344 Nuclear Instruments and Methods in Physics Research A234 (1985) 344-353 North-Holland, Amsterdam

F A B R I C A T I O N O F A M U L T I W l R E T I S S U E E Q U I V A L E N T P R O P O R T I O N A L C O U N T E R AND ITS U S E IN 14 MeV N E U T R O N D O S I M E T R Y

L.S. C H U A N G , Y.N. C H A N and H.K. W O N G

Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong

Received 13 March 1984 and in revised form 10 August 1984

Fabrication and performance details of a multiwire tissue equivalent planar proportional chamber are presented. Together with an AI-Ar dual chamber, it is then used to evaluate the fast neutron and gamma ray tissue-absorbed dose at one point of the mixed neutron-gamma field produced by a 14 MeV neutron generator. Comparison of the result with that of the Monte Carlo computer simulation of the neutron-chamber interaction shows a good agreement. Further improvement in the accuracy can be obtained by increasing the accuracy of the evaluation of the relative neutron sensitivity of the dual AI-Ar chamber.

1. Introduction

Accurate neutron dosimetry is required for applica- tion of fast neutrons in radiotherapy of cancers as well as in radiological protection of personnel. A multiwire planar tissue equivalent (TE) proportional chamber was constructed to evaluate the 14 MeV neutron tissue-ab- sorbed dose at the point of interest inside the neutron field under receptor conditions. The principle is that under 14 MeV neutron irradiation of the Shonka A150 tissue equivalent cathode plastic, the various ions pro- duced by the various neutron induced nuclear reactions are collected by the anode wires. The total ion current measured, after calibration against a standard 137Cs gamma-ray source, should give the rate of mean energy imparted by the neutrons to the unit mass of tissue which is the definition for the neutron tissue-absorbed dose rate. If a pulse height spectrum is measured instead of the ion current, additional information on the linear energy transfer (LET) spectrum can be obtained and a more realistic practical quantity of the neutron tissue dose equivalent rate can be inferred in principle.

An independent assessment of the gamma-ray dose component arising from the neutron generation or inter- action of the neutrons with surrounding objects is essen- tial. We used the dual chamber technique which utilizes an aluminum chamber identical in geometry to that of the TE chamber for evaluation of the gamma absorbed dose at the point of interest. Further, the relative neu- tron response of the AI chamber was determined so that this can be deducted from the gamma absorbed dose assessed.

The advantages of the TE multiwire chamber over the conventional spherical [1] or cyclindrical [2] ones are:

0168-9002/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

a) The gas multiplication factor increases because of the additional capacitance.

b) The sensitive volume increases by a factor N, repre- senting the number of anode wires increased, and thus the sensitivity of the chamber is improved by the same factor.

c) As the chamber is constructed in the planar shape, it simulates planar tissue like that of the abdomen.

d) It can be operated under a strong magnetic field - up to 1 T.

e) It has the capability of measuring various microdosi- metric quantities like LET, linear energy etc. of various charged secondaries produced by neutron irradiation.

f) It is also possible to detect the positions and time of traversal of these induced charged secondaries with an error less than + 1 mm and + 3.0 ns respectively if construction of the chamber is further improved [3]. The disadvantages are:

a) The electric field distribution around the anode wires is complicated because of the anisotropic geometry, which makes theoretical studies and analysis of the experimental pulse height spectrum difficult.

b) The pulse height decreases, because of the electro- static induction between the neighboring anodes wires and the additional capacitance.

c) The acceptable maximum counting rate decreases due to pulse pile up. In the presentation that follows, we will first describe

the construction details, the characteristics and perfor- mance of the multiwire TE planar chamber. Then we will show how the neutron and gamma absorbed dose rate at one point of the mixed neutron-gamma field produced by a 14 MeV neutron generator were de

L.S. Chuang et al. / Fabrication of a tissue equivalent MWPC 345

termined using the multiwire TE planar chamber and the dual AI chamber. Finally, a description of the evaluation of the relative neutron response of the AI chamber, and use of the Monte Carlo method in verify- ing the 14 MeV neutron tissue absorbed dose rate obtained will be given.

