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A prototype thermoluminescent dosimetry system for personnel monitoring
International
Journal
of Applied
Radiation
A Prototype System / R.
C.
and Isotopes,
1966, Vol.
17, pp. 399-41
I.
Pergamon
Press Ltd.
Printed
in Northern
Ireland
Thermoluminescent Dosimetry for Personnel Monitoring* U
PALMER,
D. RUTLAND,
Edgerton,
Germeshausen Santa
R. LAGERQUIST
& Crier, Barbara,
Inc., Santa California
and E. F. BLASE
Barbara
Division,
(Received 20 September 1965) .A prototype thermoluminescent system developed under contract to the U.S. Navy is described, wherein the thermoluminescent properties of CaF,:Mn are used to monitor the y-radiation dose range of 2 mrad-104 rad. Design specifications for the system are presented, together with performance data as measured by Edgerton, Germeshausen & Grier, Inc. The system has shown characteristics of accuracy, reproducibility and re-usability far superior to those of conventional personnel-dosimetry systems. UN
SYSTEME
THERMOLUMINESCENT
DOSIMETRIE
POUR
LA
PROTOTYPIQUE
SURVEILLANCE
DE
RADIOACTIVE
DU
PERSONNEL On dtcrit un systtme thermoluminescent prototypique mis a point sous contrat B la Marine des E-U, dans lequel les qualites thermoluminescentes du CaF,:Mn servent a mesurer la dose de rayonnement gamma dans la rangte de 2 mrad-104 rad. On presente les specifications de dessin du systeme, ainsi que les don&es du fonctionnement telles que mesurees par Edgerton, Germeshausen & Grier, Inc. Le systeme a fait preuve des caracttristiques de precision, de reproduisibilite et de rtutilizabilite fortement superieures a celles des systemes conventionnels de dosimetrie de personnel. IIPOTOTIIH
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PERSONAL Eine Erstausfuhrung eines im Auftrag der amerikanischen Marine entwickelten ThermoDie Thermoluminiszenz-Eigenschaften von CaF, : Mn luminiszenz-Systems wird beschrieben. werden dazu benutzt den Dosisbereich der Gammastrahlung von 2 mrad-104 rad zu uberwachen. Die konstruktiven Angaben fur das System werden aufgefiihrt, zusammen mit den erzielten Ergebnissen nach Messungen der Firma Edgerton, Germeshausen & Crier, Inc. Das System hat vie1 bessere Kenndaten hinsichtlich seiner Genauigkeit, Wiederholbarkeit und Wiederverwendbarkeit als die bekannten Systeme fur Personaltiberwachung. * This work was supported NObsr-85506.
by the Bureau
of Ships,
U.S. 399
Navy,
Department
of Defense,
Contract
No.
400
R. C. Palme)-, D. Rutland, R. Lagerquist and E. F. Blase 1. INTRODUCTION
EDGERTON, Germeshausen & Grier, Inc., recently completed the design and construction of the U.S. Navy’s prototype thermoluminescent dosimetry system (TLD), which is comprised of a Computer-Indicator CP-748(Xx-l)/PD and a Dosimeter DT-284(xX-l) PD.(l) Conceptually this system is the result of the earlier work of SCHULMAN et uL(~-‘) of the Dosimetry Branch of the U.S. Naval Research Laboratory, and stems from the desire of the U.S. Savy to use a single device to monitor the y-radiation dose ranges from near background levels to the high levels encountered in a nuclear accident. The system’s basic requirements, as envisioned by the U.S. Navy and followed in the design of the TLD system, are given in Table 1. These requirements, which are well filled by thermoluminescent dosimetry, more particularly by CaF,:Mn thermoluminescent dosimetry, preclude use of the silver-activated phosphate glass and the conventional film badge, as well as chemical dosimeters and ion chambers. vI'~~~~
1.
Requirements for personnel dosimctry system
I. Gamma dose range: 2. Accuracy : 3. Re-use: 4. Energy dependence: 5. 6. 7.
8.
9. 10.
2 mradp104 rad
&150/, 100 readings ,20% for X- or yradiation from 80 to 1300 keV Directionality: i 15 “/0regardless of direction Size of active element: 0.5 in3 less than 3 oz. (Dosilt’eight : meter) less than 40 lb (Computer-Indicator) Operating temperature: -~45”C to -55°C (Dosimeter) -10°C to -55°C (Computer-Indicator) one readout every 20 set Time: automatic readout of Automation : dose and identification code of dosimeter
An excellent introduction to the mechanisms of thermoluminescence is provided by Preparation and Characteristics of Solid Luminescent AIaterials,(B) and Thermoluminescence, Bibliography”’ is a comprehensive reference source of literature on the
subject. The preparation and properties of the CaF,:Mn phosphor have been described in a previous paper by PALMER.'~") For the present paper, it is essential only to indicate that when a thermoluminescent phosphor is exposed to ionizing radiation, electrons are promoted from the filled energy levels of the valence band into the empty levels of the conduction band of the phosphor’s crystalline lattice. Some of the electrons raised to the conduction band are trapped in metastablc states present as imperfections in the crystal lattice. 1Yhen the temperature of the CaP,:Mn crystals is raised to 28O”C, the electrons are released from the traps to combine with luminescent centers also present in the crystal, thereby causing the emission of light in the blue-green region of the electromagnetic spectrum (peak at 4950 A). By appropriate calibration, the quantity of light emitted can be related to the energy deposited in the phosphor by the ionizing radiation. 2.
