tritium and its measurements. - technical … and its measurement … · instruments for ultra low...

TRITIUM AND ITS MEASUREMENT

M. W. Overhoff, PhD

September 1999

OVERHOFF TECHNOLOGY CORPORATION 1160 US ROUTE 50, MILFORD, OHIO, USA

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TABLE OF CONTENTS FOREWORD i CHEMICAL FORMS OF TRITIUM 2 APPLICATION OF MEASUREMENT OF TRITIUM, ANALYTICAL AND ENVIRONMENTAL SAFETY INSTRUMENTAL METHODS OF TRITIUM MEASUREMENT 3 DIRECT PHYSICAL METHODS METHODS INVOLVING RADIOACTIVE DECAY SCINTILLATION DETECTORS IONIZATION DETECTORS 4 RESTRICTED AREAS 5 UNRESTRICTED AREAS STACK AND HOOD MONITORING NUCLEAR POWER PLANTS 6 NESHAP ENVIRONMENTAL INVESTIGATIONS HIGH LEVEL MEASUREMENTS CONFIGURATIONS 7 INSTRUMENTS FOR ULTRA LOW LEVEL MEASUREMENTS IN AIR FIGURE 1 8 LOW LEVEL AIR MEASUREMENTS WITH 9 PROPORTIONAL COUNTERS FIGURE 2 10 SIGNATURE ANALYSIS 11

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FIGURE 3 12 FIGURE 4 13 AIR DILUTED GAS PROPORTIONAL COUNTERS 14 LOW TO INTERMEDIATE LEVELS OF TRITIUM IN AIR MEASUREMENTS WITH LINEAR IONIZATION CHAMBERS ELECTROMETER LIMITATIONS 15 FIGURE 5 16 FIGURE 6 17 FIGURE 7 18 IONIZATION CHAMBERS FOR LOW LEVEL MEASUREMENTS 19 INSULATORS 20 RADON BACKGROUND GAMMA 21 FIGURE 8 22 FIGURE 9 23 MOISTURE / CONDENSATION 24 TRITIUM MEASUREMENT IN THE PRESENCE OF OTHER 25 GASEOUS RADIOISOTOPES FORM SPECIFIC MEASUREMENTS OF TRITIUM JESSE EFFECT WALL EFFECTS, THE MEAN FREE PATH FIGURE 10 26 FIGURE 11 27 FIGURE 12 28

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IONIZATION CHAMBERS FOR WIDE RANGE OF 29 MEASUREMENT AND FOR HIGH LEVEL MEASUREMENTS NON LINEAR RESPONSE (LOSS OF SATURATION) PLATE-OUT, CONTAMINATION FIGURE 13 30 FIGURE 14 31 PHANTOM WALL IONIZATION CHAMBERS 32 PHANTOM WALL IONIZATION CHAMBERS AS REFERENCE STANDARDS FIGURE 15 33 FIGURE 16 34 FIGURE 17 35 MOISTURE TRANSIENT RESPONSE 36 REDUCTION IN EFFECTS FROM RADIOACTIVE IMPURITIES ON THE SURFACE OF IONIZATION CHAMBER WALLS CALIBRATION OF IONIZATION CHAMBERS AND PROPERTIES OF CALIBRATORS AND GAS STORAGE VESSELS OVERALL CALIBRATION OF TRITIUM MONITORS IONIZATION CHAMBERS CALIBRATION OF THE ELECTRONICS 37 FIGURE 18 38 TRANSFER GAMMA CALIBRATION(QUICK AND DIRTY) 39 ELECTRICAL CALIBRATION FIGURE 19 40 PRACTICAL DESIGN LIMITS FOR LINEAR IONIZATION 41 CHAMBER TRITIUM MONITORS

Overhoff Technology Corporation Milford, Ohio 45150 USA

Telephone (513) 248-2400 e-mail: [email protected]

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FOREWORD This set of notes is a short compilation of facts and figures concerning tritium that I have been privileged to gather over the last twenty-five years from many scientists and engineers in government, universities and industry. Certainly much of this information is already known to the reader, and it is my hope that the text can bring you additional knowledge. The notes are not claimed to be either complete nor profound, but rather to provide basic insight to help understand the properties of tritium and the limitations and performance of practical measurement systems.



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Tritium is the third isotope of hydrogen, comprised of one orbital electron surrounding a nucleus containing one proton and two neutrons. It is weakly radioactive, ejecting one of its electrons in the form of a beta particle and decaying into helium three, which is a stable element. Pertinent basic physical radioactive properties can be summarized as follows. ACTIVITY: 9620 Curies per gram, or 21695 Curies per cc at STP HALF-LIFE: 12.33 years. BETA DECAY: 18.6 kev maximum, 5.6 kev mean energy The ejected beta particles do not all have the same kinetic energy, but range from zero to a maximum value corresponding to the Aston mass packing fraction associated with the two nuclides. Chemically, tritium is essentially indistinguishable from hydrogen, or deuterium, although there are sufficient differences to permit the use of some chemical (and electrical) methods for isotope separation or enhancement. To all extents and purposes, tritium is a man made artificial substance, although it does occur in nature in trace amounts being generated by cosmic ray interactions. It is found in water at very low concentrations, as well as in the upper atmosphere. Tritium also occurs as an unwanted byproduct in heavy water nuclear power reactors, where it appears as a contaminant in the deuterium oxide moderator fluid. Tritium is also found in light water reactors, but at much lower concentrations. I believe that it was experimentally isolated in about 1934, possibly by Harold Urey or Glen Seaborg at the University of California. Uses for tritium are few. Quite apart from its use in thermonuclear devices, chief applications are as component of radio pharmaceuticals, self powered emergency lights or similar items such as watch dials. Tritium can be manufactured in several ways, including separation from D2O or by use of the lithium six-neutron reaction.



