The authors have no acknowledgements.

1. Experimental Setup (Figure 2)
(Performed&#160;1 hr prior to the first measurements to stabilize the detector temperature and the high voltage supply.)
<ol>
	<li>Place the water tank under the treatment head, keeping in mind that the SSD will have to be increased up to 200 cm. Thus the tank should be positioned as low as achievable, depending on the ceiling height.</li>
	<li>Align the water tank with the linac (its vertical sides must be parallel to the vertical sides of the head). The laser can be used to ensure the orientation is correct; this procedure is detailed in the Physics guide14 of the system.</li>
	<li>Verify the vertical orientation of the linac by performing x&#160;and y&#160;profile measurements at two different depths, computing the beam declination and correcting using the axes of rotation of the head (see Physics guide).</li>
	<li>Replace the collimator with the telemeter accessory, and use it to accurately position the head at 78.5 cm SSD. The tip of the accessory should barely touch the water surface.</li>
	<li>Remove the telemeter and place the 60 mm collimator on the treatment head.</li>
	<li>Position the LIC reference point at 1.5 cm depth in a vertical position, i.e. with the axis of the cylindrical cavity parallel to the beam direction. This results in a distance of 80 cm between the source and the detector. Use the laser to position the LIC at the center of the beam in the lateral direction.</li>
	<li>Place a 0.125 cm3 air-filled ionization chamber (AIC) next to the LIC to be able to correct for attenuation, distance and scatter effects.</li>
	<li>Connect the LIC and the high voltage supply to the electrometer and set the voltage to 800 V. Connect the AIC to another electrometer and set the voltage to 400 V. Then wait 1 hr&#160;for stabilization purposes.</li>
	<li>To ensure the accuracy of the detector lateral positioning, perform profile measurements in both transverse directions and correct the zero of the LIC if necessary.</li>
	<li>Make sure the repetition rate of the linac is fixed (nominal value = 150 Hz).</li>
</ol>
2. Measurements
<ol>
	<li>First deliver a pre-irradiation dose of 3,000 monitor units (MU) in order to stabilize the LIC response. Then perform a zero of the electrometers.</li>
	<li>To assess leakage current and stability, perform a series of charge acquisitions with the beam off for a duration equal to that of the measurements (7.5 sec for 100 MU). Compare the average value obtained to the values measured with the beam on. A typical leakage charge of less than 0.03% of the smallest value measured with the beam on can be considered negligible.</li>
	<li>Place the treatment head at 58.5 cm SSD: use the remote controller in Cartesian mode and simply perform a 20 cm motion in the z-direction.</li>
	<li>Leave the treatment room, close the door and program an irradiation of 100 MU at the operator console. Then start both electrometers, deliver the dose and note the charges measured by the LIC and the AIC <img alt="Equation 5" fo:content-width="0.7in" src="/files/ftp_upload/51296/51296eq5.jpg" />.</li>
	<li>Repeat the process 10 times to be able to assess statistical uncertainties.</li>
	<li>After ten measurements, enter the room and move the treatment head to the next position (68.5 cm SSD). Then repeat steps 2.4 and 2.5.</li>
	<li>When the head is moved further away from the tank the distance between measuring points can be increased as the charge varies following the inverse-squared distance law. Table 1 provides an example of a list of measuring points, along with the corresponding dose per pulse.</li>
</ol>
<img alt="Table 1" fo:content-width="6in" src="/files/ftp_upload/51296/51296table1.jpg" style="width: 600px;" /> 
Table 1.&#160;List of measuring points for the two-dose rate method (A and B) with the corresponding doses per pulse.
3. Analysis
Two methods can be used to analyze the data.
Method A
<ol>
	<li>For every distance d, take the ratio of each measured LIC value with the corresponding AIC value obtained at the same distance, <img alt="Equation 5" fo:content-width="0.5in" src="/files/ftp_upload/51296/51296eq12.jpg" />.</li>
	<li>Plot the ratios <img alt="Equation 5" fo:content-width="0.5in" src="/files/ftp_upload/51296/51296eq12.jpg" />against the dose per pulse and use a linear fit to obtain the extrapolated ratio at zero dose per pulse,&#160;R0.</li>
	<li>With the assumption that the collection efficiency is equal to 1 at 0 mGy/pulse, normalize all the ratios calculated in step 3.A.1 to the extrapolated value from the previous step in order to obtain values of f (i.e. scale every ratio by a factor such that k R0 = 1).</li>
	<li>Plot the values of f against the values of the dose per pulse to represent the evolution of the collection efficiency. The error bars can be calculated by propagating the uncertainties on the LIC and AIC charges evaluated from the repeated measurements at each distance.</li>
</ol>
Method B
<ol>
	<li>Take the ratios <img alt="Equation 6" fo:content-width="0.5in" src="/files/ftp_upload/51296/51296eq6.jpg" /> of the LIC readings at 200 cm (198.5 cm SSD) and 60 cm (58.5 cm SSD), and of the AIC readings, <img alt="Equation 6" fo:content-width="0.5in" src="/files/ftp_upload/51296/51296eq6.jpg" />.</li>
	<li>Numerically solve the equation below for u200. 
	<img alt="Equation 7" fo:content-width="1.5in" src="/files/ftp_upload/51296/51296eq7.jpg" /></li>
	<li>Inject the value of u200 in the following relation to obtain the collection efficiency,&#160;f at 200 cm distance. 
	<img alt="Equation 8" fo:content-width="1.5in" src="/files/ftp_upload/51296/51296eq8.jpg" /></li>
	<li>The efficiency can be calculated using the ratios with other distances than 60 cm, as long as the charge ratio is higher than 3. This procedure allows testing the uncertainty on the calculated u and f values.</li>
	<li>Calculate the parameter u for all measurement points, using the following relation (choosing the distances so that <img alt="Equation 10" fo:content-width="1in" src="/files/ftp_upload/51296/51296eq10.jpg" />) 
	<img alt="Equation 9" fo:content-width="1.5in" src="/files/ftp_upload/51296/51296eq9.jpg" /></li>
	<li>Calculate all collection efficiencies,&#160;fd, from the relation 
	<img alt="Equation 11" fo:content-width="1.2in" src="/files/ftp_upload/51296/51296eq11.jpg" /></li>
	<li>Plot the values of f against the values of the dose per pulse to represent the evolution of the collection efficiency. The error bars can be calculated by propagating the uncertainties on the LIC and AIC charges evaluated from the repeated measurements at each distance.</li>
</ol>

