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The calibration of the MAST neutron yield monitors
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Nuclear Instruments and Methods in Physics Research A 562 (2006) 521–530 www.elsevier.com/locate/nima
Technical note
The calibration of the MAST neutron yield monitors Keith Stammers, M.J. Loughlin EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK Received 27 January 2006; received in revised form 6 March 2006; accepted 7 March 2006 Available online 5 April 2006
Abstract Several neutron detectors have been installed on MAST to monitor the temporal production of neutrons during neutral beam injection. This paper describes the detectors, their calibration and applications of the data. The main neutron diagnostic is a guarded fission chamber, with processing electronics that allow data collection in three modes of operation, and covers the whole range of neutron production rate to be expected from current operations and future upgrades. The scalar mode of operation is calibrated with a 252Cf source inside the vacuum vessel and then MCNP modelling is used to relate this calibration to an extended plasma source. Plasma neutron data are used to extend the calibration to the Campbell and ion-current modes, with final uncertainties of approximately 8% in each case. Corroborative evidence for the accuracy of the calibration, obtained from neutron activation, indicates that the method is satisfactory. The neutron data are used routinely to keep track of the radio-activation of key components of the MAST tokamak. r 2006 Elsevier B.V. All rights reserved. PACS: 52.70.m Keywords: Neutron; Diagnostics; Calibration; MAST; Tokamak
1. Introduction The MAST tokamak is designed for the investigation of the characteristics of low aspect ratio, magnetic confinement devices. The vacuum vessel is in the form of a right cylinder of 4 m diameter and 4.4 m height, and the tokamak is capable of plasma currents in excess of 1 MA. The purpose of MAST is to contribute to the resolution of key issues for ITER and to address outstanding issues specific to the spherical tokamak concept that have a bearing on its viability as a test facility or power plant. A full description can be found in Ref. [1]. A major feature of MAST is the presence of two neutral beam injectors (NBIs), one (originally two) on loan from the Oak Ridge National Laboratory, and one JET-style PINI. This paper includes discharges with up to 3 MW of injected power with a beam energy of about 50 keV. The injection of high-energy neutral particles affects the behaviour of most MAST plasmas in ways that are beneficial to the majority of our experimental programmes, so NBI is a usual requirement. In particular, the current Corresponding author. Tel.: +44 1235 46 6367; fax: +44 1235 46 6379.
E-mail address: [email protected] (K. Stammers). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.03.012
drive and the effects of increased plasma temperature relax the demands on the power supplies, allowing longer plasma discharge times; up to 0.7 s for this study. The higher plasma temperature available with neutral beam injection allows better access to plasma regimes of reactor relevance. Almost all injection is of deuterium beams into deuterium plasmas, so there is significant production of 2.45 MeV neutrons via the D(d, n)3He reaction and hence a need to monitor the neutron production. Typically the neutron yield is 1013 s1 in discharges of about 300 ms duration. Neutron diagnostics provide a direct measure of fusion performance and, when combined with data from other diagnostics and with modelling, can yield useful information about fundamental plasma behaviour; for example, fast ion losses during saw-tooth crashes. This paper describes the calibration of the neutron counters that provide time-resolved measurement of neutron production. 2. The detectors There are five detectors for monitoring neutrons produced on MAST. They are a 3He proportional counter;
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a small fission counter; a BF3 proportional counter; an activation foil; and a large fission chamber. The 3He proportional counter is a Centronics 15He3/152/ 25 fitted directly with a matching preamplifier. The combination is mounted inside a 50-mm-thick, highmolecular-weight polythene cylinder that is in turn wrapped in 1-mm cadmium sheet and 2-mm of lead shielding. The reaction exploited in this detector is 3