DOSIMETER
DT-284(XN-l)/PD
Figure 1 is a sketch of the thermoluminescent dosimeter. The unit consists of a detector and a case, each of which is composed of subparts. 2.1 Detector The detector is composed of the active CaF,:Mn element to record the exposure information, and the coded handle to automatically identify the detector and to seal the detector inside the case. 2.1.1 rlctiue element of CaF,:Mn. The active element is basically a vacuum tube in which the filament is coated with the thermoluminescent CaF,:Mn. During production, the element is made by dipping the heater coil into a mixture of phosphor, potassium silicate and water, drying out the excess water, and then cncapsulating the coil in a vacuum tube at a pressure of 1OW mm torr. Two Kovar feed-throughs, terminated in the heater contacts, provide a current path for resistance-heating of the phosphor. In practice, a constant current of 6.5 A at 3 V is supplied to the unit to raise the phosphor’s temperature to 300°C in 12 sec. 2.1.2 Coded handle. The coded handle is attached to the detector by means of a spacer. It contains the detector’s identification number
A prototrpe
PLASTIC
thermoluminescent
CASE
DIGIT
( PHOSPHOR GLASS
: POTASSIUM
BUTTONS
SILICATE
)
ENVELOPE
FIG. 1. Thermoluminescent in the form of eight digit buttons. The digit buttons present a l-2-4-2 code as notches and lands on each digit button, which are automatically sensed by the computer-indicator. A tamper-proof catch seals the unit inside the case to physically protect the detector. 2.2 Case The case serves two purposes: (1) it provides a light-tight detector housing to protect the phosphor from light, and (2) it contains an energy compensation shield to correct for overresponse of the CaF,:Mn phosphor in the region below 500 keV. The shield shown in Fig. 2 was designed and constructed to ensure essentially unity response relative to a Co60 exposure. The energy-response curve shown in Fig. 3 is obtained for the dosimeter with the shield removed from the plastic case. Figures 4 and 5 show the energy response in various orientations in different radiation fields (Co60, CS~~~, 250-kV filtered X-ray machine) with the shield in place. All of the irradiations were monitored with a Victoreen 1 cm3 air-equivalent ion chamber, which had been calibrated at the U.S. Bureau of Standards. As can be seen from Fig. 5, the dosimeter’s response is uniform within i-10 percent when it is irradiated, either normal to the longitudinal axis or normal to the end cap in the energy range of 80 to 1250 keV. As shown in Fig. 5, the only serious 3
401
dosimetry system for personnel monitoring
dosimeter.
deviation in sensitivity is encountered at 45” from the coded handle and at low energies. The handle, of course, precludes some directional dependence, which will become progressively worse as even lower energies are encountered. 3.
COMPUTER-INDICATOR CP-748(XN-l)/PD
The computer-indicator consists of a mechanical section and an electronic section, as shown in Fig. 6. The mechanical section contains the mechanisms for holding the detector portion of the dosimeter, drawing it into reading position, electrically reading the detector’s identification (ID) number, chopping the light emitted by the and introducing optical filters to detector, attenuate the light output. The mechanical section also includes the photomultiplier (PM) tube, which senses the light, and its voltagedivider network and high-voltage supply. The electronic section, completely transistorized except for one current-regulating tube in the heater supply, receives the necessary signals from the mechanical section to operate the unit. Some of these signals contain information on the detector identification number and filter-disc position, and other signals are used for sequencing the operations. The filter switches are cam-operated from the filter-disc shaft and indicate which filter is inserted in front of the
402
R. C. Palmer, D. R&and,
\
Y
energy
00065”
TIN
-‘-0018”
LEAD
PLASTIC
FIG. 2. Cross-sectional
FIG. 3. Dosimeter
\
//
R. Lagerquist and E. F. Blase
view
response
of plastic
normal
case,
showing
to longitudinal
energy-compensation
axis (shield
shown in Fig.
shield
2 removedj.
A prototype thermoluminescent
FIG. 4. Dosimeter
energy normal
dosimetry system for personnel
response to end
normal to longitudinal cap (shielded).
403
monitoring
axis
and
404
R. C. Palmer, D. Rutland R. Lagerquist and E. F. Blase
FIG. 5. Dosimeter
directional
energy
response
(shielded).
l-
I
I
----
START
r,
HEATER TlMER
,
STOP
L
---------____ MECHANICAL UNIT
-I
CONTROL
DOSEIPI;
REFLECTOR
SI
DC
I
POWER 4
GO-CYCLE TENS/UNITS SIGNAL
CIRCUITS
1
function diagram.
q&
!t&;\
POWER SWITCH
NOT
NIXIE --
ELECTRONIC UNIT
TRANSFORMER
TO ALL
FIG. 6. Overall
;
I-2-4-2 CODE TENS+ “NIT5
c____-,
1
I
1
IC bISPLAY
J-
117”
GOOW
PART OF THE COMPUTER-INDICATOR
LAMP
1.
RECORD
PRINTED
RANGES
10 OPTICAL A TEN ,T,LTER
b
406
R. C. Palmer, D. Rutland, R. Lagerquist and E. F. Blase
detector. The drive commutator makes a halfturn as the detector is going in, and another half-turn as the detector is coming out. The signals from the drive commutator control the printing functions and the heating of the detector’s active element. The signals from the identification switches are translated by the electronic section into decimal digits, which are entered into the printer. A person reading a dosimeter removes the detector from its case and inserts it into the computer-indicator through the hatch shown in Fig. 7, and the following cycles are then performed by the computer-indicator (refer to Fig. 6).
3.1 Cycle 1 The detector is driven into its reading position as the identification number is sensed and printed. This action is initiated by the READ switch on the front panel (Fig. 7). When the READ switch is depressed, it applies voltage to the solenoid clutch in the detector-drive mechanism (not shown). This causes the clutch to make one complete revolution and draw the detector past the switches used for identificationnumber reading. The drive commutator controls the transistor digit gates during this cycle so that the l-2-4-2 code signals are directly connected to the transistor decoder. The decoder interprets the l-2-4-2 code and supplies a voltage on one of ten wires representing the digits 0 through 9. Each of the ten wires drives one of ten solenoid drivers, which in turn, supply current to operate the solenoids on the serial keyboard-printer. As the handle moves past the identificationnumber reading switches, each digit button is read in turn, and the button operates the printer via the digit gates, decoder, printer solenoid drivers, and keyboard solenoids. This places the identification number in the printer keyboard. After the last digit has been placed in the keyboard, the drive commutator operates the print-back solenoid in the printer, and causes the identification number to be printed. When Cycle 1 is completed, the active element of the detector is centered in the reflector.
3.2 Cycle 2 During this cycle, current is supplied to the active element, heating it and causing it to emit light. When the drive commutator arrives in the IX position, it energizes the detector heater supply and starts the detector heater timer. The constant 6.5 A heater current passes through the detector heater contacts and the heater coil. As the active element is heated, the temperature of the CaF,:Mn phosphor is increased to 300°C in 12 set; and the phosphor emits light, which the reflector directs through the infrared (ir.) filter, the filter-disc holder, and the chopper disc, onto the PM tube. At the end of the 12 XC heating period, the heater timer energizes the detector-out switch, and the drive commutator turns off the detector heater supply.