<ol>
	<li>Das, I. J., Din, G. X., Ahnesj&#246;, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Small+fields:+non-equlibrium+radiation+dosimetry.">Small fields: non-equlibrium radiation dosimetry.</a> Med. Phys. 35 (1), 206-215 (2008).</li><li>Wickmann, G., Nystrom, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+use+of+liquids+in+ionization+chambers+for+high+precision+radiotherapy+dosimetry.">The use of liquids in ionization chambers for high precision radiotherapy dosimetry.</a> Phys. Med. Biol. 37 (9), 1789-1812 (1992).</li><li>Chung, E., Soisson, E., Seuntjens, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dose+homogeneity+specification+for+reference+dosimetry+of+nonstandard+fields.">Dose homogeneity specification for reference dosimetry of nonstandard fields.</a> Med. Phys. 39 (1), 407-414 (2011).</li><li>Francescon, P., Kilby, W., Satariano, N., Cora, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Monte+Carlo+simulated+correction+factors+for+machine+specific+reference+field+dose+calibration+and+output+factor+measurement+using+fixed+and+iris+collimators+on+the+Cyberknife+system.">Monte Carlo simulated correction factors for machine specific reference field dose calibration and output factor measurement using fixed and iris collimators on the Cyberknife system.</a> Phys. Med. Biol. 57 (12), 3741-3758 (2012).</li><li>Wagner, A., Crop, F., Lacornerie, T., Vandevelde, F., Reynaert, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Use+of+a+liquid+ionization+chamber+for+stereotactic+radiotherapy+dosimetry.">Use of a liquid ionization chamber for stereotactic radiotherapy dosimetry.</a> Phys. Med. Biol. 58 (8), 2445-2459 (2013).</li><li>Johansson, B., Wickman, G., Bahar-Gogani, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=General+collection+efficiency+for+liquid+iso-octane+and+tetramethylsilane+in+pulsed+radiation.">General collection efficiency for liquid iso-octane and tetramethylsilane in pulsed radiation.</a> Phys. Med. Biol. 42 (10), 1929-1938 (1997).</li><li>Andersson, J., T&#246;lli, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Application+of+the+two-dose-rate+method+for+general+recombination+correction+for+liquid+ionization+chambers+in+continuous+beams.">Application of the two-dose-rate method for general recombination correction for liquid ionization chambers in continuous beams.</a> Phys. Med. Biol. 56 (2), 299-314 (2010).</li><li>Sjgren, R., Wendelsten, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+two-dose-rate+method+for+general+recombination+correction+for+liquid+ionization+chambers+in+pulsed+beams.">A two-dose-rate method for general recombination correction for liquid ionization chambers in pulsed beams.</a> Phys. Med. Biol. 55 (15), 4247-4260 (2010).</li><li>Boag, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ionization+measurements+at+very+high+intensities:+part+I.+Pulsed+radiation+beams.">Ionization measurements at very high intensities: part I. Pulsed radiation beams.</a> Br. J. Radiol. 23 (274), 601-611 (1950).</li><li>Boag, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+saturation+curve+for+ionization+measurements+in+pulsed+radiation+beams.">The saturation curve for ionization measurements in pulsed radiation beams.</a> Br. J. Radiol. 25 (300), 649-650 (1952).</li><li>Stewart, K. J., Elliott, A., Seuntjens, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+of+a+guarded+liquid+ionization+chamber+for+clinical+dosimetry.">Development of a guarded liquid ionization chamber for clinical dosimetry.</a> Phys. Med. Biol. 52 (11), 3089-3104 (2007).</li><li>Yin, Z., Hugtenburg, R. P., Beddoe, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Response+corrections+for+solid-state+detectors+in+megavoltage+photon+dosimetry.">Response corrections for solid-state detectors in megavoltage photon dosimetry.</a> Phys. Med. Biol. 49 (11), 3691-3702 (2004).</li><li>Griessbach, I., Lapp, M., Bohsung, J., Gademann, G., Harder, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dosimetric+characteristics+of+a+new+unshielded+silicon+diode+and+its+application+in+clinical+photon+and+electron+beams.">Dosimetric characteristics of a new unshielded silicon diode and its application in clinical photon and electron beams.</a> Med. Phys. 32, 3750-3754 (2005).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Accuray Inc., Physics Essentials Guide P/N 032515A-ENG. Accuray Inc. , (2010).</li></ol>