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CHEMICAL FORMS OF TRITIUM Tritium is chemically identical to protium and is simply one of the three isotopes of hydrogen. It can be found in any chemical combination normally associated with hydrogen. It can be found in combination with other isotopes, forming HT, or DT as well as T2. When combined with oxygen, it will exist as HTO, DTO or T2O. In the oxide form it is essentially indistinguishable from ordinary water which therefore poses potential radiological health hazards. APPLICATIONS OF MEASUREMENT OF TRITIUM, ANALYTICAL AND ENVIRONMENTAL SAFETY Analytical measurements of tritium are made in association with production processes of tritium and its compounds, such as in the purification and storage of high purity tritium, as well as in its use in the manufacture of luminous devices or in the production or assay of radio pharmaceuticals. Other applications include the manufacture of neutron generators used in the oil well logging industry, in weapons production, or general scientific studies of the physics and chemistry of tritium and its compounds. Environmental safety measurements are required to protect both workers as well as the general public from radioactive hazards associated with accidental absorption of tritium. Analytical measurements and environmental measurements are generally quite different. The former involving high level measurements, including that of live tritium, where as the latter often involve the measurement of exceedingly low concentrations, as low as a few disintegrations per minute per cc of liquid or cubic meter of gas (air). There are quite a few technical challenges encountered in the quest of accuracy or sensitivity in measuring tritium over this enormous range of concentration. It is the purpose of this monograph to identify and explore many of these challenges and the methods and techniques that are used to overcome them.



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INSTRUMENTAL METHODS OF TRITIUM MEASUREMENT Detection methods sometimes depend on the forms of tritium to be measured. Tritium can be found in liquid form, in gaseous form, or in solid form, chiefly as contaminant on solid surfaces. However, as shall be seen, detection methods for one form are often used for the other physical forms of tritium. DIRECT PHYSICAL METHODS There are direct physical methods for the determination of tritium. These include: Mass-density measurements Coulometry, measuring the heat of disintegration. Gas chromatography

Raman spectroscopy and other optical methods. Electrochemical methods used for the separation or enhancement of the hydrogen isotopes.

METHODS INVOLVING RADIOACTIVE DECAY The two major methods of detection of the radioactive decay are scintillation detection, and the detection of charge or current produced from ionization generated by the beta particles. At low measurement levels, it becomes necessary that the beta particles which are generated by decaying tritium are somehow distinguished from other forms of energy, such as natural radioactive background including cosmic radiation. At high levels of measurement, it may be difficult to separate individual beta events. Loss of measurement accuracy due to pulse pile up or to chamber saturation can occur. SCINTILLATION DETECTORS These detectors utilize photo optical amplification of the light pulses (scintillations) which are generated when beta particles exchange their kinetic energy with liquid, solid or gaseous materials.



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Scintillator detectors are predominantly used for the assay of the oxide of tritium in water or other liquids, or for the assay of low levels of airborne tritium in the environment. Scintillation detector methods are used chiefly for very low level measurements of tritium in liquids. IONIZATION DETECTORS The two types of direct ionization are linear ionization chamber detectors and proportional counters. Both detectors use a closed volume to measure the charge generated when a beta particle expends its energy in the gas inside the chamber. In the case of the proportional counter, controlled electronic gas amplification is used to enlarge the ionization charge from each individual beta event to a size sufficient to be distinguishable by conventional electronics. This is similar to the techniques used for scintillation detection, where photomultiplier tubes are used for the same purpose. The primary ionization charge of a single tritium beta, or the equivalent scintillation is so weak that ordinary electronic amplifiers cannot be used for direct detection. “Linear” ionization chambers can be used where the concentration of tritium is sufficient to produce a smooth current that can be detected by conventional means such as electrometer amplifiers. In the following pages some of the theoretical and practical difficulties associated with each of these techniques will be discussed. Before that, it is useful to digress into the realm of health physics, which is a major reason for tritium measurement in the environment, and ultimately determines the limits of sensitivity which the instrumentation needs to attain. Airborne tritium can take the form of elemental tritium, that is T2, HT or DT as well as the oxide form HTO, T2O or DTO. In the environment, the oxide form is bound to predominate, especially since the elemental form will rapidly combine with ambient moisture in the air to form oxides of tritium. Since it is radioactive it can be a safety concern depending on the way living things can absorb it. When breathed into the lungs, the oxide form is essentially totally absorbed where as the elemental form is mostly expelled. The oxide form is considered to be several thousand times more dangerous than the elemental form of tritium.



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Once absorbed by the lungs, the way in which tritium is retained and ultimately expelled is of interest. The body burden half-life of tritium as HTO is of the order of two weeks, and can be accelerated by drinking lots of liquids. Tritium absorbed elsewhere in the body, such as in tissue or in bone will have much longer residence times, and can therefore be of greater concern. Tritium oxide can also be absorbed through the skin, but wearing gloves or other protection gear is an easy precaution. Duration of exposure also plays a role. Forty hours a week is a lower exposure than twenty-four hours continuously seven days a week. Finally, political factors can force a distinction between exposure to trained nuclear workers and that of the unwitting general public. As a result, different environments call for different tolerances for maximum permitted concentrations of tritium. RESTRICTED AREAS 10 to 20µCi/m3 300kBq/m3 to 1MBq/m3 For “restricted” environments in dedicated work areas such as laboratories or CANDU reactors, the maximum average concentrations are often limited to 10 or 20 µCi/m3 for a forty-hour per week exposure. UNRESTRICTED AREAS 1µCi/m3 37kBq/m3 Warehouse storage facilities or other areas that may be frequented by untrained personnel and that may be exposed continuously when maintaining the same dose, the maximum average concentration drops by the ratio of one week continuous to forty hours, or to 1.19 µCi/m3. STACK AND HOOD MONITORING 0.1µCi/m3 The USNRC (United States Nuclear Regulatory Commission) and, presumably, the agencies of other countries do not limit total emissions of radioactive materials to the environment, but instead, restrict the concentration of the mixture being released. These limits are often only a few micro Curies per cubic meter, or fractions of one µCi/m3.