3.3 Cycle 3 The peak light output is measured and its value is stored digitally. The light from the detector is chopped by the chopper disc, which produces equal periods of illumination and darkness on the PM tube at a 90 c/s rate. The output of the PM tube, a 90 c/s square wave, is amplified by the a.c. amplifier, and the amplified signal is changed to a filtered d.c. level by a phase-lock rectifier, which is driven by a synchronizing signal. The synchronizing signal is generated in synchronism with the PM tube signal by means of a magnetic pickup attached to a steel wheel on the chopper disc shaft. The rectifier’s d.c. output is converted to a decimal number by a digital voltmeter consisting of a two-decade counter, a diode current switch, a d.c. amplifier, and a pulse generator. When the detector’s highest light emission is reached, the counter remains at its count and stores the dose information in digital form. The value appearing in the counter is displayed on two nixie lamps showing 10’s and units. The counter operates in a l-2-4-2 code for each decade. These codes are translated to decimal form by the decoder, which handles only one decimal digit at a time and which the digit gates switch at a 60-cycle rate between the 10’s and the units of the decimal counter. The nixie
FIG. 7. Front
paw1
of computer-indicator.
A proto@c
thermoluminescent
dosimetry system
lamp voltages are switched in synchronism to produce the effect of a two-digit display. At its peak, the light from the detector may exceed that corresponding to the maximum two-digit display of 99 mrad. When this happens, the a.c. amplifier gain is reduced by a factor of exactly 10, and the counter is reset to zero by means of the control logic. The counter reading must now be multiplied by 10 to indicate the true dose. This is accomplished by turning on a zero in the nixie lamp immediately to the right of the unit readout-nixie lamp. When the detector dose exceeds 990 mrad, the light to the PM tube is attenuated by a factor of exactly 10, by rotating a neutral-density filter into the light path. The control logic energizes the solenoid in the filter-advance mechanism, and the filter-disc is advanced one position. If the detector light increases to higher and higher values, the filter-advance mechanism is successively activated each time the reading in the counter exceeds 99, thereby introducing denser filters which attenuate the light by factors of 100; 1,000; and 10,000. The maximum reading corresponds to a dose of 9,900,OOO mrad as shown in the table in Fig. 6. The factors of 10 introduced by the neutraldensity filters are displayed by adding further zeroes in the nixie lamps to the right of the unit nixie lamp. These zeroes are controlled by the cam-operated filter switches on the filter-disc shaft. 3.4 Cycle 4 With the heating current turned off, the detector is driven out to its original position and the dose is printed out. The heater timer allows 12 set for the light emission to reach its peak, and the detector-drive mechanism is then actuated, which causes an additional half-turn of the drive commutator and moves the detector out to its original position. As the drive commutator revolves, it produces a signal to perform the dose printout. The 10’s and units of the counter are printed first, followed by the zeroes representing the decade scale changes (Fig. 6). The detector is removed from the computer-indicator and returned to its case. The computer-indicator is now ready to accept another detector.
for personnel 4.
4.1 Dosimeter
monitoring
407
CALIBRATION
calibration
The dosimeters are calibrated by exposing them to known radiation doses, as measured with a 1 cm3 air-equivalent ion chamber, and reading the detector part in the computer-indicator, whose sensitivity is set to an arbitrary value. The dosimeter readings are then averaged, and the sensitivity of the computer-indicator is adjusted by means of the CALIB knobs (coarse and fine) on the front panel (Fig. 7) so that a 1 : 1 correspondence is obtained between dose readings : a 1 mrad reading per 1 mrad dose. Although all of the dosimeters may be used by merely assigning calibration factors, in actual dosimeters selected for the lNavy practice program varied by no more than f 10 percent of the average, with no calibration factor being assigned. This, of course, introduces error, but the system is easier to use and it is still within the allowable error of good personnel-dosimetry practices. 4.2 Computer-indicator
calibration
As described in 4.1, the computer-indicator is calibrated by using dosimeters exposed to known doses. In the field, however, this procedure may be cumbersome and difficult since a calibrated source is not always readily available. A standard light, therefore, is included with each computer-indicator (Fig. 7) to permit easy calibration. The standard light is a dosimeter that is identical to all others except that 40 ,uc of lYi(j3 (0.067 MeV I-) have been electroplated onto the heater coil prior to its being coated with a mixture of phosphor, potassium silicate, and water. Since the CaF,:Mn fluoresces upon irradiation, a constant light signal is generated, depending only on the half-life of the Ni63 (90 yr) and the radiation damage to the phosphor (less than 1 percent over a 1-yr period with 40 PC of activity). Because the light by fluorescence is of the same wavelengths as that emitted by thermoluminesit is unnecessary to correct for the cence, different spectral responses of the computer indicators that may result from variations in the photocathode response of the PM tubes, or the optical characteristics of the neutral density filters between computer-indicators. The output
408
R. C. Palmer, D. Rutland, R. Lagerquist and E. F. Blase
1
CoF2 : Mn
TLD
SYSTEM
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GLASS
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SYSTEM
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RANGE
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Stability
IC
Excellent
Cobalt-activatedI
Pocket ionization
Chemicals
Glass
Glass
Good
Fair
Few Times
Few Times
Fair
Fair
Fair
Excellent
&6%
ClO%
16%
55%
?15%
&15%
?15%
i15%
Chambers Fair
Not Reusability
Ease of read-out
Excellent
Thermal R/n/cm 5.4 Mev 14
MeV
Rate Dependence R/xc
9
None below 10
3.54 x 10-10 0.066 x 10-9
-9 3.63 x 10 __________
5 x 10-11 _________
_________
0.88
1.3 x 10-9
____-----
_____----
x 10-9
None below lo9
L
FIG. 8. Comparison
of CaF2:Mn
No Limit
reusable
system
9
1\~onebelow 10
None below lo9
Yes
L
L
TLD
________-
and
other
personnel
dosimetry
systems.
A prototype thermoluminescent
dosimetry system for personnel
monitoring
409
a 6
.
-
CHEMICAL
m--
GLASS
A-
TLD
DOSIMETER MICRODOSIMETER
4-
2 t
1
I
II
11
INCREASING
1
I
I
I
I
I
I
I
DISTANCE -
FIG. 9. Field study of three dosimetry systems. of the light is calibrated against the dosimreading in millieters, and an equivalent roentgens is assigned to it. Calibration of the computer-indicator is then accomplished as follows : 1. Remove standard light from its case and insert in computer-indicator. 2. Place three-position switch on the front panel (Fig. 7) to CALIB position. 3. Press READ switch. 4. Observe equivalent milliroentgen reading. 5. Adjust computer-indicator by means of CALIB knobs (coarse and fine) to proper milliroentgen reading indicated on standardlight case. 5.