The authors have nothing to disclose.

The methods presented above allow evaluating the recombination effects in a LIC over a large range of dose rates (0.14 to 1.58 mGy/pulse). Method A is simple but is associated with more uncertainties than method B, which provides fairly accurate (and absolute) values of the collection efficiency,&#160;f. Recombination is responsible for approximately 2% loss in signal over the whole range investigated, but this range is larger than what is usually spanned during routine measurements. The largest error on an output factor is 0.35%, and it reaches 1% for a percentage depth dose measurement as was shown in the results section.
The critical element for conducting the protocol is the initial setup of the experiment, as all measurements are performed relative to the initial position of the treatment head. Thus one should be careful about the accurate measurement of the initial SSD to be able to relate the detector readings to each dose per pulse. This is also true for the placement of the detector in the water; care should be taken that the effective point of measurement (situated 1 mm behind the entrance window in the case of the microLion detector) is positioned at 1.5 cm below the surface. The 1 hr delay and the pre-irradiation dose are also essential in order to stabilize the 800 V supply and the temperature.
The repetition rate of the linac directly impacts the dose per pulse. At 800 MU/min and with a frequency of 150 Hz, the dose per pulse is 0.89 mGy/pulse. This frequency should be fixed for all measurements to make sure the distance is the only variable factor having an influence on the dose per pulse. The method can be used in the case of a continuous beam with some adaptations7. On other devices where the SSD cannot be varied by directly moving the treatment head, the repetition rate could be modified to introduce the dose per pulse variation. If this parameter is fixed as well, the SSD can still be modified by moving the LIC and the water surface in the tank, but the accuracy of this approach would likely be lower than the treatment head movement used in the present study.
The next step in the characterization of the LIC for its use in small field dosimetry is to investigate the other factors that induce perturbation of the response, such as the materials of the detector and the volume effect (i.e. the fact that the sensitive volume is not small compared to the dimensions of the beam). This is possible through the use of Monte Carlo simulations5. With those aspects taken into account, global correction factors can be applied to the LIC readings obtained in clinical routine measurements (output factors, percentage depth doses, dose profiles) in order to fully eliminate the perturbations.
After the complete characterization and correction of these perturbating effects, the LIC can be used as an additional detector for small beam dosimetry, allowing independent verification of profiles, percentage depth doses and output factors measured by other detectors. Its very high spatial resolution in the longitudinal direction would also be suited for the dosimetry of rectangular fields with only one small dimension (e.g. Tomotherapy.)