2. Chamber structure and electronics

2.1. Fabrication of the multiwire planar TE chamber

The simplified schematic diagram of the chamber is shown in fig. la and a photograph of the chamber is shown in fig. lb. The multiwire anode plane is inserted midway between the two cathode planes, which are insulated from the anode by two teflon and two bakelite spacers. This structure allows one to take into account the direct incidence as well as backscattering of the neutrons.

The cathode planes have an effective detection area of 10 cm by 10 cm so as to simulate a planar soft tissue surface like those of the abdomen and the back of the human body. Both are made of Shonka A150 TE plastic * of 3 mm thickness. They are maintained at ground potential with respect to the anode wires by intimate contact with aluminum frames so that the background electric field can be shielded out properly.

The anode stainless-steel wires ** are soldered on a bakelite plane with one end insulated from the other and the other in common to give electrical signals. The solder points are minimal in size and far from the edges, otherwise electrical breakdown will be apt to occur. The anode wires have a radius of 29 #m and are maintained with a tension greater than 40 gwt.

Ross tissue equivalent gas ÷, which has as compo- nents nitrogen, carbon-dioxide and methane (4.0%, 31.4% and balance, respectively), is used as the counting gas as well as tissue simulating material. The purified TE gas flows into the chamber through a gas handling system which maintains uniformity in distribution and true quasi-static state of the gas. It then flows out from the gas-outlet or diffuses through the chamber walls to open air with a constant flow rate of approximately 30 cm 3. rain- 1.

A guard-strip of square shape is inserted between the anode and cathode planes to eliminate end effects. To avoid too high a gradient of the electric field near the edges of the cathode plane, successively thicker wires should be used for the last few wires [4]. Alternatively,

* Supplied by Physical Sciences Laboratory, Illinois Benedic- tine College, Lisle, Illinois, 60532, USA.

** Available from the California Fine Wire Co, PO Box 446, Grover City, USA.

+ Obtained from SOXAL Special Gases, Singapore Oxygen Aire Liquide 'Pte' Ltd.

avoiding the use of any anode wires near the edges for approximately 3 cm as in the chamber constructed presently, may serve the same purpose.

2.2. Electronics

There is basically no difference between the connec- tion of a positive high voltage to the anode or an equal amount of negative high voltage applied to the cathode planes (figs. 2a and b). We used the former setup to observe the pulse height spectrum and the latter to measure the mean ionization current.

The positive high voltage is applied through a 22 MI2 protection resistor to the anode wires and guard strip from the preamplifier. Thus the high voltage power supply is protected from possible damage due to a sudden large current flow when anode and cathode are shorted (e.g. electrical breakdown, gaseous discharge or a broken wire touching the anode). Nevertheless, the FET of the charge-sensitive preamplifier may still be damaged under such circumstances because it is so sensitive to large voltage fluctuations. Thus it is sug- gested to perform some electrical tests to ensure that no sparking occurs and that it has a minimum leakage current. Better elimination of the end effect and avoi- dance of breakdown become possible when the guard- strip are at a suitable potential [5].

When a negative high voltage is applied to the cathode planes, the problems of safety and leakage current become significant. A good insulator, e.g. teflon, can be used to shield the chamber completely but this will slightly disturb the neutron field to be measured. Nevertheless, we shield the chamber properly. To mini- mize the leakage current arising from background elec- tromagnetic field, dirt and moisture, the chamber is cleaned thoroughly and wrapped with an aluminized mylar foil to provide a good external common ground shield. Also, a dehumidifier and dry nitrogen gas jet were used to dry the air around the chamber.

The signal wire from the chamber to the preamplifier and electrometer were kept as short as possible to reduce the input capacitance and stray noise so that a large signal to noise ( S / N ) ratio can be achieved.