He þ n ! 3 H þ p
ðQ ¼ 0:765 MeVÞ:
This assembly was installed early in the operation of MAST when neutral beams of only hydrogen were being injected. A small quantity of neutralised plasma deuterium would invade the NBI injectors, become ionised and injected, and produce an easily measurable neutron flux. Present operation of MAST NBI, injecting deuterium into deuterium, produces such a copious neutron flux as to saturate this detector during most of the discharge. The small fission chamber, called here FC2, (GE ReuterStokes type RS-P6-0805-134) is part of a Bonner sphere assembly loaned by Princeton Plasma Physics Laboratory (PPPL). Although it was retained in early calibration exercises, its high sensitivity precluded its use during normal operations but data for this detector are reported here to demonstrate the accuracy of the modelling. The BF3 proportional counter (Centronics 13.5EB/20/ 25M) was installed originally in a moderator identical to that of the 3He. The reaction used is 10
B þ n ! 7 Li þ a
ðQ2:5 MeVÞ:
Although purposely less sensitive than the 3He detector, it was still much too sensitive for routine use when deuterium NBI power became significant, even when situated as far as possible from the vessel. To combat this, we designed a moderator/absorber assembly using boric acid to reduce the sensitivity by a factor of 200. This arrangement works well at low neutron production, although saturation begins at about 5 105 counts/s, as does the fission chamber scalar signal. The BF3 calibration factor with the detector attached to a vessel support is 4.17 107 source neutrons per count, so this detector can be used with a plasma source of up to 2 1013 neutrons per second. The large fission chamber, called here FC3, is the primary neutron diagnostic on MAST and is the focus of
this report. It is a cylindrical, guarded chamber (GE Reuter-Stokes type RS-C3-2510-114) with 1.34 g 235U and sits inside a concentric electrostatic shield, this combination being surrounded by a 50-mm-thick, high-molecularweight, polythene moderator. The assembly is positioned as close to the vessel as conveniently possible, 0.7 m from the vessel wall and 0.57 m above the mid-plane. There are two connections (high-voltage and signal) each of which is used in the associated signal-conditioning unit that is on loan from the PPPL and that was used successfully on TFTR [2]. The conditioning unit is a Gamma-Metrics model RCS 100 that provides output signals for three modes of operation:
Pulse (proportional counter) mode - or ‘‘scalar mode’’. Campbell mode. DC (ion current) mode.
A fourth signal, log10(count-rate), is not used. The Campbell [3] mode of operation uses the statistical variations in the pulse rate to extend the useful range of the pulse data well beyond the onset of saturation. In practice, the Campbell signal is provided as a voltage proportional to log10(equivalent, linearised Campbell count-rate) with an appropriate offset. The DC mode, in which the signal is a voltage proportional to the ion current, is intended for only the highest neutron fluxes and may not be required on MAST, but is always included for completeness in calibrations. Fig. 1 shows raw data from the three signals for a typical plasma shot. The MAST neutron production rate is too low at present for the detector to enter a truly valid range of operation of the DC mode, with typical signal levels at current neutron rates of 10 mV compared with an available, full-scale range of 10 V. Although the DC data appear unpromising, smoothing to remove 50 Hz pick up is effective at the expense of much reduced time resolution, and in the case of failure of the Campbelling electronics we would at least have some data available, although with larger errors. The HT and signal leads to the fission chamber are both 50 O, super-screened, co-axial cable (type MM15/50LSF) which is very flexible and far easier to install than the rigid cable provided with the signal-conditioning unit. The electrostatic screen around the fission chamber is not
Fig. 1. Raw data signals from the large fission chamber.