Dosimetry in radiation therapy has been performed using gaseous ionization chambers for many years. These detectors perform well as far as &#8220;conventional&#8221; radiation therapy is concerned, i.e. large homogeneous (or slowly varying) fields are used. However many recent devices, such as the Cyberknife (Figure 1) system studied in this work, offer the possibility of using very small fields (down to 5 mm). Other devices produce highly modulated beam profiles such as in Intensity-Modulated Radiation Therapy (IMRT). Conventional air-filled detectors are not well-suited for these techniques1; in order to reach an acceptable spatial resolution the volume of the cavity would have to be reduced to a size where the chamber response would become too low. Diodes offer the advantage of smaller sensitive volumes and they are extensively used in small beam dosimetry. However they present other limitations such as scattering effects arising from their metallic shielding12,13.
In a liquid ionization chamber2 (LIC), the ionization density is much higher and thus the reduction of the sensitive volume is possible without compromising the detector response. Moreover the sensitive medium has a density close to that of water, reducing the fluence perturbations associated with an air cavity. These aspects make the LIC an interesting candidate for small beam dosimetry3-5.
There are nevertheless some issues to address before being able to perform routine dosimetric measurements with LICs. First, due to the higher ionization density the recombination effects are more important than in air-filled chambers6-8. Recombination can either be initial (an electron recombines with its mother ion) or general (two ions coming from different ionization events recombine). The latter is dependent on the dose rate incident on the detector; this means that relative dose measurements (i.e. dose profiles, percentage depth doses, output factors) can potentially undergo deviations due to the change in dose rate. Recombination is characterized by the general collection efficiency, defined as the ratio of the measured charge to the charge produced by the incident radiation and escaping initial recombination: f = QC/Q0. In gaseous detectors recombination effects are evaluated using the two-voltage method from the theory of Boag9,10, which cannot be applied in LICs11.
An alternative can be found in the use of the two-dose-rate method8, consisting of varying the dose rate to isolate the influence of general recombination and measure the general collection efficiency through the relation 
<img alt="Equation 1" fo:content-width="1in" src="/files/ftp_upload/51296/51296eq1.jpg" />
where u is defined as 
<img alt="Equation 2" fo:content-width="1in" src="/files/ftp_upload/51296/51296eq2.jpg" />
with &#945; being the recombination coefficient, Q0 the amount of charge that escapes initial recombination, h the electrode separation, e the elementary charge, V the sensitive volume of the chamber, k1 and k2 the mobilities of the positive and negative charges, and U the applied voltage. By measuring at different doses per pulse it is possible to obtain the parameter u and thus the collection efficiency,&#160;f. The dose per pulse is given by the relation 
<img alt="Equation 3" fo:content-width="3in" src="/files/ftp_upload/51296/51296eq3.jpg" />
All measurements are performed at the reference conditions of the Cyberknife (Source-Surface Distance SSD = 78.5 cm, 1.5 cm depth, 60 mm collimator). The use of a large collimator allows avoiding the volume effects associated with small beams. Given the dose rate is 800 MU/min and the repetition frequency is 150 Hz, this results in a dose of 0.89 mGy/pulse (at the reference conditions, 1 MU corresponds to a dose of 1 cGy). When the pulse repetition frequency is kept constant, the dose per pulse only depends on the dose rate in Gy/min, which is related to the SSD through the inverse-squared distance law: 
<img alt="Equation 4" fo:content-width="1in" src="/files/ftp_upload/51296/51296eq4.jpg" /> 
for two SSDs d1 and d2.

<table border="0" cellpadding="0" cellspacing="0" width="788">
	<tbody>
		<tr>
			<td>MicroLion chamber</td>
			<td>PTW</td>
			<td>31018</td>
			<td>http://www.ptw.de/2263.html</td>
		</tr>
		<tr>
			<td>Unidos Webline dosimeter</td>
			<td>PTW</td>
			<td></td>
			<td>http://www.ptw.de/unidos_webline_dosemeter_rt0.html</td>
		</tr>
		<tr>
			<td>HV supply</td>
			<td>PTW</td>
			<td></td>
			<td>http://www.ptw.de/2265.html</td>
		</tr>
		<tr>
			<td>MP3 water scanning system</td>
			<td>PTW</td>
			<td></td>
			<td>http://www.ptw.de/2032.html</td>
		</tr>
		<tr>
			<td>0.125 cm3 SemiFlex chamber</td>
			<td>PTW</td>
			<td>31010</td>
			<td>http://www.ptw.de/semiflex_chambers0.html?&#38;cId=6069</td>
		</tr>
		<tr>
			<td>Cyberknife</td>
			<td>Accuray</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

characterization of recombination effects in a liquid ionization chamber used for the dosimetry of a radiosurgical accelerator