3. Chamber characteristics and pedormance

Investigation of the major characteristics of each chamber is essential for an absolute determination of the 14 MeV neutron tissue absorbed dose because the method involved is chamber dependent.

3.1. Counting rate plateau curve

Using an Am-Be neutron source of neutron emis- sion 2.5 X 106 n / s (obtained from the Radiochemical

346 LS. Chuang et al. / Fabrication of a tissue equivalent MWPC

Centre, Amersham, England) a curve of the counting rate versus high voltage applied was obtained (see fig. 3) for the 3-wire TE chamber with Ar as chamber gas. An optimum operation voltage of 1550 V was then chosen for all the subsequent investigations.

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S C A L E I J I ¢m Fig. la. Schematic diagram of the tissue equivalent multiwire planar chamber (vertical cross section view).

3.2. Counting gas pressure

The counting rates were found to increase by less than 30% while the maximum pulse heights decreased by less than 5% when the Ar counting gas pressure applied to the 3-wire TE chamber decreases from 35 mm Hg (with respect to atmospheric pressure) to 1 mm Hg step by step. The optimum counting gas pressure was then taken to be 1.2 mm Hg for reasons of economy and stable operation.

3.3. Gas amplification

The gas amplification factor A, which is defined as the ratio of the final number of secondary ion pairs due to avalanche processes in the counting gas to the num- ber of primary ion pairs, has a value which depends on the electric field distribution and the counting gas used in the gas cavity, etc. The important factors affecting A for the 3-wire TE chamber with a wire spacing of 2.8 cm include:

(a) The high voltage V, applied to the anode wires: In A versus V has a good linear relationship as indicated by fig. 4. This is analogous to the case in the single-wire cylindrical proportional counter because the multiwire chamber can be conceived as a simple superposition of independent single-wire chambers if the spacing be- tween the anode wires of the multiwire chamber is sufficiently large.

(b) Flow rate of the counting gas. When the flow rate of the counting gas changes from 15.4 cm 3. min -1 to

Fig. lb. Photograph of the TE and A1 multiwire chambers.

L.S. Chuang et al. / Fabrication of a tissue equivalent M W P C 347

a

+HV

22 M~ l ~/V ~.

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( Canberra 1406 ) Cathode and vacuum vessel

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Fig. 2. (a) Circuit diagram for obtaining pulse height spectra. (b) Circuit diagram for measuring mean ionization current.

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10 3

10 2

Neutron Source: Am- Be • Chamber: 3-Wire TE Type /

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10 I I I I I )m 800 I000 1200 1400 1600

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Fig. 4. Recorded maximum channel number variation with applied voltage in the 3-wire TE chamber.

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Neutron Smlroe: Am-Be Chamber: 3-Wire TE Type

/ /

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Optimum Operating Voltage (1§§0 V)

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Fig. 3. Count rate variation with applied voltage in the 3-wire TE chamber.

182 cm 3. min - l , the maximum pulse height (i.e. channel numbers) varies by less than 1.6%. The counting rate has not been found to have any statistically significant change.

(c) Geometry. It can be shown that [6]

d A / A = k z d r / r , with k z = 5-20, (1)

d A / A = k 2 d s / s , with k 2 = 5-15, (2)

d A / A = - k a d L / L , with k 3 = 8-15, (3)

where r is the radius of the anode wire, s the spacing between anode wires and L the spacing between cathode plane and anode wire plane. It was found that of the three factors above, the non-uniformity in wire spacing s gives the most variation in the output pulse height between two different wires. Since the wire spacing used in the present chamber is large (28 mm) compared with a few mm as commonly used, we expect the error in the output pulse height due to the chamber structure not to be too large.