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continued around the cables but appears not to be contributing to the small level of 50 Hz pick up on the ion-current signal. 3. Data acquisition A CAMAC crate houses all the data acquisition. The fission chamber pulse signal is taken, along with signals from the proportional counters described earlier, to a LeCroy 8590, 8-channel scalar with 8206A memory unit, and the CAMAC control computer defines the integration time that is set at present to 5 ms. The analogue signals have a potential, full-scale range of 0 to +10 V, matching the range of the Aurora Model 14 transient recorder. The present sampling period is set to 10 ms. 4. Calibration The calibration of the fission chamber falls into four parts: 1. The use of a neutron source inside the tokamak vessel to characterise the response of the pulse mode of detection to a toroidal line-source. 2. Neutron modelling of this data to obtain a calibration factor for a bulk plasma source. 3. The use of suitable, plasma-derived, neutron data to transfer the calibration from the pulse mode to the analogue modes of operation. 4. An independent calibration using indium activation. The need for the third action is a consequence of the unacceptably strong neutron source that would be needed to activate the analogue modes and the calibrations of the Campbell and DC modes are therefore necessarily of secondary and tertiary natures, respectively. Jasby et al. [4] describe this process for TFTR but Medley and Darrow [5], at NSTX dispense with the Campbell mode despite the reduced overlap available between the scalar and current modes during plasma-derived neutrons. 4.1. Calibration of the pulse-mode The preferred source for calibration work is Californium-252 that has a mean neutron energy of about 2.5 MeV, very close to that of d–d neutrons. The procedure for this calibration, as in many other laboratories (see for example Ref. [6]), is to move the 252Cf neutron source around the major axis of the torus, measuring the count rate at discrete points. Despite the popularity of this method, Jarvis et al. [12] ‘‘ y warn against the uncritical use of 252Cf for calibration purposes’’ since in their experience 252Cf neutrons can be ‘‘disproportionately moderated and absorbed, relative to 2.5 MeV neutrons, by massive diagnostic equipment y interposed between the fission chamber and the diagnostic port’’. In their case, however, the source of neutrons was leakage through the
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diagnostic ports (since the remainder of the vessel was well shielded), whereas for MAST there is a relatively small mass of diagnostic equipment at most sectors and the fission chamber, mounted above the mid-plane (where the diagnostics are) has a good view through the unshielded vessel wall of the plasma or calibration source. We feel, therefore, that our use of a 252Cf source is justified and is supported by MCNP calculations, detailed below, in which detection efficiency for 252Cf and plasma neutron sources are compared. Strachan et al. [7] have proposed a calibration protocol that would facilitate comparison between laboratories, recommending in particular the use of a 252Cf neutron source placed at 60 toroidal locations, the count rate of each neutron detector being recorded. Hendel et al. [2] describe an extension of this procedure performing 930 measurements both toroidally and poloidally, whilst Heidbrink et al. [8] employ a model train that moves constantly along the toroidal axis. This latter method has the twin advantages of simplicity and providing the toroidal integral directly but lacks the value of spatial information gained from discrete measurements. Although we have a device for performing the physical movement with 101-toroidal resolution, its use has been precluded so far due to the short time available for its installation and the calibration work. The compromise is to introduce the source on a string through ports at the top of the vessel. On MAST there is a ring of six ports at a radius of 950 mm on the even-numbered sectors that have a view between the poloidal field coils. The new divertor restricts access through these ports to an 18-mm slot. The poor spatial resolution is far from ideal but the method is programmatically pragmatic. The procedure adopted is to lower the source to the midplane and collect sufficient counts for acceptable statistics; the process being repeated at each of the remaining toroidal positions. In addition a vertical scan is performed at 30 cm intervals in at least one toroidal position. Fig. 2 shows the results of the first calibration in which the effects of the centre column on the mid-plane scan and the internal poloidal coils on the vertical scan are clearly visible. The data from this procedure can be used to obtain a crude estimate of the calibration factor (source neutrons per count), but a better value can be obtained by using the data in an MCNP model. 4.2. MCNP calculation of the pulse-mode calibration factor MCNP is a Monte-Carlo radiation transport code maintained by United States’ Los Alamos National Laboratory [9]. We have used it to calculate the neutron transport through the MAST vacuum vessel to three detectors and to determine the neutron fluxes and spectra at these positions. To calculate the neutron transport using MCNP, one must first define a ‘‘geometry’’. This is done by describing a number of cells using standard surfaces (e.g. planes, cones)
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Mid-plane scan - FC3 0.25
Nett cps
0.20
0.15
0.10
0.05
0.00 0
2
4
6 8 10 Port number
12
14
(a)
Vertical scan - Port 10 - FC3 0.25
Nett cps
0.20
0.15
0.10
0.05
-200
0.00 -100 0 100 Height above mid-plane, cm
200
Fig. 2. Representative calibration data from fission chamber FC3 located at sector 11. MAST has 12 sectors with three ports on the top plate at each sector. Alternate sectors have one of these ports at 950-mm radius, close to the nominal toroidal axis and useful for calibration purposes, see Fig. 4 also.
