# Some unexpected behaviours of PTW/Nucletron well-type ionization chambers

Guilhem Douysset, Aimé Ostrowsky, Frank Delaunay

► To cite this version:

Guilhem Douysset, Aimé Ostrowsky, Frank Delaunay. Some unexpected behaviours of PTW/Nucletron well-type ionization chambers. Physics in Medicine and Biology, 2008, 53 (14), pp.N269-N275. 10.1088/0031-9155/53/14/N01. cea-05471457v2

HAL Id: cea-05471457

https://cea.hal.science/cea-05471457v2

Submitted on 16 Feb 2026

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# Some unexpected behaviours of PTW/Nucletron well-type ionization chambers

Guilhem Douysset, Aimé Ostrowsky and Frank Delaunay

CEA, LIST, LNE-Laboratoire National Henri Becquerel, F-91191 Gif-sur-Yvette, France

E-mail: guilhem.douysset@cea.fr

## Abstract

Some unexpected behaviours of PTW/Nucletron well-type ionization chambers have been discovered. A significant undesired detection volume is present in the region surrounding the electrical terminals. A finite-element code has been used to compute electrical fields in this region, and it showed that the magnitude of the undesired volume was dependent on the electrical connector type. An experimental confirmation of this assumption has been obtained. A source activity dependence of the calibration coefficients has also been revealed. A set of recommendations for the optimal use of this detector is proposed.

## 1. Introduction

The Laboratoire National Henri Becquerel (LNHB) has developed the French primary dosimetric standard for HDR brachytherapy and is currently in charge of the calibration of transfer instruments for the medical physicists community. Some unexpected behaviours of PTW/Nucletron well-type ionization chambers have recently been discovered and are described in this note.

Well-type ionization chambers are widely used instruments for the routine calibration of brachytherapy sources. For the calibration of high dose rate and pulse dose rate (HDR, PDR) 192Ir brachytherapy sources, PTW 33004 chambers are commonly used. These instruments are also distributed by Nucletron under reference 'Source Dosimetry System' 077.0XX. They comprise a cylindrical air-vented active volume of about 200 cm³, a collecting electrode, an external electrode and a guard electrode (see figure 1). Depending on the electrical connecting system, several models are identified (see table 1). In the TNC and BNT schemes the collecting electrode and guard are connected to high voltage (±300 V nominal), whereas in the M system the collecting electrode and guard are at ground potential.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Schematic drawing of a PTW/Nucletron well-type ionization chamber (1: collecting electrode, 2: external electrode, 3: guard electrode, 4: undesired collection volume, 5: detachable base plate, black: aluminum, dark grey: plastic, light grey: air).</figcaption></figure>

<table><caption>**Table 1.** Identification of PTW/Nucletron well-type ionization chambers.</caption><thead><tr><td>Connecting system</td><td>PTW</td><td>Nucletron</td></tr></thead><tbody><tr><td>Triaxial TNC</td><td>TW33004</td><td>077.090 and 077.091</td></tr><tr><td>Triaxial BNT</td><td>TN33004</td><td>077.089 and 077.092</td></tr><tr><td>Triaxial M</td><td>TM33004</td><td>077.087 and 077.094</td></tr></tbody></table>
## 2. An undesired detection volume

A detachable base plate is located at the base of the ionization chamber. After the removal of this plate, one can gain access to the three electrical terminals (collecting electrode, guard and external electrodes) (see figure 2).

In this region there is a combination of high voltage (on bare terminals), air and γ-rays. This defines an undesired ionization volume. This extra collecting volume may not be a drawback of the detector if its influence is not reliant on the electrical connection scheme.

A finite-element code has been used in order to estimate the E-field in this region and its influence on chamber calibration coefficients. Results and experimental verification are presented thereafter.

### 2.1. E-field calculation

As a first approximation, a simplified modelling of the E-field has been performed using the 2D finite-element code QUICKFIELD™ 5.1 (Tera Analysis Ltd). A cut-view has been done in a plane orthogonal to the chamber longitudinal axis. Electrodes and chamber body are metallic and air is filling the cavity (relative electric permittivity = 1). Electric fields have been estimated for the two electrical connection schemes.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Bottom view of a Nucletron 077.091 (TNC connector) well-type chamber (base plate removed, C: collection electrode, G: guard electrode, E: external electrode, gw: ground wire).</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 3.** E-Field simulation around electrode terminals. Right: TNC/BNT connection. Left: M connection. Equipotential lines: 10 V. polarization voltage: +300 V. (C: collection electrode; G: guard electrode; E: external electrode.)
(This figure is in colour only in the electronic version)</figcaption></figure>

It is obvious in figure 3 that in the TNC/BNT connection scheme a wider undesired collection volume is defined. Thus, the calibration coefficient of this type of chamber should depend on the connection scheme. However, the magnitude of the effect is difficult to predict theoretically. Therefore, following the procedure for cable replacement described in the operating manual (see Nucletron 2002, Section 3), the same well-type chamber has been calibrated in both configurations.