(d) Total number of anode wires. When the number of anode wires soldered, in parallel and in common, increases the output pulse height decreases as [7]

relative amplitude - n - 0 .4(n - 1) , n

where n is the number of anode wires in parallel. In fact, it was found that when more anode wires were used, which is useful in large area low neutron flux dosimetry, the output pulse heights become so low that a higher applied voltage a n d / o r electronic amplification

348 L.S. Chuang et al. / Fabrication of a tissue equivalent M W P C

,°, t ! _g i 104

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Fig. 5. Count rate variation with distance squared in the 3-wire TE chamber.

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Start of Neutron Date

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Fig. 6a. Pulse height spectrum measured by the 3-wire TE chamber under 14 MeV neutron irradiation.

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Channel Number Fig. 6c. Pulse height spectra measured by a Rossi TE propor- tional counter under 1.4 MeV neutron and 6°Co ~-ray irradia- tions.

LS. Chuang et al. / Fabrication of a tissue equioalent MWPC 349

were needed to give an observable pulse height spec- trum.

(e) Counting gas. Of the two types of counting gas used, namely Ar and TE gases, Ar gas gives a higher gas amplification factor as shown clearly by the higher S / N ratio of the output current measured by the electrometer under identical conditions (see sect. 4). For the TE 3-wire chamber, with TE gas supplied, the S / N ratios were about 1.06 and 3.6 for TE gas and Ar gas as the chamber gas, respectively.

3.4. Distance between the A m - B e neutron source and the chamber

Varying the distance between the Am-Be neutron source and the multiwire TE chamber along the symme- try axis, the counting rates recorded at different dis- tances squared is plotted in fig. 5. In general, this counting rate should change according to the inverse- square law. However, our results deviated from this law because of the high background counting rate due to the small dimensions of the counting room (a pit of size 2.4 m × 3 m × 2 m) and the obstacles inside it.

3.5. Pulse height spectrum obtained under 14 M e V neu- tron irradiation

A pulse height spectrum of the 3-wire TE chamber resulting from the 14 MeV neutron irradiation is shown in fig. 6a with the irradiation geometry as shown in fig. 6b.

The 14 MeV neutron source was the Kaman Science Model A-711 neutron generator which is a miniature sealed-tube accelerator capable of producing quasi- monoenergetic 14.3 MeV neutrons of a total flux in excess of 10 H n / s [8]. By means of a liquid organic scintillation detector of type NE213 coupled with a pulse shape analyzer system, the relative differential

flux density spectrum, S(E) , of the neutrons from the neutron generator has been obtained. The absolute dif- ferential flux density, F ( E ) , is related to the relative one simply by a proportionality constant b, i.e., F ( E ) = bS(E) . This constant b can be determined easily by neutron activation of an aluminum foil placed at the position of the TE multiwire chamber utilizing the nuclear reaction 27Al(n, a) 24 Na [9].

For the present experimental conditions, as shown in fig. 7, b was determined to be 4.01 x 103 ns -1 cm -2 at a beam current of 2.2 mA of the neutron generator and the neutron flux incident normally on the TE planar chamber surface was 7.5 × 105 ns -~.

The collimator as shown in fig. 7 is constructed of paraffin filling a hollow cylindrical iron can.

The typical pulse height spectrum derived from a Rossi-type (spherical) tissue equivalent proportional counter [10] is shown in fig. 6c. The minimum at point 1 can be selected as the lower limit of neutron events. The starting point of the proton dropoff at point 2 is defined as the proton drop point. This corresponds to a slow proton recoil having the highest linear energy transfer or stopping power traversing the diameter of the spherical cavity. Although the geometry of the present TE cham- ber is quite different from that of the Rossi type the general shape of the pulse height spectrum and the two characteristic points can still be observed. In fact, Kellerer [11] has used the 'dose mean energy imparted per event' as a criterion and proved that a cylinder with mass equal to that of a sphere, of diameter and height 0.87d, is nearly equivalent to a sphere of diameter d for unidirectional irradiation perpendicular to the axis of the cylinder. In the case of a multiwire proportional chamber, each wire can be treated very roughly as one independent cylindrical single-wire chamber if the sep- aration between wires is sufficiently large [12]. However, the proton drop point of our pulse height spectrum is smeared out by the dependence of stopping powers and

Neutron Generator

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Unit: cm Fig. 7. Arrangement of the 14 MeV neutron generator, the collimator and the detector.