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NUCLEAR POWER PLANTS Emissions from heavy water nuclear reactors not only contain tritium, but also carry varying amounts of other radioactive gases including carbon 14, Krypton 85, Argon 41, and Xenon 133. NESHAP 0.1µCi/m3 and less Restrictions under NESHAP regulations are even more restrictive, and can entail measurements of tritium to fractions of 0.1 µCi/m3. ENVIRONMENTAL INVESTIGATIONS 1 nanoCi/m3 and less Using passive sampling techniques, it is possible to measure even lower levels of tritium activity. Such measurements are made for “plume” studies, or for scientific purposes to study atmospheric distribution of tritium. Tritium oxide in lakes or rivers, whether naturally occurring, or due to man made causes, can be found in concentrations as low as 10 to 100 Pico Curies. HIGH LEVEL MEASUREMENTS At the opposite end of the measurement spectrum, concentrations can vary all the way up to live tritium, sometimes at pressures even above ambient. High concentrations of tritium are encountered during manufacture, storage and processing of tritium. Typical systems include: Recovery Systems Storage beds Recovery systems Self powered lighting systems Fusion reactors Thermonuclear devices Pharmaceutical manufacturing Hospital nuclear laboratories



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CONFIGURATIONS Tritium monitors can be configured to suit the applications. This ranges form small hand held devices to large “fixed” installations permanently mounted or on carts. Monitors for liquid effluent are large and cart mounted for field use. Ultra low level environmental monitors such as passive samplers are often mounted to small outdoor enclosures. In conclusion, we are concerned with the measurement of tritium in all possible concentrations from pure tritium to pico Curies per liter. The tritium is often found in association with other radioactive materials, or, as will be mentioned later, in the presence of gamma radiation or of readily ionizable atmospheric pollutants that make tritium detection more difficult. So far, we have dealt with background facts mainly to show that all possible concentrations of tritium can be encountered for measurement purposes. We may add the fact that many applications involve substantial temperature and pressure variations, which make accuracy of measurement ever more demanding. The major types of instrumentation can show each he described, with emphasis on engineering of scientific challenges encountered in design or use. INSTRUMENTS FOR ULTRA LOW LEVEL MEASUREMENTS IN AIR Figure 1 There are no direct, real time, measurement instruments for tritium concentrations of the order of 1 x 10-10 Curies per cc (1 x 10-4 Curies per m3) (1 Bq per cc). It is common to use passive sampling techniques, generally using a small pump which draws a controlled flow of the environment through a series of small vials, which traps the airborne tritium. An oxidizer is interposed between the first two sets of vials, so that both the elemental and the oxide form of tritium can be assayed. The first two vials collect the oxide form, and the next two collect the oxidized elemental form of tritium. The last vial is filled with indicating sodium hydroxide, which is used to trap carbon dioxide for measurements of carbon 14. Vials are usually chosen which fit directly into a liquid scintillation counter; transfer from one vial into another is thereby avoided.

PASSIVE SAMPLING OF TRITIUM IN AIR

Oxidizer is used for simultaneous measurement of boththe elemental and the oxide form

an “ascarite” trap for 14C can be included

ANALYSIS IS BY USE OF A SEPARATE SCINTILLATION COUNTER

SENSITIVITY to about 10-10 Ci/cc (10-4 Ci/m3)

FLOWMETER OXIDIZER ASCARITEPUMP

8 FIGURE 1

µµ



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The reason for using pairs of vials is that the water mixture evaporating out of the first vial is then reabsorbed by the second vial. Over time, the liquid level in the first vial slowly descends; but the level in the second vial stays more or less constant. Under extreme circumstances, the first vial can deplete before routine servicing. To avoid this situation; the vials can be filled with a mixture of water and glycol. Desiccant such as silica gel can be used, it is easily dissolved in water for subsequent liquid scintillation assay. The tacit assumption made is that the tritium in the sampled air is absorbed with 100% efficiency, and that large changes in background natural relative humidity do not appreciably affect the outcome of the assay. LOW LEVEL AIR MEASUREMENTS WITH PROPORTIONAL COUNTERS Figure 2 At first glance, proportional counters look like an ideal way for low level tritium detection. After all, the counter tube acts as a collector, and appears to offer high signal gain at low cost and great simplicity. While this is partly true, proportional counters do have drawbacks, not the least of which in that they use a consumable gas supply. When properly designed, the level of sensitivity of a proportional counter can reach about one nano Curie per cubic meter, earlier versions of proportional counters, using air/gas mixtures, show serious short comings, limiting sensitivity to about 100 times lower, or about 0.1 µCi/m3. To understand their limitations it is helpful to first understand how they work. A proportional counter is comprised usually of a long slender conductive cylinder with a very thin wire in the middle. A high voltage is applied between the wire and the wall thereby creating an electric field which draws ions (and electrons) to the wire and to the wall of the counter tube. The field inside the tube is very non uniform, the field gradient in the vicinity of the control wire is very high, but shrinks rapidly away from the wire. The field near the wire is high enough that electric charges are sufficiently accelerated to produce further ionization as they collide with the molecules of the gas with which the counter is filled. In this way, an ionizing event anywhere inside the counter tube is greatly amplified, producing a current pulse at the wire of sufficient magnitude to permit further signal processing by conventional electronics. The gas collision

SIGNAL PROCESSING ELECTRONICS

RATE METER COUNTER

OUT

COUNTER

AIR SAMPLEOUT

FLOWMETERPUMP

MEMBRANE

SEMIPERMEABLE

COUNTER GAS

FLOWMETER

COMPENSATIONCOUNTER

ACTIVE METHOD FOR LOW LEVELS OF TRITIUMPROPORTIONAL COUNTER

STABLE SENSITIVITY to 10-9 Ci/ccBalanced chambers for background gamma subtraction