SYSTEM
LINEARITY
The linearity of the light emission of the CaF,:Mn phosphor as a function of dose has
been presented in an earlier paper, which showed that this particular phosphor has a linear response from 10-4-10+5 rad. In terms of system linearity, the primary problem, therefore, was one of selecting neutral-density filters with optical densities of exactly 1, 2, 3, and 4, to allow the computer-indicator to cover the seven decades of information required wherein the largest possible dose is 10’ times the lowest measurable dose. The system’s response from 10e3 to lo4 rad was a linear function of exposure over the same range. 6.
SYSTEM
STABILITY
System stability is dependent on two main factors: (1) the electronic stability of the computer-indicator, in particular the PM tube, and (2) the storage stability of the dosimeters.
410
R. C. Palmer, D. Rutland, R. Lagerquist and E. F. Blase TABLE
2. Performance
of CaF,:
Mn TLD
system
1. Gamma dose range : 1 mr-104 rad 2. Rate dependency : none to at least log rad/sec 3. Energy independence : & 10 o/, from 80 keV to at least 1300 keV 4. Neutron sensitivity in rad/n/cm2 a. Thermal: 1.41 x 10-l” b. 5.4 Me\‘: 0.11 x lo-” c. 14 MeV: 1.47 x 1O-s 5. Particle sensitivity: none to at least 1 MeV 6. Rc-usability : at least 100 readings 7. Readout time : 12 set ?‘he dosimeter may be read out at any time after exposure with negligible loss 8. Stability: of information (less than 5 o/oloss per 2 month period when stored in the case where temperature does not exceed specification). The computer-indicated is stable to 52 “/ standard deviation a. 100 readings of one dosimeter (1) 5 mrad S, = t4”/, (2) 40 mrad S, = &,2% (3) 400 mrad S, =: + 1 0/O b. 10 readings of 10 dosimeters (1) 5 mrad S,i: = &4’% (2) 40 mrad S,? = ~2% (3) 400 mrad S, x 5 1 ‘A 10. Variance among dosimeters: f 10 % of average at 99 “/,confidence limit. 11. Accuracy: ‘I’he accuracy that can be achieved using the overall system will be a function of two factors: (a) the calibration procedure, and (bj the degree of difference between user’s exposure and an NBS-calibrated exposure. 9. Percentage
6.1
Cvmputer-indicator
stabi@
Because of the wide range of temperature over which the computer-indicator is required to operate (Table l), difficulty with stability was initially encountered in selecting a temperatureinsensitive PM tube. At 50°C most of the tested PM tubes became noisy and erratic, and a large percentage of them failed completely. This problem was solved by installing an Ascop 541-D PM tube, used primarily in oil-well logging and rated as being able to withstand temperatures up to 125°C. This tube is a low-noise, high-gain PM tube with an extended blue response. \t’ith this tube adapted to the assembly, the unit was stable to within 32 percent over the required temperature range for a 1 month period. 6.2
Dosimetry
stability
The dosimeters have been found to be stable between the temperatures indicated in Table 1, to within f5 percent over a 2 month period. When the dosimeters are checked for stability outside their cases, however, a rapid loss of
information occurs. Under normal fluorescent room-lighting, a loss of 5 percent of the stored information is encountered in 1 hr. To maintain accurate readings, therefore, the detectors must not be removed from their cases until they are ready to be inserted into the computer-indicator. 7.
DOSIMETER
RE-USE
Under normal circumstances, a dosimeter may be re-used at least 100 times (the extent to which they have been tested); however, a dosimeter that has a past history of over lo5 rad will show a change in sensitivity due to color center formation in the detector’s glass envelope, while one with a past history ofover lo8 rad will show a loss of sensitivity, as a result of radiation damage of the phosphor. Moreover, the total information stored by a dosimeter is not erased during a single reading cycle. In the 12 set reading cycle used in the system, only about 99 percent of the trapped electrons are released. To return the remaining 1 percent to the initial level requires that the detector be baked in an oven at 35O’C for 1 hr. (The detector is designed to withstand this temperature
A proto@e
thermoluminescent dosimetry system for personnelmonitoring
indefinitely.) Both the glass envelope and the phosphor are annealed by this procedure, with no resulting change in dosimeter sensitivity. 8. CaF,:Mn
COMPARISON TLD SYSTEM
PERSONNEL
OF THE WITH OTHER
DOSIMETRY
SYSTEMS
The table and bar graph in Fig. 8 give a comparison between the CaFz:Mn TLD System It is and other personnel-dosimetry systems. readily seen that the TLD system covers a wider range than other systems and, in particular, covers the entire personnel monitoring range. Figure 9 shows a comparison among three dosimetry systems during a field study at the Atomic Energy Commission’s Las Vegas Test Site, during an atomic test. Agreement among them is very close. 9.
SUMMARY
AND
CONCLUSIONS
The TLD system that Edgerton, Germeshausen & Grier, Inc., designed and constructed for the U.S. Navy, adequately fulfills the requirements set forth in Table 1, and although the Navy has not accomplished the actual evaluation, the system should easily serve to replace both
411
the film badge and the DT-60 silver-activated phosphate glass now in use. Table 2 is a summary of the performance of the CaF,:Mn TLD System. REFERENCES 1. Technical Manual for Computer-Indicator, Radiuc, CP-748(XN-1)lPD and Detector, Radiac, D T-284 (XN-I)/PD, NavShips No. 95677 (1963). 2. GINTHERR. J. J. Electrochem. Sot. 101,248 (1954). 3. GINTHER R. J. and KIRK R. D. Report of NRL Progress, U.S. Naval Research Laboratory, Washington (1956). 4. GINTHER R. J. and KIRK R. D. J. Electrochem. Sot. 104, 365 (1957). 5. SCHULMAN J. H., GINTHER R. J., KIRK R. D. and GOULART, H. S. NRL Report I\io. 5326, U.S. Naval Research Laboratory Washington (1959). 6. SCHULMANJ. H., GINTHER R. J. KIRK R. D. and GOULART H. S. Nucleonics 18, No. 3, 92 (1960). 7. SCHULMANJ. H., ATTIX F. H., WEST E. .J. and GINTHER R. J. Rea Gent. Instrum. 31, 1263
(1960). 8. Preparation and Characteristics of Solid Luminescent Materials, Wiley, New York (1948). 9. ANGINO E. E. and GROGLER N. Thermoluminescence, Bibliography, TID-39 11 (1962). 10. PALMER R. C., BLASE E. F. and POIRIERV. ht. J. appl. Radiat. Isotopes 16, 737 (1965).
Journal
of Applied
Radiation
A Prototype System / R.
C.
and Isotopes,
1966, Vol.