and/or volumes (spheres, cylinder cubes, etc.). Each cell is filled with a material of a specific isotopic composition and density. The model used in these calculations includes the concrete floor, walls and ceiling of the MAST area but a wooden floor above the pit is not included. The steel vacuum vessel is described as a simple cylinder without port details. The central column, the PF coils, and TF coils that demarcate 12 sectors are described but other internal components of the vessel, such as the coil supports, are not (see Fig. 3). The new divertor was not in place for this work. The responses of the BF3, and the two fission chambers were calculated. Their locations are indicated in Fig. 4. To determine the calibration, the number of reactions in the detector volume per source neutron must be computed. This is done directly in the Monte-Carlo calculation by
(b) Fig. 3. (a) MCNP model of MAST. Model components external to the MAST vessel, including the biological shield. The roof is not displayed, for clarity. The nominal position of the large fission chamber FC3 is shown. (b) MCNP model of MAST. A section of the vessel wall is removed to show modelled components inside the vessel. The positions of the small fission chamber FC2 and a representative calibration port are also shown.
multiplying the neutron flux f(E) by the reaction crosssection sn,x(E) and the number of atoms in the sensitive volume using MCNP’s track length estimator of the particle flux. By further multiplying the reaction rate by the source strength one can directly predict the experimental detector count rate. Two types of neutron source were modelled, a point californium source and a d–d MAST-plasma, fusion neutron source.
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TF coils FC3
12
1
11
BF3
2
10
3
9
4
Ports used for introducing the neutron source.
1.0 0.8 0.6 0.4 0.2 0.0 0
2
4
6 8 Sector number
10
12
14
5 7
6 Sector numbers
Fig. 4. Plan view of the of the detector locations relative to MAST. Table 1 Calculations and measurements of detector count-rate using a
2 4 6 8 10 12
1.2
FC2 8
Sector
We modelled the production of neutrons from a deuterium plasma using a specially written MCNP subroutine. This subroutine has been used successfully to model neutron production on other tokamaks including JET and NSTX. The subroutine simulates a neutron emission profile with D-shaped contours of constant emission distributed throughout the vessel. The energy spectrum is Gaussian with a full-width at half-maximum of 320 keV that is appropriate to a beam-heated plasma in which beam-plasma neutron production dominates. Table 2 shows the parameters for the model, and Table 3 the results expressed as the numbers of counts in the detector per source neutron. The calibration factor is defined as R/(counts per plasma neutron) where R is the ratio of the calculated sensitivity to the californium source (counts per neutron) to the measured sensitivity, summed over the measurements in each sector: P C R ¼ Psectors . sectors E
Normalised cps
Six MCNP runs were required to reproduce the calibration experiment. Twenty-five million histories and typically 16 h were required for each run. The results are shown in Table 1. The errors are statistical which arise from the stochastic nature of the Monte-Carlo calculations. The agreement between calculation and experiment is excellent for the BF3 and FC2 detectors, the average countrate being only 20% low for the BF3 and 9% high for FC2. The large disagreement between the calculated and measured counts in FC3 (a factor of 27) is due to the setting of a high threshold in the processing electronics that is used to eliminate pulses from a particles and g-rays. However, the toroidal dependence demonstrated in the experimental data is reproduced in the calculations. This is shown in Fig. 5. The MCNP results have been normalised to the experimental measurements to demonstrate the agreement in toroidal variation. In addition, the BF3 data and the FC3 data in Fig. 5 (both experimental and calculated) have been scaled by factors of 5 and 10, respectively, for convenience of display. This validates the model which we then used to determine the response to an extended plasma neutron source; the ratio of the calculated to experimental responses observed with the californium source being used to provide a correction to the model results.
525
FC2(exp)
FC2 (MCNP)
FC3 (exp)
FC3 (MCNP)
BF3 (exp)
BF3 (MCNP)
Fig. 5. Normalised comparison of experiment with MCNP modelling.