µ

10

HTOandair

P-10 HTOandP-10

FIGURE 2



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multiplication is a function of the geometry of the counter, the nature and pressure of the gas filling, and of the applied voltage. When the gas becomes infine, then the tube is called a Geiger-Müller tube. Generally overlooked is the fact that this type of proportional counter actually combines two functions into one unit. Referring to figure 3 we need to be aware that there are actually two phenomena that take place in the counter, one of which is gas amplification, and the second one is the drift of the ionization charges along the track left by the incident ionizing particle. By reference to figure 3 which shows the electric field and the gradient distribution, it is to be noted that decreasing the diameter of the central wire counter will increase the gain, but at the expense of the counter’s ability to drift the ionization packets towards the counter wire itself. Slow moving, or almost stationary ions of opposite charge will often recombine before they have a chance to be segregated through action of the electrostatic field. This is known as recombination, and is a direct function of the population density of the average number of ions present at anytime. Thus, a proportional counter works best only at low levels of measurement, becoming non-linear for higher activity levels. Since the charge amplification can, in principle, be made quite stable, it is theoretically possible to offer pulse height analysis to distinguish pulses of tritium from pulses of different origins, such as cosmic or terrestrial gamma radiation interacting with the counter gas or with the walls of the counter tube. SIGNATURE ANALYSIS A combination of pulse height and pulse duration analysis is used to select only those pulses that are presumed caused by decaying tritium. Looking at figure 4 it is seen that tritium beta pulses are both weak, and short in length. Thus the pulse duration is also small and short in duration. Pulses due to external gamma radiation are also weak, but much longer in track length and subsequent signal pulse length is much longer and can therefore be discriminated against.

TWO DISTINCT PHENOMENA OF A PROPORTIONAL COUNTER

12 FIGURE 3

region of drift

region of charge

multiplication

radius of counter tube

0.005”

electric field

gradient

COUNTER TUBE

radius

radius of wire

DIFFERENCES IN PULSE SHAPES FOR BETA AND GAMMA RADIATION ON A

PROPORTIONAL COUNTER

13 FIGURE 4

COUNTER TUBE

GAMMA

TRITIUM BETA

GAMMA

200ns

TRITIUM

50ns



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Signature analysis is thus used to discriminate against cosmic radiation and Auger electrons produced at the wall of the counter tube. Further reduction in undesirable background counts is achieved by using materials of construction of low atomic weight for the tube (plastic) and using a carbon coating for the wall conductor. Also, shielding the tube with progressive layers of lead, copper and aluminum to reduce Compton effects is helpful. In this way, using balanced counters, one for background subtraction, it is possible to count as low as 100 Bq/m3 of tritium. AIR DILUTED GAS PROPORTIONAL COUNTERS Although prevalent, a counter system that employs direct mixing of air sample streams with counter gas does NOT really work, despite claims to the contrary. Air contains oxygen, which is an electronegative gas, which highly enhances ion-electron recombination. Referring to figure 3, we are faced with the fact that, in a practically realizable tube we can either promote counter gain by making the wire as small as possible, or to maintain a reasonable drift field by making the wire larger, but one cannot do both at the same time. Commercially available air-gas counters attempt a compromise, but it is easy to see that substantial recombination of changes occur, many ion pairs are simply lost. An obvious conclusion is that such a counter is no longer “proportional” since the loss due to recombination is random. Without proportionality, it is no longer possible to properly apply signature analysis since the essential nature of the signature has been lost. Thus, claims that are made for the use of proportional counters that can distinguish tritium in the presence of other radioactive gases by use of signature analysis must be considered optimistic at best. For these reasons, Air Gas Mixture counters have very poor sensitivity. Furthermore, changes in relative humidity in the sample air will further affect recombination which, results in random changes in counter efficiency. LOW TO INTERMEDIATE LEVELS OF TRITIUM IN AIR MEASUREMENTS WITH LINEAR IONIZATION CHAMBERS As a whole, the use of linear ionization chambers and relatively simple low current amplifiers (electrometers) is enticing. Compared to proportional counters, there is no need for a supply, of gas for the semi-permeable membrane or for



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any other method for tritium transfer, the electronics is essentially far more stable, and simpler. However, linear ionization chamber systems are far less sensitive. Figure 5 shows an ionization chamber consisting of an ionization collecting electrode surrounded by an electric field defined by the enclosing surface of the chamber (wall). The ions collected by the electrode are transferred to the electrometer whose task it is to convert the ionization current into a significant signal voltage. ELECTROMETER LIMITATIONS Until the last decade, it was difficult to construct sufficiently sensitive, reliable and high speed electrometers. Figure 6 shows the arithmetic and the “physics” which gives the relationship between tritium activity, in Curies (or micro Curies) per unit volume (cubic meter) and the ionization current, or net charge per unit time (seconds). Fortunately, the ionization current corresponding to a tritium concentration of 1 µCi/m3 generates an ionization current of almost exactly 1 x 10-15 amperes in a one liter ionization chamber, or if one wished to measure down to 0.1µCi/m3, the current decreases to 1 x 10-16 amperes. Or one could increase the size of the ionization chamber to 10 liters, but, as we shall see later, this is counter productive. Concerning electrometers, there are a few essential properties, which affect performance. The basics are shown in figure 7. The ultimate sensitivity of the electrometer is limited by the size and stability of the two input bias currents, when compared to the size of the ionization current we wish to measure. These two currents are seriously affected by changes in temperature. A common rule of thumb says that thermally induced current changes in semiconductors are of the order of doubling of current for every 10o centigrade. Since the market for electrometer amplifiers is somewhat less than say, TV sets or cell phones, there are few semiconductor manufactures who have taken the trouble to design or produce semiconductors for ultra low current applications. In practice, the best currently available devices can be used to measure to about 1 x 10-16 ampere, provided the amplifier offset bias to each of the inputs are very carefully watched for the temperature range of operation. While good results are

LINEAR IONIZATION CHAMBERS

Measurement of charge (current) produced by the decay of tritium in an enclosed volume

16 FIGURE 5

ELECTROMETER

(-)HT+ +

- -

INSULATOR

INSULATOR

FIGURE 6

Beta decay energy: PEAK 18.6 kev, MEAN 5.6 kevW value, air: 33ev per collision

Mean number of secondaries per primary beta particle5600 ev = 175 ion pairs32 ev

charge of the electron is 1.5921 x 10-19 coulombssecondary charge per beta event:

175 x 1.5921 x 10-19 = 2.78 x 10-17 coulombs per countThe Curie is defined as 3.71 x 1010 events per second

= 3.71 x 1010 Bqthus, a Curie of Tritium generates a current of

2.78 x 1017 x 3.71 x 1010 = 1.03 x 10-6 amperesThe ionization current in an ionization chamber of

1 liter volume, with a tritium concentration of 1µCi/m3

is only 1.03 x 10-15 amperes!