Most modern radiation therapy devices allow the use of very small fields, either through beamlets in Intensity-Modulated Radiation Therapy (IMRT) or via stereotactic radiotherapy where positioning accuracy allows delivering very high doses per fraction in a small volume of the patient. Dosimetric measurements on medical accelerators are conventionally realized using air-filled ionization chambers. However, in small beams these are subject to nonnegligible perturbation effects. This study focuses on liquid ionization chambers, which offer advantages in terms of spatial resolution and low fluence perturbation. Ion recombination effects are investigated for the microLion detector (PTW) used with the Cyberknife system (Accuray). The method consists of performing a series of water tank measurements at different source-surface distances, and applying corrections to the liquid detector readings based on simultaneous gaseous detector measurements. This approach facilitates isolating the recombination effects arising from the high density of the liquid sensitive medium and obtaining correction factors to apply to the detector readings. The main difficulty resides in achieving a sufficient level of accuracy in the setup to be able to detect small changes in the chamber response.

In Figure 3 the collection efficiency f obtained from method A is plotted against the dose per pulse, which ranges from 0 to 1.6 mGy/pulse where a 2% loss in signal can be seen. The points follow a linear behavior. The error bars show important uncertainties which seem inherent to the method and can be greatly reduced with the use of method B. It is also worth noting that in this method the AIC response is assumed to undergo no recombination effects, which is not necessarily entirely true. To verify this one can simply perform similar measurements using the AIC alone in a build-up cap (no water tank) and correct for the inverse square distance; small deviations can be observed and included in the uncertainties values.
Figure 4 shows the collection efficiency calculated from the second method (B). It proves more precise and has the advantage of providing absolute values of f. The deviations from the linear behavior are small and the loss in signal is slightly lower than with method A.
As a direct application of method B, factors can be calculated to correct for general recombination at a given dose per pulse, by simply taking the inverse of the collection efficiency,&#160;f. Then these factors can be applied to relative depth dose measurements. Figure 5 shows relative depth dose measured with a diode (not subject to recombination effects) and with the LIC before and after recombination correction. When the curves are normalized at a depth of 240 mm (where recombination effects vanish), they coincide, meaning that the corrections compensate for recombination effects in the buildup area (where the dose per pulse and thus the correction factors are the highest). This suggests that the calculated correction factors are accurate and can serve as a validation of the two-dose rate method.
<img alt="Figure 1" src="/files/ftp_upload/51296/51296fig1highres.jpg" /> 
Figure 1.&#160;Cyberknife sytem. A view of the Cyberknife system used for the measurements, with the accelerator head pointing downwards. The water tank can either be placed on the floor, or on the robotic couch visible at the back of the room, depending on the space available above the head.
<img alt="Figure 2" src="/files/ftp_upload/51296/51296fig2highres.jpg" /> 
Figure 2.&#160;Experimental setup. The setup is represented here with the AIC and LIC placed next to each other inside the water tank (1.5 cm depth), at the center of the beam, which is directed downwards. The arrows indicate the accelerator head movement between each series of measurements, starting at 60 cm distance (58.5 cm SSD) and ending at 200 cm (198.5 cm SSD).
<img alt="Figure 3" src="/files/ftp_upload/51296/51296fig3highres.jpg" /> 
Figure 3.&#160;General collection efficiency, method A.&#160;Graph of the evolution of the general collection efficiency,&#160;f,&#160;with respect to the dose per pulse (in mGy/pulse) obtained from method A.
<img alt="Figure 4" src="/files/ftp_upload/51296/51296fig4highres.jpg" /> 
Figure 4.&#160;General collection efficiency, method B. General collection efficiency plotted against the dose per pulse, following results from method B.
<img alt="Figure 5" fo:content-width="5in" src="/files/ftp_upload/51296/51296fig5highres.jpg" width="500" /> 
Figure 5.&#160;Application to relative depth dose measurements. The relative depth dose obtained from the diode measurements is shown in blue. The results from the LIC measurements are represented by the red (uncorrected) and yellow (corrected) curves. <a href="https://www.jove.com/files/ftp_upload/51296/51296fig5highres.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator at JoVE.com

Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

A growing number of radiation therapy devices offer the advantage of delivering the dose through very small beams to the tumor, allowing for increased conformity and higher doses per fraction. Many different detectors can be used for the dosimetry of these small fields. In the present study, the effect of ion recombination is investigated for a liquid ionization chamber using a stereotactic radiation therapy system.

