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deck. These are usually considered by the m anufacturer of the Instruments. However, for the present purpose, to ensure no personnel get access into the high voltage enclosure, the region of which is isolated by a fence with one door, a switch is mounted such that the power supply can be connected ohly when the door has been shut. A red bulb is lit when the high voltage is on. An irradiation timing control is also included. The arrangement is shown in Fig.3.8.
3.2 Alpha Irradiation Arrangement
Several different types of arrangement for alpha particle irradiation in cell studies have been reported. For an ideal device of alpha irradiation, two criteria are of special importance (Roos & Kellerer, 1986). First, the fluence has to be reasonablly uniform over an extended sam ple. Secondly, the energy needs to be sufficient for penetration of a monolayer of mammalian cells, and the dose m ust not vary strongly through the region of interest.
To fulfil the first condition, one could use a uniform and sufficiently large source. Its diameter would have to be three times as large as that of the sample. This ensures a uniform fluence but is necessary to also ensure uniform energy deposition and so a
collimator is introduced to perm it only normal incidence to the cells. For example, Simmons et al. used an active area of diameter of 10 cm source for the studies of hum an lung cell lines (Simmons
et al.r 1983; Min et al., 1985). A collimator was used to further
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typical collimator, reported by Roos and Kellerer, has a height of 15 mm and 3 mm diameter of its channels).
Another method is to separate source and sample by a large distance in vacuum, such that an approximately parallel beam can be obtained and yet the alpha particles will not Idse much of their energy in transit to the target {e.g., Barendsen & Beusker, 1960).
A m ost effective method, according to Roos and Kellerer, is to design a moving collimator, the fluence inhomogenicity may be
reduced to 3% while the transparency of the collimator can be over 80%. To reduce energy loss in the channels of the collimator, source and collimator can be mounted in a container which is flushed with helium under normal pressure (no vacuum is needed).
In the present work, to enable a quick preliminary test of the damage model, it was considered reasonable to dispense with collimator and to perform the irradiation under fixed geometrical conditions. This means that care m ust be taken to calculate the interaction parameters (LET, Ij etc.) for the spectrum of alpha particles that traverse the cellular targets.
Two alpha sources are utilized for the experiment. These are so used to obtain data th at can be used to test the proposed DNA-rupture model. Also a comparison can be made to the low LET result. Of the two sources, one (^"^^Cm) is used to deliver an acute and the other (^"^^Am) is used to deliver a chronic irradiation. The arrangement of the two sources is given in Fig,3.9. The basic data of the two disc sources are as follows:
ü 2 L O CD Q T CD ro to C\J CL u ü < 03 O ^ O) j9 - co en JQ UJ ü g en
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Isotopes 2'^CmgQ
Energy spectra 5.801 MeV, 76.7%; 5.477 MeV, 85.1%; 5.759 MeV, 23.3%; 5.435 MeV, 12.6%;
5.378 MeV, 1.7%;
Activity 505 jiCi 1.35 pCl
Both isotopes have long half-life (18 and 433 years respectively) and are disc sources of 6 mm diameter. The curium is vacuum
sealed with a 4 jim titanium safety window; the americium is a conventional source and its gamma-ray contribution is known to deliver a negligible dose.
3.3 Methods of Dosimetry
In m ost circum stances, dosimetry is the m easurement of absorbed dose (or abbreviated simply as dose) by m eans of dosimeters. The measurement can involve basically determining the energy spectra and/or the fluence rate of the incident particles to be measured. There are a num ber of different dosimetric methods and they have been extensively discussed (see Attix et al,
1966-1969).
Dosimeters which depend on the collection of electric charge include ionization cham bers, p ro p o rtion al and Geiger-Miiller counters. The solid state or sem iconductor dosimeters are relatively new. They have better energy resolution and perm it accurate measurements of energy spectra. Indirectly methods by cloud chambers and photographic emulsions may allow dosimetry of ionizing radiation over a wide range of dose, a wide range of time and a wide range of area.
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In this discussion, only the dosimetric methods used for the electron and alpha particle detection in the present work will be discussed. As in the present situation, experiments on biology may be considered preliminary.