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1Bq27pCi

1kBq27nCi

1MBq27µCi

1GBq27mCi

1TBq27Ci

37Bq1nCi

37kBq

37MBq

1µCi

37GBq1Ci

37TBq1kCi

1mCi

TABLE OF PHYSICAL CONSTANTS

ELECTROMETER PROPERTIES

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FIGURE 7

Rf

-+

Rs

ib(-)

ib(+)

i

eoffset(+) = ib(+) Rs

eoffset(-) = ib(-) Rf

bias currents

eout = -i Rf + ib(+) Rs - ib(-) Rf

bias offset terrms

the bias offset terms cancel if and only if:

Rs = Rf

ib(-)

ib(+)



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relatively easy to obtain at low temperatures, such as 0o C or lower, the situation becomes progressively more difficult as the temperature rises. The satisfactory operation of field effect amplifiers above 40o C or 50o C is difficult to achieve. Instruments not designed properly with this in mind will exhibit large random drifts. Some types of field effect transistors have other unfortunate side effects. In particular, insulated gate metal oxide field effect transistors (IGFETS or MOSFETS) have the unpleasant habit that the static and dynamic electrical parameters undergo slow and pronounced change subsequent to application of power. Hand held portable tritium monitors are battery powered and the batteries are disengaged when the instrument is not in use. When power is applied, the instrument will exhibit a significantly large initial drift, sometimes lasting as long as several minutes. Worse yet, these transistors tend to be very noisy, which limits the speed of response of the tritium monitor. Bandwidth, or speed of response, of the electrometer is very important, as shall be seen later. IONIZATION CHAMBERS FOR LOW LEVEL MEASUREMENTS There are a suprising number of limitations to sensitivity or accuracy associated with physically realizable ionization chambers. To list a few: • Offset drift from insulators • Radon • Background gamma, terrestrial or artificial • Environmental pollutants • Condensation • Specific measurement of tritium in the presence of other radioactive gases. • Measurement of tritium in host gases other than air, such as helium, argon

etc., the Jesse effect. • Wall effects, pressure induced sensitivity changes.



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INSULATORS Figure 5 The insulator carrying the ionization chamber collector element must have exceedingly high insulation resistance otherwise the electrometer will drift. Insulators are characterized by several qualities, three of these, which concern us most. 1. Bulk resistivity: This is the direct shunt resistance associated with

conduction of charge through the body of the insulator. 2. Surface resistivity: This is the sneak path of conduction across the surface

of the insulator. 3. Thermally induced or mechanically induced currents, piezo electric effects. Although clean sapphire insulators have been used in the past, only PTFE or related plastic materials have adequate bulk and surface resistivity since only these materials are hydrophobic. It is the monomolecular layer of water on the surface of insulators that causes poor leakage properties. Unfortunately, PTFE is very piezo electric so that special attention is needed in the design of the shape and size of insulators. Teflon insulators can exhibit nasty temperature induced piezo electric affects if poorly designed. RADON Radon is a common radioactive constituent of air. During decay, an alpha particle with an energy of about 5 Mev is ejected. In other words, the Penning ionization produced by an alpha decay is about a thousand fold greater than that of a tritium beta decay. Noting that the tritium decay rate for a microCurie per cubic meter in a volume of 1 liter is 37.1 events per second, or 2226 events per minute, it is seen that just two alpha decays per minute will produce the same amount of ionization as a tritium concentration of 1 micro Curie per meter cubed. This is definitely not a negligible ratio when measuring low levels of tritium in air. Terrestrial radon background can vary from just a few events per minute per liter of air to many hundreds of decays per minute per liter of air. Furthermore, the activity is generally subject to meteorological conditions and can therefore change significantly in a matter of hours or even minutes. This helps to explain why most older tritium monitors show “unexplained” drift over short periods of time or from day to day.



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Figure 8 shows two electrometer responses. In one instance, the effects of radon alpha pulses have been removed by electronic means. In the second instance, the alpha pulses are fully present but have been somewhat smoothed out. In fact when using relatively small ionization chambers, the relatively massive alpha decays lead to “noisy” instrument behavior. For this reason, some older tritium monitors use large ionization chambers, up to 25 liters. This helps to smooth out the alpha “noise” BUT, the basic drift produced by changes in radon concentration will still remain. In earlier days this drift was frequently mistakenly attributed to unstable electrometers. Radon response can be recognized by using pulse recognition techniques. The ionization current produced by individual alpha decays can readily be amplified by fast electrometers. The potential penalty paid by fast electrometer response is sensitivity to continuous vibration from the instrument sampling pumps or other external seismic forces which affect the piezo electric insulators. Sometimes you just can’t win. BACKGROUND GAMMA Radon is not the only background problem. Cosmic radiation and other gamma radiation are ever present. A gamma radiation level of 1 mR per hour, produces about the same ionization in air as 90 microCuries per meter cubed of tritium. Assuming a typical value of 15 microR per hour, the effect of background radiation is already greater than the equivalent of one micro Curie per cubic meter of tritium. This is not all. Normal atmosphere contains ever-increasing amounts of pollutants and natural organic volatile components. Many of these vapors are readily ionized by cosmic radiation. Dust particles, which are ever present, are massively ionized by triboelectric effects. The obvious way of reducing the effects of background gamma is to use two identical ionization chambers, as shown in Figure 9. This works reasonably well for gamma background which is uniform, i.e. omni-directional and with a zero gradient. It must be remembered that with balanced ionization chambers, the averages will cancel, but the noise associated with the statistical nature of radiation will add.

RADONDecay energy 4 to 5 Mev compared to Tritium mean 5.6 kev

About 1,000 : 1 in energyat 1µCi/m3 in an ionization chamber with a volume of 1 liter

There are 3.71 decays per second or 2,226 decays per minuteThus, 2 radon decays per minute generate

the same mean ion current as 1 µCi/m3 of Tritium!