Most modern radiation therapy devices allow the use of very small fields, either through beamlets in Intensity-Modulated Radiation Therapy (IMRT) or via ...

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11.7K Views.  Centre Oscar Lambret. A growing number of radiation therapy devices offer the advantage of delivering the dose through very small beams to the tumor, allowing for increased conformity and higher doses per fraction. Many different detectors can be used for the dosimetry of these small fields. In the present study, the effect of ion recombination is investigated for a liquid ionization chamber using a stereotactic radiation therapy system.

Video: Characterization of Recombination Effects in a Liquid Ionization Chamber Used for the Dosimetry of a Radiosurgical Accelerator

Ultrasound Velocity Measurement in a Liquid Metal Electrode

A growing number of electrochemical technologies depend on fluid flow, and often that fluid is opaque. Measuring the flow of an opaque fluid is inherently more difficult than measuring the flow of a transparent fluid, since optical methods are not applicable. Ultrasound can be used to measure the velocity of an opaque fluid, not only at isolated points, but at hundreds or thousands of points arrayed along lines, with good temporal resolution. When applied to a liquid metal electrode, ultrasound velocimetry involves additional challenges: high temperature, chemical activity, and electrical conductivity. Here we describe the experimental apparatus and methods that overcome these challenges and allow the measurement of flow in a liquid metal electrode, as it conducts current, at operating temperature. Temperature is regulated within &#177;2 &#176;C using a Proportional-Integral-Derivative (PID) controller that powers a custom-built furnace. Chemical activity is managed by choosing vessel materials carefully and enclosing the experimental setup in an argon-filled glovebox. Finally, unintended electrical paths are carefully prevented. An automated system logs control settings and experimental measurements, using hardware trigger signals to synchronize devices. This apparatus and these methods can produce measurements that are impossible with other techniques, and allow optimization and control of electrochemical technologies like liquid metal batteries.

Ultrasound velocimetry is used to study mixing by fluid flow in liquid metal electrodes. The focus of this manuscript is to illustrate the methods used for making precise, spatially-resolved ultrasound measurements while limiting oxidation and controlling and monitoring temperature, applied current, and the heater power being supplied.

A growing number of electrochemical technologies depend on fluid flow, and often that fluid is opaque. Measuring the flow of an opaque fluid is inherently ...

Electron Channeling Contrast Imaging for Rapid III-V Heteroepitaxial Characterization

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging (ECCI) in a scanning electron microscope (SEM). ECCI allows for imaging of defects and crystallographic features under specific diffraction conditions, similar to that possible via plan-view transmission electron microscopy (PV-TEM). A particular advantage of the ECCI technique is that it requires little to no sample preparation, and indeed can use large area, as-produced samples, making it a considerably higher throughput characterization method than TEM. Similar to TEM, different diffraction conditions can be obtained with ECCI by tilting and rotating the sample in the SEM. This capability enables the selective imaging of specific defects, such as misfit dislocations at the GaP/Si interface, with high contrast levels, which are determined by the standard invisibility criteria. An example application of this technique is described wherein ECCI imaging is used to determine the critical thickness for dislocation nucleation for GaP-on-Si by imaging a range of samples with various GaP epilayer thicknesses. Examples of ECCI micrographs of additional defect types, including threading dislocations and a stacking fault, are provided as demonstration of its broad, TEM-like applicability. Ultimately, the combination of TEM-like capabilities &#8211; high spatial resolution and richness of microstructural data &#8211; with the convenience and speed of SEM, position ECCI as a powerful tool for the rapid characterization of crystalline materials.

The use of electron channeling contrast imaging in a scanning electron microscope to characterize defects in III-V/Si heteroexpitaxial thin films is described. This method yields similar results to plan-view transmission electron microscopy, but in significantly less time due to lack of required sample preparation.

Misfit dislocations in heteroepitaxial layers of GaP grown on Si(001) substrates are characterized through use of electron channeling contrast imaging ...