252
Cf source
FC2
BF3
FC3
MCNP BF3 (cps)
MCNP error (%)
Expt. (cps)
MCNP FC2 (cps)
MCNP error (%)
Expt. (cps)
MCNP FC3 (cps)
MCNP error (%)
Expt. (cps)
1.58 102 1.68 102 2.27 102 4.11 102 5.02 102 2.04 102
10.1 9.62 8.33 5.42 6.16 7.45
2.03 102 1.75 102 2.62 102 5.60 102 6.34 102 2.43 102
0.554 1.21 0.553 0.281 0.139 0.298
2.5 1.7 2.5 3.5 4.7 3.4
0.555 1.06 0.551 0.267 0.121 0.259
2.49 1.8 1.82 2.53 4.55 4.62
1.5 1.6 1.6 1.4 1.0 1.1
0.104 0.054 0.055 0.088 0.178 0.180
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4.E+06
Table 2 Parameters describing the plasma neutron source 70.0 50.0 2.5 0.3 320
Table 3 Detector sensitivities and calibration factors Detector
Sensitivity to plasma neutrons (counts/n)
R
Normalised calibration (neutrons/count)
BF3 FC2 FC3
1.928 108 3.3814 107 1.8219 106
0.804 1.087 27.169
4.17 107 3.21 107 1.49 107
The normalisation values R are described in the text.
In this way the detectors are calibrated against the standard neutron source and MCNP has effectively been used to determine the effects of extending the neutron source and differences in the energy distribution of the plasma source neutrons. The values for R can be derived from Table 1 and are listed in Table 3. The resulting calibration factor of the large fission chamber for a volume neutron source is 1.49 107 neutrons per count. 4.3. Calibration of the Campbell and current modes Since only the pulse mode of operation can be calibrated directly, one must rely upon a secondary calibration of the Campbell mode by relating signals from the pulse and Campbell modes during actual plasma neutron production. The calibration of the DC (current) mode is obtained similarly by relating these signals to those of the Campbell mode at higher neutron production rates when the pulse mode has saturated. The DC calibration is therefore of a tertiary nature. Data for this calibration exercise were taken from a range of shots in which the NBI power was scanned across its whole practicable range at the time, ensuring that there would be sufficient information in each operational mode of the fission chamber to allow comparison of signals. Fig. 6 shows the comparison of Campbell and scalar (pulse) modes for a range of NBI shots. The saturation of the scalar mode is clear and allows one to select the upper linear limit of the validity of scalar data. Note that since the raw Campbell voltage represents roughly log10 of the desired, linear signal, it must be linearised and backgroundsubtracted before use. In addition, since the analogue signals are sampled every 10 ms and the pulse counter integrates over 5 ms, the Campbell data (for this calibration exercise) are averaged over the 5 ms periods corresponding to the scalar data. Before taking the slope of the linear
Linearised campbell, a.u.
Shots 7070 - 7080
Major radius (cm) Minor radius (cm) Elongation Triangularity Spectrum width (FWHM) (keV)
3.E+06
2.E+06
1.E+06
0.E+00 0.E+00
1.E+06 Scalar, cps
2.E+06
Fig. 6. Showing the onset of non-linearity in the relative responses of fission chamber’s Campbell and scalar modes. Each point represents average count-rate over a scalar integration period (5 ms) and Campbell signal averaged over the same period. The reason for outlying points is not known.
the the the the
region, Campbell data below its region of validity must be removed. For this data, a lower Campbell limit of 2 104 is taken (most easily found from a log–log plot) and the linear plot of restricted data (Fig. 7) yields the final calibration relationship. Although the signal-conditioning electronics could be adjusted to make a zero intercept and provide a simple constant of proportionality between the two signals, this would be very time consuming and unnecessary, so we use a two-parameter calibration factor. The relationship between the Campbell and the DC (current mode) signals is derived from high-power NBI shots in a similar manner, as shown in Fig. 8, and the equations relating the three signals are Neutrons ¼ 1:49 107 P
ðby source calibrationÞ
4
CB ¼ 5:59 10 þ 0:863 P DC ¼ 7:80 105 þ 1:29 108 CB
ð1Þ
where P is the scalar pulse rate, CB is the linearised Campbell signal and DC is the current-mode signal. Since the scalar calibration uses nett counts, the intercept is zero. With a little algebra, the final calibration equations for each operation mode are n=s ¼ 1:49 107 P 11
Scalar mode 7
n=s ¼ 9:66 10 þ 1:73 10 CB n=s ¼ 8:62 1011 þ 1:34 1015 DC
Campbell mode DC mode: (2)
Conservative thresholds for the valid ranges of each mode are used as guides in the on-line analysis programme to construct a complete temporal profile of the neutron rate. The thresholds used in practice, in equivalent neutrons per
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1.E+06
Linearised Campbell, a.u.
