, and the effect of leakage current through

the filter and the power loss of the transformer

is also negligible small when they are used within

the limited temperature.

Curve G in Fig. 5 shows the temperature

Fig. 4 Temperature Characteristics of high Voltage

Rectifiers and SCR

characteristics of high voltage source which is

composed of a Toshiba high voltage rectifier OR-

06YXZ31 and NEC SCR 2SF18. The out put

voltage of the source increases gradually in ac-

cordance with the increase of surrounding tem-

perature because of the increase of SCR forwardcurrent and reaches the maximum value at 108℃,

and then decreases gradually because of the

increase of reverse current on the high voltage

rectifier. The maximum voltage drift of the

source is 2% seen at 108℃ and this causes the

gain drift of 13% on the photomultiplier EMI 9607B.3.2 Photomultiplier

A high-temperature photomultiplier EMI

9607B (maximum operating temperature is 150℃)

is used in the down hole probe. Fig. 6 is a heat-


100 Itoh: A Temperature Compensated Scintillation

Fig. 5 Temperature Characteristics of CsI (Na)- IJetector Assembly, NaI(Tl)-Detector Assem-

bly, Photomultiplier EMI 9607B, NaI (Tl) Crystal, CsI(Na) Crystal, Total Gain of Detec-

tor and dc High Voltage Source

Fig. 6 Heating Assembly of Temperature Experi- ments of the Photomultiplier

ing assembly of temperature experiments of the

photomultiplier. The photomultiplier is mounted at the center of a cylindrical heating chamber

and a standard light source is set in front of the

tube window. A cylindrical thermal insulator

and a transparent glass, which are inserted be-

tween the photomultiplier and the light source,

interrupts the thermal conducting between the

tube window and the atmosphere. Those as-

semblies are covered by an optical shielded case.

The tube temperature is measured by the

thermistor element attached to the window sur-

face. Therefore, the thermometer reading does

not always indicate the correct temperature of

inside tube elements such as cathode, anode and

dynodes. The inside elements may reach a

thermal equilibrium with the tube envelope by around of 5 minutes experienced at the tem-

perature test. Then, the photomultiplier is heated up by zigzag steps of alternation of the temperature increasing period and the flat period where the thermometer reading indicates the

constant value. The duration of the flat period is taken as long enough to cover the time lag during which the tube reaches to thermal equili-brium. Curve C on Fig. 5 is the temperature characteristics of EMI 9607B photomultiplier

(regulation of the temperature during the flatperiod is kept within ｱ2℃). This curve

has the same tendency with the results given by

R. E. Rohde9) except the temperature range.

The surface temperature of the light source

varies in the range from 15℃ to 25℃ during

the experiment. A fluorescent lamp (NEC 6977) is used as the light source because of its high stability of the light output within such a temperature range. The regulation of a heater current and an anode voltage of the lamp is keptwithin ±0.5% to reduce the drift of lightロト

lntenSlty.

3.3 Scintillation Crystal

The temperature characteristics of the scintil-lation crystal are reported by Von G. Brunnerio) and J. Menefee et al11).

Because the temperature response of detector assembly (scintillation crystal and photomulti-

plier unit) is more significant in our experiment than that of scintillation crystal itself, the detector assembly is conducted into the temperature ex-

periment. Then, the efficiency of scintillationcrystal at the given temperature is obtained as the ratio of the value of detector assembly to the

value of photomultiplier. Fig. 5 shows the re-lation between the surrounding temperature and the relative efficiency of the detector as-

sembly, (curve A and B) of the photomultiplier

(curve C) and of the scintillation crystal (curve D and E). The photo-emission efficiency of CsI(Na)

crystal increases gradually with the temperature increasing and reaches the peak value at around60℃ and, then, decreases rapidly with increase

of surrounding temperature. The position of the maximum photo-emission efficiency of CsI-

(Na) crystal shown in Fig. 5 is a little different from the results given by J. Menefee which show that the maximum efficiency of CsI(Na) crystalis obtained at around 80℃. This discrepancy

on the value of temperature range is probably


Gamma Ray Detector for Well Logging 101

Fig. 7 Photograph of Nal(Tl) Scintillation Crystals

which were Cracked at the Surface Caused

by Quick Cooling from 120℃ to Room

Temperature

caused by the difference of experimental systems,

because the same results are obtained from tem-

perature experiments of the different type ofCsl(Na) crystal (One is TEN's linφ×lin crystal

and another is Harshow's 2inφ ×2in crystal).

Although the photo-emission efficiency of NaI(Tl) crystal is much higher than that of CsI-

(Na) crystal, (around twice of CsI(Na) crystal) the temperature characteristics of its photo-emission efficiency are worse than those of CsI (Na) crystal. In addition to this disadvantage, the crystal frequently cracks when it is cooled down

quickly from 120℃ to the room temperature as

shown in Fig. 7.