17, pp. 399-41
I.
Pergamon
Press Ltd.
Printed
in Northern
Ireland
Thermoluminescent Dosimetry for Personnel Monitoring* U
PALMER,
D. RUTLAND,
Edgerton,
Germeshausen Santa
R. LAGERQUIST
& Crier, Barbara,
Inc., Santa California
and E. F. BLASE
Barbara
Division,
(Received 20 September 1965) .A prototype thermoluminescent system developed under contract to the U.S. Navy is described, wherein the thermoluminescent properties of CaF,:Mn are used to monitor the y-radiation dose range of 2 mrad-104 rad. Design specifications for the system are presented, together with performance data as measured by Edgerton, Germeshausen & Grier, Inc. The system has shown characteristics of accuracy, reproducibility and re-usability far superior to those of conventional personnel-dosimetry systems. UN
SYSTEME
THERMOLUMINESCENT
DOSIMETRIE
POUR
LA
PROTOTYPIQUE
SURVEILLANCE
DE
RADIOACTIVE
DU
PERSONNEL On dtcrit un systtme thermoluminescent prototypique mis a point sous contrat B la Marine des E-U, dans lequel les qualites thermoluminescentes du CaF,:Mn servent a mesurer la dose de rayonnement gamma dans la rangte de 2 mrad-104 rad. On presente les specifications de dessin du systeme, ainsi que les don&es du fonctionnement telles que mesurees par Edgerton, Germeshausen & Grier, Inc. Le systeme a fait preuve des caracttristiques de precision, de reproduisibilite et de rtutilizabilite fortement superieures a celles des systemes conventionnels de dosimetrie de personnel. IIPOTOTIIH
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UBERWACXUNG
SYSTEMS
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PERSONAL Eine Erstausfuhrung eines im Auftrag der amerikanischen Marine entwickelten ThermoDie Thermoluminiszenz-Eigenschaften von CaF, : Mn luminiszenz-Systems wird beschrieben. werden dazu benutzt den Dosisbereich der Gammastrahlung von 2 mrad-104 rad zu uberwachen. Die konstruktiven Angaben fur das System werden aufgefiihrt, zusammen mit den erzielten Ergebnissen nach Messungen der Firma Edgerton, Germeshausen & Crier, Inc. Das System hat vie1 bessere Kenndaten hinsichtlich seiner Genauigkeit, Wiederholbarkeit und Wiederverwendbarkeit als die bekannten Systeme fur Personaltiberwachung. * This work was supported NObsr-85506.
by the Bureau
of Ships,
U.S. 399
Navy,
Department
of Defense,
Contract
No.
400
R. C. Palme)-, D. Rutland, R. Lagerquist and E. F. Blase 1. INTRODUCTION
EDGERTON, Germeshausen & Grier, Inc., recently completed the design and construction of the U.S. Navy’s prototype thermoluminescent dosimetry system (TLD), which is comprised of a Computer-Indicator CP-748(Xx-l)/PD and a Dosimeter DT-284(xX-l) PD.(l) Conceptually this system is the result of the earlier work of SCHULMAN et uL(~-‘) of the Dosimetry Branch of the U.S. Naval Research Laboratory, and stems from the desire of the U.S. Savy to use a single device to monitor the y-radiation dose ranges from near background levels to the high levels encountered in a nuclear accident. The system’s basic requirements, as envisioned by the U.S. Navy and followed in the design of the TLD system, are given in Table 1. These requirements, which are well filled by thermoluminescent dosimetry, more particularly by CaF,:Mn thermoluminescent dosimetry, preclude use of the silver-activated phosphate glass and the conventional film badge, as well as chemical dosimeters and ion chambers. vI'~~~~
1.
Requirements for personnel dosimctry system
I. Gamma dose range: 2. Accuracy : 3. Re-use: 4. Energy dependence: 5. 6. 7.
8.
9. 10.
2 mradp104 rad
&150/, 100 readings ,20% for X- or yradiation from 80 to 1300 keV Directionality: i 15 “/0regardless of direction Size of active element: 0.5 in3 less than 3 oz. (Dosilt’eight : meter) less than 40 lb (Computer-Indicator) Operating temperature: -~45”C to -55°C (Dosimeter) -10°C to -55°C (Computer-Indicator) one readout every 20 set Time: automatic readout of Automation : dose and identification code of dosimeter
An excellent introduction to the mechanisms of thermoluminescence is provided by Preparation and Characteristics of Solid Luminescent AIaterials,(B) and Thermoluminescence, Bibliography”’ is a comprehensive reference source of literature on the
subject. The preparation and properties of the CaF,:Mn phosphor have been described in a previous paper by PALMER.'~") For the present paper, it is essential only to indicate that when a thermoluminescent phosphor is exposed to ionizing radiation, electrons are promoted from the filled energy levels of the valence band into the empty levels of the conduction band of the phosphor’s crystalline lattice. Some of the electrons raised to the conduction band are trapped in metastablc states present as imperfections in the crystal lattice. 1Yhen the temperature of the CaP,:Mn crystals is raised to 28O”C, the electrons are released from the traps to combine with luminescent centers also present in the crystal, thereby causing the emission of light in the blue-green region of the electromagnetic spectrum (peak at 4950 A). By appropriate calibration, the quantity of light emitted can be related to the energy deposited in the phosphor by the ionizing radiation. 2.