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Relative current variation after insulation of the collection electrode terminal in TNC and M mode (statistical uncertainties reported. Detector: Nucletron 077.091 S/N 25325).</figcaption></figure>

<table><caption>**Table 2.** Current comparison for the TNC/BNT and M connection schemes. Detector: Nucletron 077.091 S/N 25324.</caption><thead><tr><th>Connection scheme</th><th>Sweet-spot (mm)</th><th>Corrected current (nA)</th></tr></thead><tbody><tr><td>TNC/BNT</td><td>1334</td><td>+23.319(23)</td></tr><tr><td>M</td><td>1334</td><td>+23.032(23)</td></tr></tbody></table>
## 2.2. Influence on measured signal

The cable exchange procedure basically consists in connecting a ground wire (labelled as 'gw' in figure 2) either to the external electrode (TNC/BNT) or to the guard electrode (M).

Ionization currents are measured with an home-build high-precision current integrating device. It is based on a high-gain amplifier using a precision capacitor in an external feedback mode. For convenience the high voltage potential is not applied directly onto the signal cable. Therefore, in the TNC/BNT mode, the high voltage is applied directly onto chamber body instead of collection electrode. Except that the E-field is in the opposite direction, this configuration is strictly equivalent to the normal TNC/BNT connection scheme.

A Nucletron microSelectron V2 HDR brachytherapy source of nominal activity 119 GBq has been used for measurements. The source has been scanned vertically in the well-type chamber to determine the point of maximum response (often labelled as 'sweet-spot'). Currents have been corrected for atmospheric effects, leakage and radioactive decay. The results are reported in table 2.

Analysis of bias polarity effects and sweet-spot position did not show any significant differences for the two types of connectors. However, the measured currents differ by more than 1.2%. As predicted by E-field calculations, the highest current value is obtained for the TNC/BNT connection scheme. As a supplementary verification, an attempt has been made to reduce the influence of the extra collecting volume by covering the collection electrode terminal with a Teflon® tape. Results are reported in figure 4.

It is confirmed that in M mode there is no evidence of a significant extra collection volume in the base plate region, whereas in TNC mode about 1% of the total measured signal originates in this region. In TNC mode, the insulation of the collection electrode terminal removes the largest part of the effect, however one still cannot exclude the presence of remaining extra collection volume located between the base plate region and the collection electrode.

## 3. Activity-dependent calibration coefficients

The calibration coefficients issued by LNHB are expressed as mGy/h(1m)/C (for 1 min integration time). They are obtained by using the following equation:

$$N_{K_R} = \frac{\dot{K}_R(0) \cdot \Delta t}{k_{\text{dec}}(t) \cdot R(t) \cdot k_{\text{ion}}}, \quad (1)$$

where $\dot{K}_R(0)$ denotes the source reference air kerma rate measured during the source calibration, $\Delta t$ is equal to 1 min, $k_{\text{dec}}(t)$ is the decay correction between the source calibration and the well-chamber calibration, $R(t)$ is the electrometer reading in charge mode (corrected for atmospheric effects) and $k_{\text{ion}} = (A_{\text{ion}})^{-1}$ is the inverse of the charge collection efficiency. In principle, any source activity influence should be accounted for by the electrical calibration of the electrometer and by the recombination correction. As recommended (IAEA 2002), the ion-collection efficiencies (or volume recombination correction factors kion) are measured using +300 V and +150 V bias (Attix 1984):

$$k_{\text{ion}} = \left( \frac{4}{3} - \frac{Q_{300}}{3 \times Q_{150}} \right)^{-1}, \quad (2)$$

where $Q_{300}$ and $Q_{150}$ represent the measured charge per minute for +300 V and +150 V bias, respectively.

