Quality factor measurements at NTF [report]

K. Vaziri, F. Krueger, T. Kroc, G. Lauten, A. Lennox, T. Leveling
1993 unpublished
The dose equivalent rate in the radiation field outside of the polydoor at the Neutron Therapy Facility I (see Figure l.) has been measured, using a Chipmunk, assuming a quality factor (QF) of 5, to be 25 mremlhr. This kind of dose rate if true introduced occupancy restrictions and NTF is operating under an exemption. Based on the previous CR-39 studies2 of the neutron field around NTF,and the amount of shielding around the NTF, it was difficult to believe that a significant neutron field
more » ... neutron field exists in this area, and contributes to the measured dose rate. If the field was mostly due to gamma rays the QF setting on the Chipmunk could be reliably set to a value of one. One method of obtaining a qualitative understanding of the relative abundance of neutron and gamma contribution to the absorbed doses, is to measure the quality factor for the field. This was determined using a recombination chamber. The recombination chamber is a gas filled ion chamber that can measure the average quality factor of a radiation field of unknown composition and energy spectrum3.4. The response of the recombination chamber can be described5 as a function of the bias across it's plates as I= kVn, (1) where I is the normalized current collected by the chamber, k is a constant of proportionality, Vis the applied bias and the exponent n can be related to the quality factor of the field. To use the recombination chamber in an unknown field, one needs to have measured a calibration curve using radiation fields of known quality factor6·7,s. The individual neutron and gamma components of the radiation field were also determined in these studies by use of an Andersson-Braun counter (SNOOPY9) to measure the dose equivalent rate due to neutrons, and a CutiePielO ion chamber to measure the gamma dose rate. The neutron dose equivalent rate in this area of NTF has been estimated by Vylet 11 ·2, and is consistent with the present measurements. II. Calibration of the Recombination Chamber A schematic drawing of the recombination chamber setup is shown in Figure 2 . The calibration was performed at the neutron irradiation area of the Radiation Physics Calibration Facility (RPCF). The recombination chamber was placed on an aluminum ladder, half way between the ceiling and floor, to minimize the effects of room scattering. Radiation with at least two different known quality factors need to be measured to produce the linear calibration curve. A gamma ray source ( 6 oCo; 60-4.3-2), and a mixed neutron-gamma source (241Am-Be; 241Be-7.2-l) were used to obtain the calibration reference points. The standard quality factor for gamma rays (of any energy) is assumed to be one. The quality factor due to neutrons is a sensitive function of energyt2.The average quality factor for the mixed 241AmBe source was calculated to be 6.6 using i = 1 Total Dose 1 where (Dose)h and (QF); are dose and the quality factor due to the ith component of the field. Only neutron and gamma radiation fields are of interest in this note. The quality factor for the predominant neutron component of the AmBe source was taken to be 7.912.Tbe results of the calibration measurements are given in Tables I and II . The last column in each table shows the response of the recombination chamber to the radiation field which is calculated using where Iv is the measured current at a bias setting of V, and !saturation is the measured current at saturation bias which is taken as -1200 volts. The data columns labeled Data#!, Data#2, and Data#3 are one minute charge integrations and the net charge for each interval is The recombination chamber response for cobalt and AmBe sources, at different bias settings are plotted in Figures 3 and 4 . The power law fit with the fit parameters are also displayed on each plot. Figure 5 shows the exponent n ( defined in Equation I) plotted against the quality factor for the two sources. The linear fit to these two points, as shown on the plot, provides the calibration curve for the recombination chamber. It is in good agreement with previous calibration measurements 7 . The quality factor for any unknown field is obtained by measuring the response curve. From the fit of the response to Equation l, the exponent n can be obtained, and the quality factor for the unknown field can be read off the calibration curve Figure 5 . III. Measurements The main part of the measurements was done with the recombination chamber positioned at the geometric center of the pol ydoor at NTF, and the Chipmunk detector located on the floor outside the polydoor. The Chipmunk detector was used to provide a redundant dose normalization factor. As mentioned a SNOOPY detector was used to measure the neutron dose equivalent rate, and a CutiePie detector was used to extract the gamma ray component of the field. To simulate the worst case operation of NTF, a large (24X24 cm2) collimator was used, and the beam was scattered off a 5 gallon carboy containing water. Each run lasted about 2.3 minutes and delivered 0.65 monitor units.