+r-r1 ÷F -I ±[-i-] D e t e c t o r

350 L.S. Chuang et al. / Fabrication of a tissue equivalent MWPC

ranges on energy and recoil species. The recoil particles will lose energy in traversing the TE plastic before striking the gas cavity. In fact, for a Bragg-Gray cavity, if the initial energy of all recoil particles is to be Ep, the recoil particle energy distribution striking the cavity extends from Ep to zero and varies as 1 / S ( E r ) , where S ( E r ) is the stopping power of a recoil particle of energy E r. The geometry of the TE plastic and the angle of the recoil particle also affect this energy deposition process. Another factor is that the track length distribu- tion of particles crossing the cavity is different for different chamber geometries. The complicated electric field distribution inside the multiwire proportional chamber also gives quite a non-uniform gas amplifica- tion inside the chamber.

4. Dual chamber technique

4.1. Basic principle [13]

In general, one instrument (T) is usually constructed to have approximately the same sensitivity to neutrons and photons, whereas the second instrument (U) will have a lower sensitivity to neutrons than to photons. Thus for the same mixed field, the quotients of the responses of the dosimeters, T and U, by their sensitivi- ties to the gamma rays used for calibration, R~- and R~j, respectively, are given by

R" T = k T D N + hTDG, (4)

R ' U = k u D N + huDG, (5)

where D N and D G are the absorbed doses in tissue of neutrons and of photons, respectively, in the mixed field if the instrument T is truly tissue equivalent, k T and k U are the ratios of the sensitivities of each chamber to neutrons to the gamma rays used for calibration, and h T and h v are the ratios of the sensitivities of each cham- ber to the photons in the mixed field to its sensitivity to the gamma rays used for calibration, respectively.

Solving the simultaneous equations (4) and (5) gives

h u R ' T - kTR 'u (6) DN h u k x - h T k U '

kTRPu - - k u R ' T (7) D G - h u k T _ h T k U •

The energy spectrum of gamma rays in the mixed field is cut off at about 0.2 MeV according to the result of the measurement using an organic liquid scintillation detector-pulse shape analyzer system. This spectrum is quite similar to that of 137Cs gamma source used for calibration. Hence h T = htj = 1 is taken for simplicity as suggested [13], due to the small differences in the mass absorption coefficients, about 0.07 cm2g -1, of the com- posite materials of the two different chambers for pho-

tons of energies of about 0.16 and 0.67 MeV, as at present. For the tissue equivalent plastic chamber the sensitivities of the chamber to both neutrons and gamma rays are about the same, so k T is also set to unity. Consequently, eqs. (6) and (7) can be simplified to

R~- Rh (8) DN 1 - k U '

R'u - k u R ' T (9) DG 1 - k u

Hence the quantity ku, the relative neutron sensitivity of the neutron insensitive instrument, must be consid- ered separately in order to give D~ and D c simulta- neously.

4.2. Mean ionization current obtained under 14 M e V neutron irradiation

Using an electrical connection as shown in fig. 2b, the mean ionization currents for various combinations of chambers (TE or A1) and counting gases (TE or Ar) under 14 Mev neutron irradiation were measured. The chambers were placed in the same position as shown in fig. 6b. The results, after subtraction of the background current, are shown in table 1.

The dual A1 chamber was constructed to assess the gamma component in the mixed neutron-gamma field. In terms of elastic scattering, aluminum is less sensitive to fast neutrons than the TE plastic as it has a large atomic and mass number.

Both chambers were electrically insulated and grounded to avoid electric shock and minimize the external EM wave background. A thin hollow-thick lucite frame followed by an aluminized mylar foil were used to shield the chambers. A dry nitrogen gas jet was allowed to blow continuously on the cathode planes to minimize condensation of the water vapour on the chamber surface.