For the electron irradiation, both the calorimetric method and the Faraday cup have been used previously in this laboratory for electron energies below 10 keV (Iskef, 1981; Al-Ahmad, 1984). These two methods are reviewed and extended to the present purpose. They are discussed in Secs.3.3.1 and 3.3.2, respectively. For the alpha particles' irradiation, a sim ple extrapolation ionization chamber was used. This will be described in Sec.3.3.3. 3.3.1 Calorimetric method
3.3.1.1 Introduction
The calorimetric method has been developed as a convenient technique applied particularly to measurement of energy fluence of
x-ray and electron beams so as to be able to measure the total absorbed dose (ICRU, 1964). For a calorimeter, it is unnecessary to
consider the factors in the foregoing discussion as it measures the energy deposited in the m aterial of interest directly. Therefore, it is often used as a calibrator of other dosimeters utilizing the secondary process, te., the indirect measurement. It is known that the sensitivity of a calorimeter is insufficient for radiation protection purposes and also due to scattering of the incident particles an d /o r rejected particles, some of the energy may be
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m ismeasured. However such disadvantages can be overcome by appropriate precautions and corrections (Laughlin & Genna,
1966).
There are two methods of calorimetry, i.e., the isothermal one and non-isotherm al one. As it is difficult to establish a therm odynam ic equilibrium state to satisfy the condition of constructing an isothermal calorimeter, it is easier in practice to utilize the non-isotherm al method. Among the non-isotherm al methods, the 'constant tem perature environmental method" is simple and explicit as long as correction for leakage is made.
3.3.1.2 Temperature change in calorimeter
Incident particles of ionising radiation traverse an energy absorber and lose part or total of its energy in the absorber. The net absorbed energy is released in the form of heat energy which results in the rise of the tem perature in the absorber. The tem perature variance can be measured by a therm istor and calibrated in terms of absorbed dose, te., the energy absorbed in a unit volume, being measured.
There are two ways of measuring temperature change. One is using a thermocouple (or thermopile composed of a num ber of thermocouples), which is made of a pair of conductors of two different metals, to measure the electrom otive force (emf) generated at their junctions. The emf is about 50 mV per and is detected with a potentiometer. However, the second method, resistance thermometer reveals a higher sensitivity compared with
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the thermoelectric thermometer.
The resistance thermometer can be a platinum resistor or a sem iconductor therm ometer, know n as a therm istor. The resistance-tem perature relationship of a therm istor of platinum is sim ply linear b u t for a sem iconductor it iè approxim ately exponential, te,
(3.2) where p is the characteristic tem perature constant (°K) and can be determ ined by plotting logR versus 1/T. As the tem perature coefficient is defined by a.p=(l/R)(dR/dt), we have, by substituting Eq.(3.2) and dR/dT= (-PRo/T^)eP(^ 1 by differentiating R with respect to T
OT = - P/T^ (3.3)
for platinum, a.p (at 298 °K) is about 0.4, and for semiconductor it is about one order higher (negative). Obviously, in order tb obtain higher sensitivity in m easuring a small tem perature change, a semiconductor therm istor is preferred.
3.3.1.3 Heat transfer loss in calorimetry
It is necessary to first discuss the mechanism of the heat transfer in a calorimeter, i.e., the relation between changes in thermal energy which is transferred from the radiation energy and tem perature rise caused by the incident particle and therefore the
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design of the calorimeter is contributed to methods of tem perature change detection and, the energy leakage correction.
According to Newton’s law of cooling, the total heat transfer loss can be expressed as
(l/A)(dE/dt)= - £ h i (T-Tq) W/m2 (3.4)
where A is the concerned area in a calorimeter, i= l,2,3 in hj which represent for the heat transfer coefficients for convection, conduction and radiation, T and Tq denote the tem peratures of the
calorimeter absorber and its surroundings respectively (Laughlin & Genna, 1966). They are briefly described in the following.
i) H eat co n v ectio n is not observed when the Raleigh num ber R=mgd^AT<1620. Where d (in cm) is the separation between the two parallel plates in a specified experiment, AT (in degrees centigrade) is their tem perature difference, and m^. (in cm"® ®C"^) is the convection modulus of air. Since the convection modulus is proportional to the square of the density w hich in tu rn is proportional to the pressure. P, then no heat convection will occur when the following condition is satisfied
P/760 < [16.2/(d® AT)]1 / 2 (3.5)
P is in torr. In the present study, let d be the separation between the calorimeter and its jacket (less than 3 cm). As AT is measured always less th an 10 and P less than 10 torr, therefore, convection can be ignored.
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ii) H e at c o n d u c t io n can tra n sp o rt energy through
interm olecular collisions in a calorimeter from the therm istor to its surrounding air or enclosure. This can be described by the heat flow equation. In Eq.(3.4), hg or h^ is known in the cylindrical surfaces:
hg= k /[r ln(l+d/r)I (3.6)
where r is the cylinder radius, d is the separation, and the thermal