22 FIGURE 8

1 µCi/m3

with radon alpha pulse suppressionwithout radon alpha pulse suppression

Chart Recorder Traces of Tritium Monitor Output Signal

1 min

1 min

BEST GAMMACOMPENSATION

ELECTROMETER

+ -

+-

γGOODCOMPENSATION

γ

POORCOMPENSATION

γ

ELECTROMETER+

-

23

FIGURE 9

GAMMA COMPENSATION



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For more demanding instances, such as in the design of small portable tritium monitors for use in nuclear power plants, a cruciform design works reasonably well, unambiguous sensitivity of as high as 1µCi/m3 in the presence of even 50 mR/hr can be realized. MOISTURE / CONDENSATION As previously mentioned, moisture on the surface of the collector electrode support can engender serious drift in instrument response. This is overcome by using PTFE or similar materials for the insulators. Such insulators will maintain excellent surface resistance properties even for air moisture concentration up to 95% relative humidity at 40o C or 50o C. However, when the moisture in the sample stream actually condenses, forming liquid droplets or mist, here the ionization chamber is completely disabled. This phenomenon occurs abruptly, and is seen as a sudden violent change in instrument "zero”. Condensation is provoked by adiabatic expansion of the sample flow into the ionization chamber, which can happen if the sampling system and the relative location of the sampling pump is poorly selected. If the ambient relative humidity at the point of sampling is already very high, then transporting the sample over long paths and into a cooler environment (coupled to a pump located down stream) is an invitation to disaster. There is a tendency for system designers to save money by using a single pump and a single tritium monitor when surveying a larger number of sampling locations. This is poor practice, especially when the cost of large runs of piping is taken into account. Turning to hand held portable tritium monitors, the possibility of condensation in ionization chambers is significant if such an instrument is stored at a cold temperature and then abruptly brought into a warmer moister environment. Instruments with poorly sealed or intentionally vented gamma compensation chambers which have been left out in the sun in a highly humid environment will suffer condensation induced performance breakdown when transferred to a cooler environment.



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TRITIUM MEASUREMENT IN THE PRESENCE OF OTHER GASEOUS RADIOISOTOPES A variety of noble gases, Krypton 85, Argon 41, Xenon 133 as well as Carbon 14 are found in some nuclear power plants along with tritium. The now classic way of measuring tritium under such circumstances is shown in Figure 10. Since tritium oxide behaves chemically like water, a desiccant system is interposed between two ionization chambers that are simultaneously used to cancel low level background gamma radiation. FORM SPECIFIC MEASUREMENTS OF TRITIUM Figure 11 shows a diagram in which tritium oxide and total tritium can be measured simultaneously. Subtracting one from the other, one obtains separate measurements for the two chemical forms. JESSE EFFECT As previously described, the total ionization which is generated by an incident beta particle in gas is called Penning ionization. It is a property of the gas in which the ionization takes place. In air, the incident beta particle looses about 33 electron volts during the generation of every pair of secondary charges. In other gases this quantity, referred to as the W function, takes different values, up to 40 eV for helium. More will be said about this later, but the influence of the Jesse effect may need to be taken into account when a monitoring system is calibrated. WALL EFFECTS, THE MEAN FREE PATH This is a colloquial term to describe the phenomena that are related to the fact that the trajectory of the incident beta particle has a finite non-negligible path length. In Figure 12 we note that a beta particle originating near a wall will spend only part of its energy inside the chamber, but spend the rest of its energy in the wall which is therefore lost. This effect is quite pronounced in small chambers, such as used in portable instruments, and such chambers can generally only be calibrated experimentally. Also, if the pressure in the ionization chamber varies, so does the length of the mean free path. However, this effect is generally negligible for low level

IONIZATION CHAMBER MEASUREMENT OF TRITIUM IN THE PRESENCE OF OTHER RADIOACTIVE GASES

The electrometer measuresthe net ion currents,subtracting the currentof the first ion chamber from that of the second.

The net result is ameasurement of HTO only.The ion currents due to HTand the noble gasescancel each other out.