A Keithley 610C electrometer of sensitivity down to 10 -11 A was used to measure the mean ionization currents.

For the same chamber type (TE or A1), using Ar gas always gave a greater current value than TE gas because of the greater gas amplification of Ar (see table 1). Also, for the same counting gas used (TE or At), the A1

Table 1 Mean ionization currents for various combinations of chambers (TE/AI) and counting gases (TE/Ar), under 14 MeV neutron irradiation

Counting gas Chamber TE Chamber A1

TE 0.25 × 10- s A 0.75 X 10-10 A Ar 0.13 × 10 -6 A 0.15 × 10 - 9 A

L.S. Chuang et al. / Fabrication of a tissue equivalent MWPC 351

chamber always gave a smaller current value than the TE chamber because in the mixed neutron-gamma field produced by the 14 MeV neutron generator, the gamma rays contribute only 10% of the total kerma and the relative neutron sensitivity of the aluminum chamber with Ar (or TE) as counting gas was only around 0.23 as shown in the following section.

4.3. Relative neutron sensitivity of the A I - A r chamber

Using the arrangement of the neutron generator, the collimator, and the liquid organic scintillation detector as shown in fig. 7, the relative (normalized) differential flux density spectra of the neutrons were obtained at positions (1), (2), (3) and (4), respectively. By placing the A1-Ar chamber at these positions, the mean ioniza- tion current were measured. These current values, to- gether with the percentage fraction of the 14 MeV peak region (12-16 MeV), taking the value at location "1" to be 100%, are tabulated in table 2. It should be noted that in these spectra there are two peak regions, namely, the 14 MeV peak ranging at 12-16 MeV, and the lower energy peak below 4 MeV. The 14 MeV neutron peak is clearly due to the monoenergetic neutrons resulting from the D - T reaction. The lower energy region is mainly due to the perturbation effects of elastic, inelas- tic carbon scattering and alpha-producing reactions in the detector, which has not been duly corrected in the computer program for unfolding the neutron energy spectrum. This fact has been confirmed by Chuang and Wong [9] using a set of threshold foil-detectors.

The k U value relative to 137Cs gamma rays can be calculated as follows:

zaI t " , k U = D ~ A D N (10)

where A I is the difference in the measured mean ioniza- tion currents at positions 2 and 3, where the gamma ray contaminations were confirmed to be approximately the same (better than 10%) by counting the gamma pulses in the time spectra obtained by means of the liquid

organic scintillation detector-pulse shape analyzer sys- tem. D~ is the dose sensitivity of the AI-Ar chamber to 137Cs (= 0.21 C Gy-a). AD N is the difference in dose rates of neutrons delivered to the A1-Ar chamber at positions 2 and 3 and determined as

E . = 1 5 . 5 M e V

abN = E a,~(E).K(E), E n ~ 3 . 9 M e V

with K ( E ) (neutron kerma factor in ICRU muscle at neutron energy E) obtained from a tabulated table [13] used to approximate the dose rate to first order, and A~b(E) the difference of the neutron flux at positions 2 and 3. As explained earlier, neutron events recorded at energies lower than about 3.9 MeV are unreal due to neglect of the corrections in unfolding the neutron spectrum, and the nuclear reaction 27Al(n,p)ZTMg which contributes much ionization from the A1 cathode, has a threshold energy of 4 MeV. Thus, A/) N was determined to be equal to 3.66 × 10 -7 Gy s -1. Finally, with the current values shown in table 2, k U was evaluated to be 0.23.