Phase Diagram Characterization Using Magnetic Beads as Liquid Carriers

Magnetic beads with ~1.9 &#181;m average diameter were used to transport microliter volumes of liquids between contiguous liquid segments with a tube for the purpose of investigating phase change of those liquid segments. The magnetic beads were externally controlled using a magnet, allowing for the beads to bridge the air valve between the adjacent liquid segments. A hydrophobic coating was applied to the inner surface of the tube to enhance the separation between two liquid segments. The applied magnetic field formed an aggregate cluster of magnetic beads, capturing a certain liquid amount within the cluster that is referred to as carry-over volume. A fluorescent dye was added to one liquid segment, followed by a series of liquid transfers, which then changed the fluorescence intensity in the neighboring liquid segment. Based on the numerical analysis of the measured fluorescence intensity change, the carry-over volume per mass of magnetic beads has been found to be ~2 to 3 &#181;l/mg. This small amount of liquid allowed for the use of comparatively small liquid segments of a couple hundred microliters, enhancing the feasibility of the device for a lab-in-tube approach. This technique of applying small compositional variation in a liquid volume was applied to analyzing the binary phase diagram between water and the surfactant C12E5 (pentaethylene glycol monododecyl ether), leading to quicker analysis with smaller sample volumes than conventional methods.

Here, we present a protocol to investigate multi-component phase diagrams using externally controlled magnetic beads as liquid carriers in a lab-in-tube approach. This approach can aid in applications that seek to gather further information on phase change in complex liquid systems.

Magnetic beads with ~1.9 &#181;m average diameter were used to transport microliter volumes of liquids between contiguous liquid segments with a tube for ...

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

Quantification of Hydrogen Concentrations in Surface and Interface Layers and Bulk Materials through Depth Profiling with Nuclear Reaction Analysis

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and non-destructively reveals the hydrogen density distribution at surfaces, at interfaces, and in the volume of solid materials with high depth resolution. The technique applies a 15N ion beam of 6.385 MeV provided by an electrostatic accelerator and specifically detects the 1H isotope in depths up to about 2 &#956;m from the target surface. Surface H coverages are measured with a sensitivity in the order of ~1013 cm-2 (~1% of a typical atomic monolayer density) and H volume concentrations with a detection limit of ~1018 cm-3 (~100 at. ppm). The near-surface depth resolution is 2-5 nm for surface-normal 15N ion incidence onto the target and can be enhanced to values below 1 nm for very flat targets by adopting a surface-grazing incidence geometry. The method is versatile and readily applied to any high vacuum compatible homogeneous material with a smooth surface (no pores). Electrically conductive targets usually tolerate the ion beam irradiation with negligible degradation. Hydrogen quantitation and correct depth analysis require knowledge of the elementary composition (besides hydrogen) and mass density of the target material. Especially in combination with ultra-high vacuum methods for in-situ target preparation and characterization, 1H(15N,&#945;&#947;)12C NRA is ideally suited for hydrogen analysis at atomically controlled surfaces and nanostructured interfaces. We exemplarily demonstrate here the application of 15N NRA at the MALT Tandem accelerator facility of the University of Tokyo to (1) quantitatively measure the surface coverage and the bulk concentration of hydrogen in the near-surface region of a H2 exposed Pd(110) single crystal, and (2) to determine the depth location and layer density of hydrogen near the interfaces of thin SiO2 films on Si(100).

We illustrate the application of 1H(15N,&#945;&#947;)12C resonant nuclear reaction analysis (NRA) to quantitatively evaluate the density of hydrogen atoms on the surface, in the volume, and at an interfacial layer of solid materials. The near-surface hydrogen depth profiling of a Pd(110) single crystal and of SiO2/Si(100) stacks is described.

Nuclear reaction analysis (NRA) via the resonant 1H(15N,&#945;&#947;)12C reaction is a highly effective method of depth profiling that quantitatively and ...

Electrochemical Etching and Characterization of Sharp Field Emission Points for Electron Impact Ionization

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. The results of studies in which the etching current and the molarity of the NaOH solution used in the etching process were varied are presented. The investigation of the geometry of the tips, by imaging them with a scanning electron microscope, and by operating them in field emission mode is also described. The field emission tips produced are intended to be used as an electron beam source for ion production via electron impact ionization of background gas or vapor in Penning trap mass spectrometry applications.

A method for electrochemically etching field emission tips is presented. Etching parameters are characterized and the operation of the tips in field emission mode is investigated.

A new variation of the drop-off method for fabricating field emission points by electrochemically etching tungsten rods in a NaOH solution is described. ...

Testing of Nanoparticle Release from a Composite Containing Nanomaterial Using a Chamber System

With the rapid development of nanotechnology as one of the most important technologies in the 21st century, interest in the safety of consumer products containing nanomaterials is also increasing. Evaluating the nanomaterial release from products containing nanomaterials is a crucial step in assessing the safety of these products, and has resulted in several international efforts to develop consistent and reliable technologies for standardizing the evaluation of nanomaterial release. In this study, the release of nanomaterials from products containing nanomaterials is evaluated using a chamber system that includes a condensation particle counter, optical particle counter, and sampling ports to collect filter samples for electron microscopy analysis. The proposed chamber system is tested using an abrasor and disc-type nanocomposite material specimens to determine whether the nanomaterial release is repeatable and consistent within an acceptable range. The test results indicate that the total number of particles in each test is within 20% from the average after several trials. The release trends are similar and they show very good repeatability. Therefore, the proposed chamber system can be effectively used for nanomaterial release testing of products containing nanomaterials.