4 Gain Drift Compensation System

Curve F on Fig. 5 which is the total efficiency of CsI(Na) scintillation detector shows that the detector gain drifts in the relatively wide range according to the increase of surrounding tem-

perature. Therefore, the gain drift compen-sation system should be equipped in the logging device for precise pulse height analysis. At the laboratory measurement, the moni-

toring of detector gain is normally achieved by the combination of reference isotope attached to the crystal and multi-channel pulse height ana-lyzer12). Therefore, the deviation of detector gain from the standard value is checked by the posi-tion of photopeak of the reference gamma ray. On the well logging, the intensity of objective

gamma ray such as uranium gamma ray, carbon or oxygen inelastic scattering gamma ray is mea-sured continually and recorded on the oscil-

lograph film according to the probe going down

or coming up in the hole. In other words, the

amount of those gamma rays is not measured by

the usual pulse height analyzer system but mea-

sured by the multiple count rate meter coupled

with the individual pulse height discriminator.

On such multiple count rate meter system, the

gain of logging device should be kept at constant

during the measurement.

The method of continuous gain monitoring

used in this logging device is as follows. The

monitor has a pulse height discriminator and a

count rate meter as an independent channel and

the lower level of discriminator is set at the photo-

peak position of the reference isotope and the upper level is set at the upper edge of photopeak.

Therefore, the count rate meter readings will

be proportional to the intensity of gamma ray

having the energy of upper half section of photo-

peak. If the intensity of reference isotope is chosen as such manner as that the count rate of

reference isotope is much higher than that of back

ground gamma ray, the increase of rate meter reading indicates the increase of detector gain

and the decrease of rate meter reading means

the decrease of detector gain on the contrary.

Since the calibration curve between the detector

gain and the monitor readings is prepared in this logging device, the deviation of detector gain from the calibrated value can be canceled out by adjusting the gain of linear amplifier in ac-cordance with the monitor reading, and then the total gain of the logging device can be held at constant during the measurement. The ex-

perimental data show that the gain drift of theequipment is held within the deviation of ｱ4%

by using this compensation system.

The characteristics of the reference isotope are desired as follows. The energy distributions of the reference gamma ray is a monochromatic

and their energy level is a little higher or lower than those of objective gamma ray. Alpha ray of Am-241 (5.44 MeV and 5.84 MeV) which

Fig. 8 Energy Spectral Distributions of the Gamma Ray Radiated from Typical Sand Stone


102 Itoh: A Temperature Componensated Scintillation Gamma Ray Detector for Well Logging

correspond to the gamma ray energy of 3.2 MeV for the CsI (Na) scintillation crystal is recom-mendable as a reference isotope of gamma ray logging device. But the surface condition of the CsI(Na) crystal is not so stable as that of CsI(Tl) crystal. It is experienced that the photo-

peak of 3.2 MeV at the polished stage drops to 2.05 MeV 50 hours later, even if the crystal isleft in a dry box of -30℃ to -50℃. The satis-

factory packing techniques which hold the crystal surface to be in fresh condition for the alpha ray

permanently have not been developed at present in Japan. Therefore, Co-60 gamma ray source is used as the reference isotope instead of Am-241. In this case, the upper level of the monitor dis-criminator is set at 1.38 MeV to cut the objective

gamma ray as shown in Fig. 8.

5 Conclusion

From temperature experiments of CsI(Na) scintillation gamma ray detector, it is obtained that the gain of scintillation detector drifts in the range between +10% to -40% from the reference value when the surrounding tem-

perature changes in the range from 20℃ to 120℃,

and the gain drift compensation system canceles

out such gain drift of the detector and holds the

total gain of the logging device at constant within

the deviation of ｱ4% during the measurement.

Although the maximum operating temperature

of the down hole probe is 120℃ in this equip-

ment, the probe can be operated up to 150℃

when the high-temperature electronic parts are used in the probe.

The author wishes to express his appreciation to Japan Petroleum Exploration Co., Ltd. for

giving him an opportunity to perform the work. Appreciation is also extended to the member of Department of Atomic Energy Engineering of Tokyo University, Professor Dr. O. Nishino, Professor Dr. A. Sekiguchi and the staffs, and also to Mr. H. Iwamoto, Head of Central Labo-ratory of JAPEX for their helpful instruction and suggestions.

References

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(1968). 5) Baker, P. E., Petrol. Trans. AIME, 210, 97 (1957). 6) Caldwell, R. L., et al., Geophysics, XXVIII, (4), 617

(1963). 7) Givens, W. W. et al., The Log Analyst, SPWLA,

9, 23 (1968). 8) Causse, J. P., IRE Trans. Nucl. Scie. Scintillator

Symposium, 66 (1959).9) Rohde, R. E., Nucl. Instr. and Meth., 34, 109 (1965).

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by Toshinobu Itoh**

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