Goodman and Colvett obtained a value of k U of 0.13 at a neutron energy of 15 MeV for an 0.1 cm 3 cylindri- cal AI-Ar chamber [14]. An ionization chamber con- structed with the same wall and gas materials, but of markedly different size, configuration or gas pressure will have a different relative neutron sensitivity. Also, the dependence of k u on neutron energy is equally important. Hence a more realistic approach to evaluate k o is to use a strongly anisotropic neutron field mixed with an isotropic gamma field so as to obtain more data points at different positions. These are then used to unfold k u ( E ) by computer iteration. Thus, our present k v value is a weighted average for neutrons of energies ranging between 12-16 MeV. Substituting k u =0.23 into eqs. (8) and (9), with the current responses R: r and Rb set to those of TE-TE and A1-Ar from table 1, and the dose sensitivity of the TE-TE chamber to 137Cs gamma rays equal to 1.16 × 10 -3 C Gy - t which was independently determined, we get brq = 2.6 × 10 -6 Gy s-1 and b~ less than 10% of DN.

Table 2 Mean ionization currents for the A1-Ar chamber placed at positions (1), (2), (3) and (4) (see fig. 7) inside the neutron-gamma field produced by a 14 MeV neutron genera- tor

Position Normalized 14 MeV peak Mean ionization (12-16 MeV) intensity current (A)

(1) 1.00 8.3 x 10-s (2) 0.41 3.3 × 10- 8

(3) 0.22 1.4 × 10- s (4) 0.16 1.3 × 10- 8

4. 4. Error discussion

From eqs. (8) and (9) it is seen that errors in the responses and the relative neutron sensitivity of both chambers contribute to the fractional errors in the neu- tron absorbed dose A D N / D N and the gamma absorbed dose A D ~ / D ~ . Since our electrometer is a high sensitiv- ity type, down to 10 -11 A, the fractional errors in the measured mean ionization currents are all less than 5%, which is negligibly small as compared to the fractional error of about 25% for the relative neutron sensitivity, as will be discussed subsequently.

Differentiating eqs. (8) and (9) by first principle and

352 LS. Chuang et al. / Fabrication of a tissue equivalent MWPC

rearranging, we get

A D N A k u / k u

DN [ ( 1 / k u ) _ (1 + A k v / k o ) ] , (11)

AD C = -ADr~. (12)

Previous investigations of k v ( E ) for a Mg-Ar thim- ble chamber [15] using computer iteration showed that k U increases from a value 0.21 at E n = 13 MeV to a maximum value 0.22 at E n = 14 MeV. Hence a frac- tional error of 5% due to the energy dependence of k u is expectable for the present AI-Ar chamber. On the other hand, the detection area of the AI-Ar chamber (100 cm 2) is much larger than that of the liquid organic scintillation detector (16 cm2). Although the centres of these two detectors were set at the same position when taking the measurements, a fractional error in A k t j / k u of not more than 20% may have been introduced due to averaging of the neutron fluxes for the point of concern. Hence the total fractional error of A k u / k v is about 25% with k U = 0.23. Accordingly, from eqs. (11) and (12)

A D N / D N -- 896, A D G / D C = O.08DN/D G > 80%,

where D ~ / D N < 10% was substituted. Thus for a moderate k v value of 0.23, the overall

uncertainty in its value should be kept as small as possible to minimize the overall uncertainty in the ab- sorbed dose of neutrons. In fact, different methods of evaluating k u, angular difference, time-of-flight, associ- ated particle coincidence and lead filtration, should be used and compared in order to minimize A k U in the AI-Ar chamber as was done in the case of the energy- compensated Geiger-Mi~ller counter [16]. The large fractional error in D C is due to the fact that A D c / D G depends inversely on Do, whereas ADr~/Dr~ depends only on k u and a k u .

5. Monte Carlo simulation of 14 MeV neutron irradia- tion on a TIE planar chamber

5.1. Method of evaluation

In order to confirm the experimental results obtained in section 4.2 using the TE planar chamber, a Fortran IV computer program was devised and then executed on an IBM 3031 computer system for calculating the mean ionization current under 14 MeV neutron irradiation. This Monte Carlo method follows a neutron on its way through the TE chamber and decides all random events, e.g., energy of the incident neutron, atomic species of interaction, reaction type, angle of scattering in the centre of mass system etc., by application of a pseudo- random number generator. The resulting energy deposi- t ion due to various recoil particles is calculated using

input stopping power values and residual ranges. A summation of the contribution currents from many incident neutron events then gives the mean ionization current. Important sampling is introduced by increasing the neutron interaction frequency in the TE plastic and then dividing by a proportional increase of the incident neutron flux so that the absorbed dose per incident neutron remains constant. The history of a recoil par- title is finished when it leaves the sensitive region of the TE gas. The maximum number of histories generated was 500000.