DOSIMETER
DT-284(XN-l)/PD
Figure 1 is a sketch of the thermoluminescent dosimeter. The unit consists of a detector and a case, each of which is composed of subparts. 2.1 Detector The detector is composed of the active CaF,:Mn element to record the exposure information, and the coded handle to automatically identify the detector and to seal the detector inside the case. 2.1.1 rlctiue element of CaF,:Mn. The active element is basically a vacuum tube in which the filament is coated with the thermoluminescent CaF,:Mn. During production, the element is made by dipping the heater coil into a mixture of phosphor, potassium silicate and water, drying out the excess water, and then cncapsulating the coil in a vacuum tube at a pressure of 1OW mm torr. Two Kovar feed-throughs, terminated in the heater contacts, provide a current path for resistance-heating of the phosphor. In practice, a constant current of 6.5 A at 3 V is supplied to the unit to raise the phosphor’s temperature to 300°C in 12 sec. 2.1.2 Coded handle. The coded handle is attached to the detector by means of a spacer. It contains the detector’s identification number
A prototrpe
PLASTIC
thermoluminescent
CASE
DIGIT
( PHOSPHOR GLASS
: POTASSIUM
BUTTONS
SILICATE
)
ENVELOPE
FIG. 1. Thermoluminescent in the form of eight digit buttons. The digit buttons present a l-2-4-2 code as notches and lands on each digit button, which are automatically sensed by the computer-indicator. A tamper-proof catch seals the unit inside the case to physically protect the detector. 2.2 Case The case serves two purposes: (1) it provides a light-tight detector housing to protect the phosphor from light, and (2) it contains an energy compensation shield to correct for overresponse of the CaF,:Mn phosphor in the region below 500 keV. The shield shown in Fig. 2 was designed and constructed to ensure essentially unity response relative to a Co60 exposure. The energy-response curve shown in Fig. 3 is obtained for the dosimeter with the shield removed from the plastic case. Figures 4 and 5 show the energy response in various orientations in different radiation fields (Co60, CS~~~, 250-kV filtered X-ray machine) with the shield in place. All of the irradiations were monitored with a Victoreen 1 cm3 air-equivalent ion chamber, which had been calibrated at the U.S. Bureau of Standards. As can be seen from Fig. 5, the dosimeter’s response is uniform within i-10 percent when it is irradiated, either normal to the longitudinal axis or normal to the end cap in the energy range of 80 to 1250 keV. As shown in Fig. 5, the only serious 3
401
dosimetry system for personnel monitoring
dosimeter.
deviation in sensitivity is encountered at 45” from the coded handle and at low energies. The handle, of course, precludes some directional dependence, which will become progressively worse as even lower energies are encountered. 3.
COMPUTER-INDICATOR CP-748(XN-l)/PD
The computer-indicator consists of a mechanical section and an electronic section, as shown in Fig. 6. The mechanical section contains the mechanisms for holding the detector portion of the dosimeter, drawing it into reading position, electrically reading the detector’s identification (ID) number, chopping the light emitted by the and introducing optical filters to detector, attenuate the light output. The mechanical section also includes the photomultiplier (PM) tube, which senses the light, and its voltagedivider network and high-voltage supply. The electronic section, completely transistorized except for one current-regulating tube in the heater supply, receives the necessary signals from the mechanical section to operate the unit. Some of these signals contain information on the detector identification number and filter-disc position, and other signals are used for sequencing the operations. The filter switches are cam-operated from the filter-disc shaft and indicate which filter is inserted in front of the
402
R. C. Palmer, D. R&and,
\
Y
energy
00065”
TIN
-‘-0018”
LEAD
PLASTIC
FIG. 2. Cross-sectional
FIG. 3. Dosimeter
\
//
R. Lagerquist and E. F. Blase
view
response
of plastic
normal
case,
showing
to longitudinal
energy-compensation
axis (shield
shown in Fig.
shield
2 removedj.
A prototype thermoluminescent
FIG. 4. Dosimeter
energy normal
dosimetry system for personnel
response to end
normal to longitudinal cap (shielded).
403
monitoring
axis
and
404
R. C. Palmer, D. Rutland R. Lagerquist and E. F. Blase
FIG. 5. Dosimeter
directional
energy
response
(shielded).
l-
I
I
----
START
r,
HEATER TlMER
,
STOP
L
---------____ MECHANICAL UNIT
-I
CONTROL
DOSEIPI;
REFLECTOR
SI
DC
I
POWER 4
GO-CYCLE TENS/UNITS SIGNAL
CIRCUITS
1
function diagram.
q&
!t&;\
POWER SWITCH
NOT
NIXIE --
ELECTRONIC UNIT
TRANSFORMER
TO ALL
FIG. 6. Overall
;
I-2-4-2 CODE TENS+ “NIT5
c____-,
1
I
1
IC bISPLAY
J-
117”
GOOW
PART OF THE COMPUTER-INDICATOR
LAMP
1.
RECORD
PRINTED
RANGES
10 OPTICAL A TEN ,T,LTER
b
406
R. C. Palmer, D. Rutland, R. Lagerquist and E. F. Blase
detector. The drive commutator makes a halfturn as the detector is going in, and another half-turn as the detector is coming out. The signals from the drive commutator control the printing functions and the heating of the detector’s active element. The signals from the identification switches are translated by the electronic section into decimal digits, which are entered into the printer. A person reading a dosimeter removes the detector from its case and inserts it into the computer-indicator through the hatch shown in Fig. 7, and the following cycles are then performed by the computer-indicator (refer to Fig. 6).
3.1 Cycle 1 The detector is driven into its reading position as the identification number is sensed and printed. This action is initiated by the READ switch on the front panel (Fig. 7). When the READ switch is depressed, it applies voltage to the solenoid clutch in the detector-drive mechanism (not shown). This causes the clutch to make one complete revolution and draw the detector past the switches used for identificationnumber reading. The drive commutator controls the transistor digit gates during this cycle so that the l-2-4-2 code signals are directly connected to the transistor decoder. The decoder interprets the l-2-4-2 code and supplies a voltage on one of ten wires representing the digits 0 through 9. Each of the ten wires drives one of ten solenoid drivers, which in turn, supply current to operate the solenoids on the serial keyboard-printer. As the handle moves past the identificationnumber reading switches, each digit button is read in turn, and the button operates the printer via the digit gates, decoder, printer solenoid drivers, and keyboard solenoids. This places the identification number in the printer keyboard. After the last digit has been placed in the keyboard, the drive commutator operates the print-back solenoid in the printer, and causes the identification number to be printed. When Cycle 1 is completed, the active element of the detector is centered in the reflector.
3.2 Cycle 2 During this cycle, current is supplied to the active element, heating it and causing it to emit light. When the drive commutator arrives in the IX position, it energizes the detector heater supply and starts the detector heater timer. The constant 6.5 A heater current passes through the detector heater contacts and the heater coil. As the active element is heated, the temperature of the CaF,:Mn phosphor is increased to 300°C in 12 set; and the phosphor emits light, which the reflector directs through the infrared (ir.) filter, the filter-disc holder, and the chopper disc, onto the PM tube. At the end of the 12 XC heating period, the heater timer energizes the detector-out switch, and the drive commutator turns off the detector heater supply.