Nanoparticle release is tested using a chamber system that includes a condensation particle counter, an optical particle counter and sampling ports to collect filter samples for microscopy analysis. The proposed chamber system can be effectively used for nanomaterial release testing with a repeatable and consistent data range.

With the rapid development of nanotechnology as one of the most important technologies in the 21st century, interest in the safety of consumer products ...

Measurement of Outgassing Rates of Steels

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of steels meet the criterion of low outgassing required for high or ultrahigh vacuum applications. However, a given material can present different outgassing rates depending on its manufacturing process or the various pretreatment processes involved during the fabrication. Thus, the measurement of outgassing rates is highly desirable for a specific vacuum application. For this reason, the rate-of-pressure rise (RoR) method is often used to measure the outgassing of hydrogen after bakeout. In this article, a detailed description of the design and execution of the experimental protocol involved in the RoR method is provided. The RoR method uses a spinning rotor gauge to minimize errors that stem from outgassing or the pumping action of a vacuum gauge. The outgassing rates of two ordinary steels (stainless steel and mild steel) were measured. The measurements were made before and after the heat pretreatment of the steels. The heat pretreatment of steels was performed to reduce the outgassing. Extremely low rates of outgassing (on the order of 10&#8722;11 Pa m3 sec&#8722;1 m&#8722;2) can be routinely measured using relatively small samples.

A protocol for the measurement of outgassing rates of hydrogen from ordinary steel vacuum chambers using the rate-of-pressure rise method is presented.

Steels are commonly used materials in the fabrication of vacuum systems because of their good mechanical, corrosion, and vacuum properties. A variety of ...

Spark Plasma Sintering Apparatus Used for the Formation of Strontium Titanate Bicrystals

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with one twist grain boundary. This apparatus utilizes high uniaxial pressure and a pulsed direct current for rapid consolidation of material. Diffusion bonding of strontium titanate bicrystals without fracture, in a spark plasma sintering apparatus, is possible at high pressures due to the unusual temperature dependent plasticity behavior of strontium titanate. We demonstrate a method for the successful formation of bicrystals at accelerated time scales and lower temperatures in a spark plasma sintering apparatus compared to bicrystals formed by conventional diffusion bonding parameters. Bond quality was verified by scanning electron microscopy. A clean and atomically abrupt interface containing no secondary phases was observed using transmission electron microscopy techniques. Local changes in bonding across the boundary was characterized by simultaneous scanning transmission electron microscopy and spatially resolved electron energy-loss spectroscopy.

A viable technique for the formation of strontium titanate bicrystals at high pressure and fast heating rate via the spark plasma sintering apparatus is developed.

A spark plasma sintering apparatus was used as a novel method for diffusion bonding of two single crystals of strontium titanate to form bicrystals with ...

Recombination Dynamics in Thin-film Photovoltaic Materials via Time-resolved Microwave Conductivity

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials such as organo-lead halide perovskites is presented. The perovskite film thickness and absorption coefficient are initially characterized by profilometry and UV-VIS absorption spectroscopy. Calibration of both laser power and cavity sensitivity is described in detail. A protocol for performing Flash-photolysis Time Resolved Microwave Conductivity (TRMC) experiments, a non-contact method of determining the conductivity of a material, is presented. A process for identifying the real and imaginary components of the complex conductivity by performing TRMC as a function of microwave frequency is given. Charge carrier dynamics are determined under different excitation regimes (including both power and wavelength). Techniques for distinguishing between direct and trap-mediated decay processes are presented and discussed. Results are modelled and interpreted with reference to a general kinetic model of photoinduced charge carriers in a semiconductor. The techniques described are applicable to a wide range of optoelectronic materials, including organic and inorganic photovoltaic materials, nanoparticles, and conducting/semiconducting thin films.

A Time Resolved Microwave Conductivity technique for investigating direct and trap-mediated recombination dynamics and determining carrier mobilities of thin film semiconductors is presented here.

A method for investigating recombination dynamics of photo-induced charge carriers in thin film semiconductors, specifically in photovoltaic materials ...