y = 0.8632x - 55942 8.E+05
6.E+05
4.E+05
2.E+05
0.E+00 0.0E+00
4.0E+05 8.0E+05 Scalar, cps
1.2E+06
Fig. 7. Data selected from the linear, overlapping ranges of the Campbell and scalar modes of operation. The fitting parameters are used in the generation of the final calibration equations. 0.05
Fig. 9. Showing the analysed data from all three modes combined into a single, temporal record. Data from each mode are separated by horizontal lines, for clarity and to demonstrate matching at the thresholds. In this figure, the lower limit to the heavily smoothed DC data has been set arbitrarily to 2 1013 n/s just for this demonstration, although this is below the acceptable range of routine operation for the DC mode.
y = 1.29E-08x + 7.80E-05
DC, Volts
0.04
R2
= 9.93E-01
0.03
0.02
0.01
0 0.E+00
1.E+06
2.E+06
3.E+06
4.E+06
Linearised campbell, a.u. Fig. 8. Data selected from the linear, overlapping ranges of the Campbell and DC modes of operation. The fitting parameters are used in the generation of the final calibration equations
second, are:
operational mode are combined to provide a temporal profile of neutron rate. The selected data ranges displayed in Figs. 7 and 8 show considerable scatter. The analogue threshold for the pulse counter is set very high to be sure of not counting the 235U alpha decays and gamma rays (at the expense of neutron sensitivity) so the pulse data is unlikely to be the source of scatter. The cause of this behaviour is not known at present and the noise signal, when there is no neutral beam injection, is negligible in all signal modes. 5. Treatment of errors There are several potential contributions to the uncertainty in the calculation of plasma neutron rate:
Lower limit for Campbell mode : 5 1012 n=s Lower limit for DC mode : 2 1014 n=s: The lower DC limit has been set to a value that will not be exceeded at present since the onset of non-linearity in the Campbell signal has not been observed. Indeed, the maximum neutron rate seen on MAST in these experiments is 8 1013 n/s and the DC mode data are not used. The DC threshold may be revised when the neutral beam system is upgraded to 75 kV at which the predicted neutron rate of 3 1014 n/s may be reached. In Fig. 9 is shown an example of how the data from the valid range of each
calibration of the source, Monte-Carlo calculation of the detector scalar response coefficient, statistics of the detector signals, transfer of the scalar calibration to the other modes of detector operation.
Our 252Cf neutron source was calibrated last about twenty years ago using the manganese bath technique (see for example Ref. [10]). This method is known to be accurate to about 2% for the original, nominal source
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strength so we use the published half-life to correct for decay and assume 2% uncertainty. The Monte-Carlo code MCNP is used, with the scalar calibration data as described earlier, to generate a response coefficient for volume, plasma neutron sources. The accuracy of the calculation depends upon the detail of the model and the time available for running the code. The error calculated by MCNP for the scalar response is 8% and dominates the error for the scalar calibration with the neutron source. While the error on the scalar signal is taken to be ON, the noise on the Campbell and (heavily smoothed) DC signals are negligible at typical neutron rates. In addition, the three signal modes are not independent, so we cannot use conventional error analysis to derive the uncertainties on the calibration equations. The scatter on the points in Figs. 7 and 8 is very much larger than indicated by counting statistics and noise. Without attempting at this stage to explain this feature, we treat the errors as instrumental in origin and analyse them to determine the errors on the calibration equation parameters. Referring to Fig. 7: we note that the deviations of the points from the fitted line appear to be independent of the signal magnitude. Since these mode signals are not independent, one can take either signal as the independent variable (in this case the scalar signal) and examine the distribution of deviations from the fitted line—Fig. 10. Even though the distribution may not be normal, we fit a Gaussian that yields a value for a standard deviation and use this value as the instrumental uncertainty on each point, following the treatment of Bevington [11]. For the DC vs. Campbell data: analysis of the deviations from the fitted line (Fig. 8) shows them to be independent of signal magnitude (the larger apparent spread at higher data values is due to the higher density of points). We therefore treat the data in the same way as the Campbell vs.