3.3 Cycle 3 The peak light output is measured and its value is stored digitally. The light from the detector is chopped by the chopper disc, which produces equal periods of illumination and darkness on the PM tube at a 90 c/s rate. The output of the PM tube, a 90 c/s square wave, is amplified by the a.c. amplifier, and the amplified signal is changed to a filtered d.c. level by a phase-lock rectifier, which is driven by a synchronizing signal. The synchronizing signal is generated in synchronism with the PM tube signal by means of a magnetic pickup attached to a steel wheel on the chopper disc shaft. The rectifier’s d.c. output is converted to a decimal number by a digital voltmeter consisting of a two-decade counter, a diode current switch, a d.c. amplifier, and a pulse generator. When the detector’s highest light emission is reached, the counter remains at its count and stores the dose information in digital form. The value appearing in the counter is displayed on two nixie lamps showing 10’s and units. The counter operates in a l-2-4-2 code for each decade. These codes are translated to decimal form by the decoder, which handles only one decimal digit at a time and which the digit gates switch at a 60-cycle rate between the 10’s and the units of the decimal counter. The nixie
FIG. 7. Front
paw1
of computer-indicator.
A proto@c
thermoluminescent
dosimetry system
lamp voltages are switched in synchronism to produce the effect of a two-digit display. At its peak, the light from the detector may exceed that corresponding to the maximum two-digit display of 99 mrad. When this happens, the a.c. amplifier gain is reduced by a factor of exactly 10, and the counter is reset to zero by means of the control logic. The counter reading must now be multiplied by 10 to indicate the true dose. This is accomplished by turning on a zero in the nixie lamp immediately to the right of the unit readout-nixie lamp. When the detector dose exceeds 990 mrad, the light to the PM tube is attenuated by a factor of exactly 10, by rotating a neutral-density filter into the light path. The control logic energizes the solenoid in the filter-advance mechanism, and the filter-disc is advanced one position. If the detector light increases to higher and higher values, the filter-advance mechanism is successively activated each time the reading in the counter exceeds 99, thereby introducing denser filters which attenuate the light by factors of 100; 1,000; and 10,000. The maximum reading corresponds to a dose of 9,900,OOO mrad as shown in the table in Fig. 6. The factors of 10 introduced by the neutraldensity filters are displayed by adding further zeroes in the nixie lamps to the right of the unit nixie lamp. These zeroes are controlled by the cam-operated filter switches on the filter-disc shaft. 3.4 Cycle 4 With the heating current turned off, the detector is driven out to its original position and the dose is printed out. The heater timer allows 12 set for the light emission to reach its peak, and the detector-drive mechanism is then actuated, which causes an additional half-turn of the drive commutator and moves the detector out to its original position. As the drive commutator revolves, it produces a signal to perform the dose printout. The 10’s and units of the counter are printed first, followed by the zeroes representing the decade scale changes (Fig. 6). The detector is removed from the computer-indicator and returned to its case. The computer-indicator is now ready to accept another detector.
for personnel 4.
4.1 Dosimeter
monitoring
407
CALIBRATION
calibration
The dosimeters are calibrated by exposing them to known radiation doses, as measured with a 1 cm3 air-equivalent ion chamber, and reading the detector part in the computer-indicator, whose sensitivity is set to an arbitrary value. The dosimeter readings are then averaged, and the sensitivity of the computer-indicator is adjusted by means of the CALIB knobs (coarse and fine) on the front panel (Fig. 7) so that a 1 : 1 correspondence is obtained between dose readings : a 1 mrad reading per 1 mrad dose. Although all of the dosimeters may be used by merely assigning calibration factors, in actual dosimeters selected for the lNavy practice program varied by no more than f 10 percent of the average, with no calibration factor being assigned. This, of course, introduces error, but the system is easier to use and it is still within the allowable error of good personnel-dosimetry practices. 4.2 Computer-indicator
calibration
As described in 4.1, the computer-indicator is calibrated by using dosimeters exposed to known doses. In the field, however, this procedure may be cumbersome and difficult since a calibrated source is not always readily available. A standard light, therefore, is included with each computer-indicator (Fig. 7) to permit easy calibration. The standard light is a dosimeter that is identical to all others except that 40 ,uc of lYi(j3 (0.067 MeV I-) have been electroplated onto the heater coil prior to its being coated with a mixture of phosphor, potassium silicate, and water. Since the CaF,:Mn fluoresces upon irradiation, a constant light signal is generated, depending only on the half-life of the Ni63 (90 yr) and the radiation damage to the phosphor (less than 1 percent over a 1-yr period with 40 PC of activity). Because the light by fluorescence is of the same wavelengths as that emitted by thermoluminesit is unnecessary to correct for the cence, different spectral responses of the computer indicators that may result from variations in the photocathode response of the PM tubes, or the optical characteristics of the neutral density filters between computer-indicators. The output
408
R. C. Palmer, D. Rutland, R. Lagerquist and E. F. Blase
1
CoF2 : Mn
TLD
SYSTEM
FILM I COBALT ACTIVATED GLASS TRICHLORETHYLENE TOSHIGA SILVER ACTIVATED PHOSPHATE
I .
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GLASS
CHEMICAL
SYSTEM
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,
.
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I7
R USABLE
RANGE
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Stability
IC
Excellent
Cobalt-activatedI
Pocket ionization
Chemicals
Glass
Glass
Good
Fair
Few Times
Few Times
Fair
Fair
Fair
Excellent
&6%
ClO%
16%
55%
?15%
&15%
?15%
i15%
Chambers Fair
Not Reusability
Ease of read-out
Excellent
Thermal R/n/cm 5.4 Mev 14
MeV
Rate Dependence R/xc
9
None below 10
3.54 x 10-10 0.066 x 10-9
-9 3.63 x 10 __________
5 x 10-11 _________
_________
0.88
1.3 x 10-9
____-----
_____----
x 10-9
None below lo9
L
FIG. 8. Comparison
of CaF2:Mn
No Limit
reusable
system
9
1\~onebelow 10
None below lo9
Yes
L
L
TLD
________-
and
other
personnel
dosimetry
systems.
A prototype thermoluminescent
dosimetry system for personnel
monitoring
409
a 6
.
-
CHEMICAL
m--
GLASS
A-
TLD
DOSIMETER MICRODOSIMETER
4-
2 t
1
I
II
11
INCREASING
1
I
I
I
I
I
I
I
DISTANCE -
FIG. 9. Field study of three dosimetry systems. of the light is calibrated against the dosimreading in millieters, and an equivalent roentgens is assigned to it. Calibration of the computer-indicator is then accomplished as follows : 1. Remove standard light from its case and insert in computer-indicator. 2. Place three-position switch on the front panel (Fig. 7) to CALIB position. 3. Press READ switch. 4. Observe equivalent milliroentgen reading. 5. Adjust computer-indicator by means of CALIB knobs (coarse and fine) to proper milliroentgen reading indicated on standardlight case. 5.