* III.A RF and X-ray Backgrounds A large RF background can inundate the recombination chamber and make the measurements useless. The RF levels in the area around NTF have been surveyed in the past13, using an RF meter. However, that measurement was only concerned with the health hazards, and the qualitative conclusion was that the levels were below the health hazard threshold. We measured the background using the recombination chamber to see if there were any X-ray and RF pickup effects. This was done a day before the actual measurements and the neutron beam had been off for at least a day. The recombination chamber was operated at the recombination voltage (-65V) and the saturation voltage (-1200V) and also with and without a Faraday cage surrounding the chamber. Our background measurements revealed three important points: First, there was no measurable RF interference. Second, the measurements showed that the background levels are the same as those measured at RPCF. Third, the background was about 0.2% of the dose rate we expected to measure with the beam on, and thus of negligible effect. III.B Background measurements During the day of studies, with the neutron beam off, a background measurement was performed with all the detectors used in this study. The results are shown in Table III . Later measurements with the neutron beam showed that the measured backgrounds for the CutiePie, SNOOPY, and the recombination chamber were less than 0.5% of the dose that was measured with the beam on. After the measurements were completed the background was measured again using the Chipmunk and the recombination chamber. The results are shown in Table IV . The Chipmunk's pre-and postirradiation backgrounds are the same, the recombination chamber measured a higher post-irradiation background due to it's sensitivity to residual induced activity. • The neutron beam is calibrated such that one monitor unit delivers to tissue (or its equivalent) one Gray at a depth of lOcm when a lOXlO cm2 collimator is used. 3 W.C Beam-On Measuremenp; During the beam-on measurements the polydoor was surveyed with a CutiePie; the exposure rate was highest in the middle of the door (4 -6 mR/hr•• ). A dose integration was done with the CutiePie, SNOOPY and a Chipmunk at the middle of the door and on the floor . The results of these measurements are given in Tables V and VI. Data from Radiation Physics Note 8614, and additional tests15 show that the CutiePie has an efficiency of 100% for the gamma rays, and about 23% for the detection of neutrons. Actually the neutron detection efficiency of the CutiePie increases with energy and 23% is a conservative estimate which is true for neutrons of average energy 4. 1 MeV. The SNOOPY is sensitive to neutrons only, and measures the dose equivalent rate directJy9. As shown in Table VI , the neutron dose equivalent on the floor is 1.5 times that at the middle of the door, but the absolute value of the neutron dose equivalent is about 20% of the gamma dose equivalent. The SNOOPY's resolving time is 1 µs. For the LINAC's 57 µs pulse width, and frequency of 15Hz, the dead time correction is 0.2% for the SNOOPY measurement at the center of the door. The correction for the dose rate on the floor is 0.35%. No corrections were applied to the data, since the SNOOPY dead time correction is performed internally. The recombination chamber's response was measured at 17 different bias setting from -30 volts to -1200 volts, as shown in Table Vll . For each one of these bias settings one run was integrated. During these measurements the Chipmunk was placed on the floor and simultaneously integrated the runs, using a scaler. The Chipmunk's measurements indicated the LINAC delivered a fixed dose each run corresponding to the same number of protons. It is seen from the data that the dose delivered was very stable, and the small variation in the chipmunk count-rate ( ±2 counts/run) was due to variation in the background and the reaction time in manually starting and stopping the scaler.
doi:10.2172/10119667 fatcat:g57bu7gfyvhf7lq4zoatlorrjm