This method of evaluation has the advantage of true gamma ray discrimination because only neutron in- duced events were taken into account, except the (n,r) inelastic reactions which emit 0.1-1 MeV gamma rays of little probability of energy deposition in the chamber. Also the running cost is much lower than that of the experimental method. The CPU time used for 500000 incident neutron events was about 30 min.

5.2. Results

The essential assumptions and approximations in the computer program were the following: a) A 4-element tissue model (i.e. assuming that the

tissue consists of oxygen, carbon, hydrogen and nitrogen only) was used.

b) Some nuclear reactions, e.g., 14N(n,2n)13N, 160(n, d)lSN etc., which contribute less than 0.1% of the kerma in tissue, were neglected.

c) The energy of the respective residual nuclear levels within an energy range of 1 MeV was grouped and represented by one level at an average energy [17].

d) All neutrons were normally incident upon the TE plastic.

e) The relative differential flux density distribution be- tween neutron energies 13 MeV and 15 MeV was used to sample the initial energy of the incident neutrons.

f) The average energy expended to create an ion pair by recoils in TE counting gas per unit electronic charge, w / e , was assumed to be a constant (= 3.15 J C-1) with a percentage error less than 5% according to ICRU Report 26. The effective mass stopping power of the TE plastic relative to TE gas for 14 MeV neutrons, (Sm,g)n, was also assumed to be unity. Since the tissue absorbed dose rate br~ is defined to

be the rate of mean energy imparted per unit mass of tissue, we have

b,~ = ~ (13) pVe '

where Q is the mean ionization current, ~ / e is the average energy required to create an ion pair per unit electronic charge, p is the density of the tissue equiva-

L.S. Chuang et aL / Fabrication of a tissue equivalent MWPC 353

lent material, V is the gas sensitive volume. Hence b N = 3.6 X 10 -6 J k g - l s -1, for ~) = 2.43 × 10 -1° A

evaluated by the computer program, ~ / e = 31.5 J C -1 and # V = 2.04 × 10 -3 kg. The result of Monte Carlo calculations for b N is higher than that obtained by the experiment, of 2.6 × 10 -6 J k g - l s -1. In view of the fact that in the Monte Carlo method only monoenergetic neutrons at 14.3 MeV are sampled which are normally incident upon the TE chamber, this excess of the calcu- lated value over the experimental one is expected.

sensitive detection area are increased, and it has the capability of extension to measure some microdosimet- ric quantities like LET, linear energy etc.

The authors would like to thanks Mr. K.S. Sin for his contribution concerning some of the technical aspects of fabrication of the multiwire tissue equivalent chamber.

References

6. Conclusion

The multiwire tissue equivalent proportional planar chamber fabricated can be used as another possible instrument, apart f rom the conventional Rossi type and cylindrical single-wire type, to evaluate the macroscopic fast neutron tissue absorbed dose for radiotherapy or radiological protection of fast neutrons. It is not, nor was it ever intended to be, the most accurate, sensitive example of this type. Yet simplicity in construction, convenience in usage and consistency in the results were realized (Monte Carlo simulation evaluation of the re- suits shows good agreement). Further improvement in the accuracy requires more subtle fabrication, conscien- tious operation and accurate measurement and calibra- tion procedures. Even more important is the improve- ment in the accuracy of the relative neutron sensitivity of the dual AI chamber.

The advantages of the presently constructed TE chamber are that it is unique in shape to simulate a planar tissue, both the gas amplification factor and the

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