SYSTEM
LINEARITY
The linearity of the light emission of the CaF,:Mn phosphor as a function of dose has
been presented in an earlier paper, which showed that this particular phosphor has a linear response from 10-4-10+5 rad. In terms of system linearity, the primary problem, therefore, was one of selecting neutral-density filters with optical densities of exactly 1, 2, 3, and 4, to allow the computer-indicator to cover the seven decades of information required wherein the largest possible dose is 10’ times the lowest measurable dose. The system’s response from 10e3 to lo4 rad was a linear function of exposure over the same range. 6.
SYSTEM
STABILITY
System stability is dependent on two main factors: (1) the electronic stability of the computer-indicator, in particular the PM tube, and (2) the storage stability of the dosimeters.
410
R. C. Palmer, D. Rutland, R. Lagerquist and E. F. Blase TABLE
2. Performance
of CaF,:
Mn TLD
system
1. Gamma dose range : 1 mr-104 rad 2. Rate dependency : none to at least log rad/sec 3. Energy independence : & 10 o/, from 80 keV to at least 1300 keV 4. Neutron sensitivity in rad/n/cm2 a. Thermal: 1.41 x 10-l” b. 5.4 Me\‘: 0.11 x lo-” c. 14 MeV: 1.47 x 1O-s 5. Particle sensitivity: none to at least 1 MeV 6. Rc-usability : at least 100 readings 7. Readout time : 12 set ?‘he dosimeter may be read out at any time after exposure with negligible loss 8. Stability: of information (less than 5 o/oloss per 2 month period when stored in the case where temperature does not exceed specification). The computer-indicated is stable to 52 “/ standard deviation a. 100 readings of one dosimeter (1) 5 mrad S, = t4”/, (2) 40 mrad S, = &,2% (3) 400 mrad S, =: + 1 0/O b. 10 readings of 10 dosimeters (1) 5 mrad S,i: = &4’% (2) 40 mrad S,? = ~2% (3) 400 mrad S, x 5 1 ‘A 10. Variance among dosimeters: f 10 % of average at 99 “/,confidence limit. 11. Accuracy: ‘I’he accuracy that can be achieved using the overall system will be a function of two factors: (a) the calibration procedure, and (bj the degree of difference between user’s exposure and an NBS-calibrated exposure. 9. Percentage
6.1
Cvmputer-indicator
stabi@
Because of the wide range of temperature over which the computer-indicator is required to operate (Table l), difficulty with stability was initially encountered in selecting a temperatureinsensitive PM tube. At 50°C most of the tested PM tubes became noisy and erratic, and a large percentage of them failed completely. This problem was solved by installing an Ascop 541-D PM tube, used primarily in oil-well logging and rated as being able to withstand temperatures up to 125°C. This tube is a low-noise, high-gain PM tube with an extended blue response. \t’ith this tube adapted to the assembly, the unit was stable to within 32 percent over the required temperature range for a 1 month period. 6.2
Dosimetry
stability
The dosimeters have been found to be stable between the temperatures indicated in Table 1, to within f5 percent over a 2 month period. When the dosimeters are checked for stability outside their cases, however, a rapid loss of
information occurs. Under normal fluorescent room-lighting, a loss of 5 percent of the stored information is encountered in 1 hr. To maintain accurate readings, therefore, the detectors must not be removed from their cases until they are ready to be inserted into the computer-indicator. 7.
DOSIMETER
RE-USE
Under normal circumstances, a dosimeter may be re-used at least 100 times (the extent to which they have been tested); however, a dosimeter that has a past history of over lo5 rad will show a change in sensitivity due to color center formation in the detector’s glass envelope, while one with a past history ofover lo8 rad will show a loss of sensitivity, as a result of radiation damage of the phosphor. Moreover, the total information stored by a dosimeter is not erased during a single reading cycle. In the 12 set reading cycle used in the system, only about 99 percent of the trapped electrons are released. To return the remaining 1 percent to the initial level requires that the detector be baked in an oven at 35O’C for 1 hr. (The detector is designed to withstand this temperature
A proto@e
thermoluminescent dosimetry system for personnelmonitoring
indefinitely.) Both the glass envelope and the phosphor are annealed by this procedure, with no resulting change in dosimeter sensitivity. 8. CaF,:Mn
COMPARISON TLD SYSTEM
PERSONNEL
OF THE WITH OTHER
DOSIMETRY
SYSTEMS
The table and bar graph in Fig. 8 give a comparison between the CaFz:Mn TLD System It is and other personnel-dosimetry systems. readily seen that the TLD system covers a wider range than other systems and, in particular, covers the entire personnel monitoring range. Figure 9 shows a comparison among three dosimetry systems during a field study at the Atomic Energy Commission’s Las Vegas Test Site, during an atomic test. Agreement among them is very close. 9.
SUMMARY
AND
CONCLUSIONS
The TLD system that Edgerton, Germeshausen & Grier, Inc., designed and constructed for the U.S. Navy, adequately fulfills the requirements set forth in Table 1, and although the Navy has not accomplished the actual evaluation, the system should easily serve to replace both
411
the film badge and the DT-60 silver-activated phosphate glass now in use. Table 2 is a summary of the performance of the CaF,:Mn TLD System. REFERENCES 1. Technical Manual for Computer-Indicator, Radiuc, CP-748(XN-1)lPD and Detector, Radiac, D T-284 (XN-I)/PD, NavShips No. 95677 (1963). 2. GINTHERR. J. J. Electrochem. Sot. 101,248 (1954). 3. GINTHER R. J. and KIRK R. D. Report of NRL Progress, U.S. Naval Research Laboratory, Washington (1956). 4. GINTHER R. J. and KIRK R. D. J. Electrochem. Sot. 104, 365 (1957). 5. SCHULMAN J. H., GINTHER R. J., KIRK R. D. and GOULART, H. S. NRL Report I\io. 5326, U.S. Naval Research Laboratory Washington (1959). 6. SCHULMANJ. H., GINTHER R. J. KIRK R. D. and GOULART H. S. Nucleonics 18, No. 3, 92 (1960). 7. SCHULMANJ. H., ATTIX F. H., WEST E. .J. and GINTHER R. J. Rea Gent. Instrum. 31, 1263
(1960). 8. Preparation and Characteristics of Solid Luminescent Materials, Wiley, New York (1948). 9. ANGINO E. E. and GROGLER N. Thermoluminescence, Bibliography, TID-39 11 (1962). 10. PALMER R. C., BLASE E. F. and POIRIERV. ht. J. appl. Radiat. Isotopes 16, 737 (1965).