26 FIGURE 10

AIR SAMPLE

IN

HT & HTOand noble gases

+ POLARIZATION HTand noble gases

no HTO

- POLARIZATION

SAMPLE OUT

HTOMEASUREMENT

ELECTROMETER

DESICCANTTRAPS

HTO

SIMULTANEOUS IONIZATION CHAMBER MEASUREMENTOF TRITIUM OXIDE AND TOTAL TRITIUM

TO DETERMINE QUANTITY OF ELEMENTALTRITIUM

27

FIGURE 11

HTOMEASUREMENT

AIR SAMPLE

IN

HT & HTO

+ POLARIZATION

HTno HTO

- POLARIZATION

ELECTROMETER

DESICCANTTRAPS

HTO

HT & HTO

+ POLARIZATION GAMMACOMPENSATION

- POLARIZATION

SAMPLE OUT

ELECTROMETER

HT + HTOMEASUREMENT

(HT + HTO) - HTO = HT MEASUREMENT

28

FIGURE 12

+

-

RECOMBINATION

+

+

-

-

+-

--

-+

+ -

+

+ + -+

-

+

WALL EFFECT

IONIZATIONCHAMBER

WALL

B

HITS WALL

A

DOES NOT

HIT WALL

MORETHAN 5mm

TOTAL PATH LENGTH

APPROX. 5mm



29

atmospheric measurement applications. In any case there is actually a remedy for the problem of wall effects. IONIZATION CHAMBERS FOR WIDE RANGE OF MEASUREMENT AND FOR HIGH LEVEL MEASUREMENTS Ionization chambers in use for process measurement in tritium processing plants, or small chambers used as detectors for gas chromatographs for tritium, as well as chambers which are required to measure over 5 decades or more of tritium activity can suffer from loss of linearity and from “plate-out”. NON LINEAR RESPONSE (LOSS OF SATURATION) In air ionization chambers, the assumption is made that all ions, both positive and negative, are separated from each other as they progress to the electrodes (wall and collector element) of the ionization chamber. As the activity (concentration) of tritium increases within a chamber, so does the population density of the generated ionization pairs. With increasing ionization pair density, the likelyhood that some of the ions of opposite polarity will collide and neutralize will then increase. The ionization current that is actually obtained is then less than what it would be if there were no recombination. Recombination of ions can only be prevented by ensuring that the electric field is sufficiently high and relatively uniform throughout the ionization chamber. Figure 13 shows a view of a typical ionization chamber with a chubby collecting electrode to overcome poor field distribution. It is to be noted that in ionization chambers with rectangular cylindrical geometry, there will certainly be some field degradation in the “corners” of the cylinder. Figure 14 shows a view of the Kanne design which employs a tri-axial arrangement to enhance uniform field distribution. Ionization chambers using a tri-axial configuration will give linear response all the way up to pure tritium. PLATE-OUT, CONTAMINATION Tritium oxide will occlude on most metal surfaces. The walls of ionization chambers exposed to high tritium activity will gradually absorb tritium, which will migrate into the metal. Tritium located at the surface will emit beta particles into the volume of the ionization chamber and will then produce an ionization current even though the ionization chamber is filled with otherwise inert atmosphere.

FIGURE 13

IONIZATION CHAMBER,TYPICAL RECTANGULAR CYLINDER DESIGN

30

- HT

INSULATOR

ELECTROMETER

POOR FIELD WITH SQUARE

“TOP”

BETTER FIELD DISTRIBUTION

WITH ROUNDED “TOP”

TOP

FIGURE 14

IONIZATION CHAMBER,TRI-AXIAL KANNE DESIGN

31

HIGH VOLTAGEELECTRODES

COLLECTINGELECTRODE



32

The effect is more pronounced in small ionization chambers, the surface to volume ratio being higher. Numerous schemes have been tried to remedy this problem. A common method is to apply heat to try to evaporate the tritium, but this can backfire by actually promoting tritium migration even further into the walls of the ionization chamber. Figure 15 shows a chamber which uses an external heater and fins to keep the electrometer section cool so that the unit can actually be operated at elevated temperatures. Figure 16 shows a chamber design with a coiled flat heater strip, which serves as a wall in the ionization chamber. This can be heated almost red hot to drive off the tritium. Reduction in plate-out of the order of two magnitudes was reported for this design. PHANTOM WALL IONIZATION CHAMBERS Figure 17 While hard to put into practice, probably the best way to reduce plate-out is simply to reduce the true surface area of the chamber design by replacing the solid surfaces of a conventional ionization chamber by means of a wire mesh. This mesh constitutes a phantom wall, with chamber properties identical to a solid wall design. The phantom wall wire mesh absorbs all beta particles, and all secondaries from the solid wall of the chamber, the wall effect is gone. Improvement in plate-out is now simply the ratio of the surface of a solid wall as compared to that of the wire mesh. Using stiff wires, a reduction of at least three orders of magnitude is possible. Furthermore, plating the exposed surfaces with gold also reduces plate-out. Gold plating is not expensive and worth the effort. PHANTOM WALL IONIZATION CHAMBERS AS REFERENCE STANDARDS The wire mesh, by itself, in a phantom wall ionization chamber actually defines precisely the true volume of the tritium gas being measured. In a solid walled ionization chamber the mean free path of betas change with ambient pressure and the carrier gas present. The actual response of a solid wall ionization chamber will be seen to vary with these parameters. In contrast, in virtual wall ionization chambers the mean free path no longer plays a role since, primaries and secondaries will propagate uniformly in all directions, both into and out of the electrically defined ionization chamber.

FIGURE 15

IONIZATION CHAMBER DESIGN USES EXTERNAL HEATERSFOR DECONTAMINATION

33

FIGURE 16

IONIZATION CHAMBER:THE OUTER WALL IS A COILED HEATER

34

FIGURE 17

IONIZATION CHAMBER: PHANTOM WALL DESIGN

35



36

Such an ionization chamber can therefore be used as an accurate means of determining the true activity of an unknown gas mixture, rather than the other way around in which a “calibration gas” is used to determine chamber response! MOISTURE TRANSIENT RESPONSE The layer of tritium oxide trapped directly on a solid surface of the ionization chambers does vary somewhat with the relative humidity of the sample flowing through the chamber. The background ionization current, due to plate-out, will change with sample RH and will cause errors in instrument reading. This phenomenon has been reported by AECL as being of concern when contaminated chambers are used for measurement at low tritium concentrations. The phantom wall construction eliminates this problem and is specially recommended for use in tritium specific noble gas monitors such as used in heavy water reactor power plants or D2O storage facilities. REDUCTION IN EFFECTS FROM RADIOACTIVE IMPURITIES ON THE SURFACE OF IONIZATION CHAMBER WALLS The phantom wall mesh acts as a screen to eliminate much of other radiation induced ionization from the solid wall of the ionization chamber. Alpha particles from radon daughters clinging to the solid wall are reduced or eliminated as are Auger electrons arising from external gamma radiation. CALIBRATION OF IONIZATION CHAMBERS AND PROPERTIES OF CALIBRATORS AND GAS STORAGE VESSELS OVERALL CALIBRATION OF TRITIUM MONITORS IONIZATION CHAMBERS Apart from phantom wall ionization chambers, whose physical dimensions accurately describe their true response to tritium, all other ionization chambers can be calibrated using simple methods. In essence, calibration consists of determining or verifying the relationship of the electrically measured ionization current to the activity of the tritium contained by the chamber. As previously mentioned, ionization chambers can suffer from a variety of apparent imperfections such as non linearity due to recombination