Two well-type ionization chambers (Nucletron 077.091 and Standard Imaging HDR1000+) are used at LNHB as constancy check instruments. Both detectors are associated with the same electrometer (Standard Imaging MAX4000) which is periodically calibrated in terms of electrical units. The two assemblies are submitted together to periodic calibration during the source lifetime (activities range from 370 GBq down to 18 GBq) and immediately after each source renewal. The four-year history (12 calibrations) of the Nucletron 077.091 chamber (S/N 25324) indicates a clear correlation ($r = 0.852, p < 0.0001$) between the source activity and the calibration coefficient: the lower the activity, the lower the calibration coefficient (see figure 5). Typically the calibration coefficient decreases by as much as 0.7% between 36 mGy/h (1 m) (typical HDR source output) and 2 mGy/h (1 m) (typical PDR source output). Regarding the calibration uncertainty (1.3% for $k = 2$), the magnitude of this effect is significant.

No similar behaviour is observed with the other well-type chamber: the calibration coefficient remains constant within ±0.1% (see figure 6).

Several explanations for this effect are proposed. According to (1), activity dependence of $N_{K_R}$ can only result from the erroneous determination of two terms: $R(t)$ and/or kion. The calibration records show that, as determined by (2), $k_{\text{ion}}$ values decrease as source activity decreases. Typical range is 1.0004–1.0001 for chambers fitted with M connectors and 1.0017–1.0010 for TNC/BNT connectors. However, if only volume recombination

<figure><img src="image_6.png" /><figcaption>Figure 5. Calibration history of the Nucletron 077.091/MAX4000 dosimeter (TNC connector) as a function of source air kerma rate. The calibration coefficients were determined using six different Nucletron microSelectron V2 HDR $^{156}$Ir sources between 2004 and 2008 and were normalized to the running mean (lines are displayed only for the clarity of the view).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Figure 6. Calibration history of the HDR1000+/MAX4000 dosimeter. The calibration coefficients were determined using six different Nucletron microSelectron V2 HDR $^{156}$Ir sources between 2004 and 2008 and were normalized to the running mean (same range of source activities as the one reported in figure 5).</figcaption></figure>
was present, kion would always be proportional to the source activity. Since this assumption is not verified, it is likely that the initial recombination is also present. A global treatment of recombination mixing initial and volume recombination can be used to derive the kion value from currents measured for two polarizing voltages and variable source activities. For TNC/BNT configuration, the obtained kion value ranges between 1.0040 and 1.0034. This value is significantly higher than the one obtained by (2), however its variation is too small to explain the observed decrease of the calibration coefficients (figure 5).

$R(t)$ might also be underestimated for high activity sources. This cannot be explained by an electrometer-related phenomenon first because the electrometer is periodically calibrated and second because no drift is observed with HDR1000+ chambers. In the undesired collection volume region, the electrical field has not been optimized (see figure 3), therefore the charge collection might be inefficient resulting in a space charge problem. This would be enhanced by the source activity magnitude. Thus, the net influence of the undesired volume might decrease as the source activity increases resulting in a lower calibration coefficient.

4. Conclusion and recommendations

Two unexpected characteristics of PTW/Nucletron well-type chambers have been revealed. Neither of them prevents the use of the detector as a transfer instrument for source calibration. However, precautions should be taken for optimal use of the instrument.

The presence of an undesired collection volume has been discovered. The magnitude of this volume depends on the electrical connection scheme. The current change upon cable exchange is as high as 1.2%. To our knowledge, other types of well-type chambers currently in use for brachytherapy are not similarly affected.

A recommendation—not given in the detector operating manual—is therefore to recalibrate systematically any chamber whose cable type has been changed (physical change or the use of a connector converter). Indeed the magnitude of the calibration coefficient change is comparable to the calibration uncertainty.

It is also wiser to limit Monte Carlo simulations of this kind of detector to the instruments fitted with an M connector for which the influence of the undesired detection volume is the lowest. Otherwise, a precise definition of this volume is needed and it would imply a full 3D calculation of the electrical field in this region.

Awaiting an update of the chamber design by the manufacturer, it has been shown that a simple way to remove most of the influence of the extra collection volume in TNC/BNT configuration is to insulate the collection electrode terminal.

A source activity dependence of the calibration coefficient has also been revealed. The more active the source, the higher the calibration coefficient. Care must therefore be taken when using the same ionization chamber for the calibration of HDR and PDR brachytherapy sources. Ideally, as recommended in the PTW version of the user manual (PTW 2001), the chamber should be calibrated at the dose rate typical for the application.

References

Attix F H 1984 Determination of Aion and Pion in the new AAPM radiotherapy dosimetry protocol Med. Phys. 11 714–6

International Atomic Energy Agency (IAEA) 2002 Calibration of photon and beta ray sources used in brachytherapy TECDOC-1274 (Vienna)

Nucletron 2002 Selectron Source Dosimetry System–User Manual 092910ENG-02 PTW 2001 HDR chamber type 33004–Instruction Manual D432.131.0/1