37

(known as loss of saturation), as well as variable wall effects, and effects due to differences in work function (the Jesse effect). To completely describe the ionization current sensitivity of any given ionization chamber it is desirable to “calibrate” or test over a whole range of tritium concentration, of different pressures, temperatures as well as for different host gases. Individual sets of tests for each of these parameters can be carried out usually assuming linear independence of these quantities. Such an assumption might not always be entirely justified. A relatively easy way of verifying linearity (loss of saturation) is by the method of consecutive dilution. Figure 18 shows a tritium monitor in a closed loop with a second vessel, together with valves such that the second vessel can be emptied systematically so that the activity of the tritium in the loop is exponentially reduced. Plotting the ionization current versus the number of dilutions on a semi logarithmic paper, any departure from a straight line plot will immediately reveal non-linearity. In doing this test, it is not even necessary to accurately know the activity of the tritiated sample gas used. If this test is carried out over a wide range of measurement, a departure from linearity will be observed at low concentrations, which is due to plate-out or contamination by tritium oxide. The tacit assumption is that the calibration gas used is genuinely what it purports to be. Again, this is not always a valid assumption. Tritiated gas contained in carbon steel vessels will not only permeate through the walls, but also react with the carbon to form formaldehyde! There have been instances where very old certified calibration gas appeared to have lost up to 20% of its expected activity after normal decay was taken into account. Calibration gas can be prepared by using gravimetric principles, with support from analysis for purity using a mass spectrometer. Alternatively, a phantom wall ionization chamber can also be used to assay the gas to be used. CALIBRATION OF THE ELECTRONICS Figure 19 More properly we are concerned with the span and zero stability of the electrometer as time goes by. Previously discussed was the thermally and mechanically induced drifts in zero, but the span, or gain, of the electrometer can also vary with temperature and humidity. Gain is, to all extents and purposes,

FIGURE 18

VERIFICATION OF LINEARITY OF AN IONIZATION CHAMBERBY METHOD OF CONSECUTIVE DILUTION

38

3-WAYVALVE

3-WAYVALVE

TRITIUM SAMPLE IS CIRCULATED THROUGHOUT LOOP

SECOND VESSEL IS EMPTIED

PUMP

3-WAYVALVE

3-WAYVALVE

CLEANAIR IN

SAMPLEOUT

NO FLOW

IONIZATION CHAMBER

UNDER TEST

FLOW

SECOND VESSEL, DILUTION VOLUME

FLOWNO FLOW

PUMP

3-WAYVALVE

3-WAYVALVE

IONIZATION CHAMBER

UNDER TEST

FLOW

SECOND VESSEL, DILUTION VOLUME

FLOW

FLOW

FLOW

FLOW

FLOW



39

determined by the feedback resistor in the transimpedance amplifier of the electrometer. To measure ionization currents as low as one tenth of a femto ampere, the resistance of the feedback resistor is of the order of a terrra ohm. There are few commercial manufactures of such resistors. Tests carried out on such resistors show temperature sensitivity, and, above all, sensitivity to changes in humidity. Relative humidity can change significantly with small changes in temperature. On reaching 100%, condensation will take place on its surface, the conductivity of these resistors can abruptly change by factors of a million! The sensitivity, or calibration, of poorly designed tritium monitors can change unpredictably from day to day. TRANSFER GAMMA CALIBRATION (QUICK AND DIRTY) Since calibration with actual tritium sample gas is a somewhat arduous and time consuming task, it is desirable to perform gas calibration only a single time, followed by other means for later verification. Tritium monitors for room monitoring or for other environmental monitoring, with relatively high sensitivity, can be checked by means of a small gamma radiation source held close to the ionization chamber. Once a monitor has been gas calibrated, and its equivalent response to the gamma check source (in a defined location on the surface of the ionization chamber) has been determined, then the response of the monitor can be confirmed at any time, and it should, in principle, always give the same response. It must be remembered that this gamma response is subject to changes in conditions of temperature and pressure. In practice, small corrections can easily be calculated. ELECTRICAL CALIBRATION Figure 19 A current simulating an ionization current can be introduced into the electrometer in parallel with the ionization chamber electrode. Having once established calibration or a monitor with tritium calibration gas, its direct equivalent electrical response can be determined. Such a “simulation” current is injected by attaching a high meg ohm resistor, and calculating the current on the basis of Ohm’s law. Of course, it is assumed that the high Meg ohm resistor itself is free from drift in value.

ELECTROMETER CALIBRATION

40 FIGURE 19

simulated ionization current is:

is =ein

Rc

Rf

-+

Rcis

eineout

eout Rf

ein Rc(-) =



41

PRACTICAL DESIGN LIMITS FOR LINEAR IONIZATION CHAMBER TRITIUM MONITORS The forgoing discussions really centered about various undesired influences as well as noise and drift in the electronics. Noise can be analyzed statistically, at least some of the time. The nature of stray cosmic or terrestrial gamma radiation and that of radon alpha radiation is vastly different from a signal point of view. The radon alpha pulses are of sufficient energy that they can be electronically recognized and electronically eliminated. Older designs for tritium monitors do not feature such strategies, and use electronics that try to smooth out the effects of alpha pulses. This imposes a limit of the apparent “noise level” of the instrument. Worse than that, with changing radon background, the instrument “zero” will proceed to drift! Instruments without alpha pulse suppression are limited to measurements only well above 100 kBq/m3 (2-3µCi/m3). Response limits for instruments that are capable of distinguishing against radon alphas are much better. Here the limits are now set only by the statistical nature of gamma radiation. It is remembered that a system with dual (or multiple) ionization chambers for gamma compensation will exhibit noise fluctuations corresponding to the entire set of chambers, even if the averages happen to cancel out. The chamber background noise is reduced by simple electronic filtering which, in the final analysis, imposes limits of rate of response for any practical monitoring system. As a first approximation, based on statistics, increasing chamber volume by a factor of ten will provide only a factor of 10 in average noise. Thus, to raise sensitivity by an overall factor of ten one needs, for example, to increase volume by a factor of ten as well as increasing the damping time constant by a factor of ten. This process obviously has practical limits. In general, we have found that, with lead sheathed dual ionization chambers each with a volume up to ten liters, and electronic instrument time constants of the order of one minute, it is possible to obtain instrument sensitivities of the order of 0.1µCi/m3 (4 k B q/m3). This requires very high quality dust filters and very stable noise free electronics.

tritium and its measurements. - technical … and its measurement … · instruments for ultra low...

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