scalar data and Eqs. (1) now become Neutrons ¼ 1:49 107 ð0:087Þ P ðby source calibrationÞ CB ¼ 5:59 104 ð0:116Þ þ 0:863ð0:0121Þ P DC ¼ 7:80 105 ð1:49Þ þ 1:29 108 ð4:98 103 Þ CB
where the errors are expressed here as fractional errors. Rearranging these equations we arrive at Eq. (2a): Scalar mode n=s ¼ 1:49 107 ð1:30 106 Þ P Campbell mode n=s ¼ ð9:66 1011 1:37 1011 Þ þ ð1:73 107 1:52 106 Þ CB DC mode n=s ¼ ð8:62 1011 1:29 1012 Þ þ ð1:34 1015 1:18 1014 Þ DC: ð2aÞ 6. Applications of the data Since these neutron diagnostics are dependent upon MCNP for ultimate calibration, the results from each of them are not truly independent, but a comparison is useful for confidence in the application of the modelling. Shown in Fig. 11 are data from the fission chamber and the neutron activation diagnostics. The agreement shown here is very good and gives us confidence in the modelling and calibration methods. An indium foil of 13 g is positioned at the end of a re-entrant port close to the mid-plane, the induced activity being measured with a calibrated germanium detector. The calibration of the activation diagnostic will be reported elsewhere. Measurements of the time-resolved neutron emission can be correlated with the instabilities in the plasma. Fig. 12
0.14
2.0E+14
0.12 1.6E+14
Indium Foil, neutrons
Normalised deviation frequency.
ð1aÞ
0.1 0.08 0.06 0.04
y = 1.01E+00x - 2.78E+12 R2 = 9.93E-01
1.2E+14
8.0E+13
4.0E+13
0.02 0.0E+00 0.0E+00
0 0.E+00
5.E+04
1.E+05
5.0E+13 1.0E+14 1.5E+14 Fission Chamber, neutrons
2.0E+14
2.E+05
Campbell signal bin value Fig. 10. Distribution of deviations from the fitted line of Fig. 7. The standard deviation for the fitted Gaussian is 2.42 105.
Fig. 11. Comparison of fission chamber and indium foil activation data. Each point represents the integral of a whole day’s shots. The point close to the origin represents just two NBI shots in the morning, with the foil being analysed in the evening.
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Fig. 12. Example plasma shot data. The sudden decreases of the neutron signal correlate well with the sharp rises in Da emission due to saw-teeth.
shows the neutron emission as a function of time during shot 11835 that was a 550 kA, beam-heated discharge. The drops in the global neutron emission of nearly 40% are correlated with saw-tooth crashes, indicated by the rise in the Da signal. Although the neutron production on MAST is insufficient at present to cause health physics concerns, it is prudent to consider at this stage the consequences of upgrading the neutral beam injection facilities. To this end, we have a suite of computer codes that accumulates integrated neutron production from each shot and calculates the current radio-activity of the vessel steel and the Inconel centre column, and the decay time at which a specified, total activity in Bq/kg will be reached. The activities of the main isotopic constituents are provided individually. When neutron activation becomes a problem and the movement of activated components must be controlled, the codes can estimate the current activity of stainless-steel components near the vessel wall and of the Inconel centre column. In particular, the activity of items that have been exposed for a known period or range of shots can be estimated, again with a prediction of decay time to some prescribed level. 7. Summary and conclusions The main neutron diagnostic on MAST is a guarded fission chamber with processing electronics that allow data
collection in three modes of operation: scalar (pulse), Campbell and ion current. The scalar mode of operation is calibrated with a 252Cf source inside the vacuum vessel and then MCNP modelling used to relate this calibration to an extended plasma source. Neutral beam-heated plasmas are then used to extend the calibration to the Campbell and ion-current modes. Corroborative evidence for the accuracy of the calibration, obtained from neutron activation, indicates that the calibration method is satisfactory. The neutron data are used routinely to keep track of the radio-activation of key components of the MAST tokamak. Acknowledgements This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by the European Communities under the Contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission. We are indebted to Lane Roquemore of the Princeton Plasma Physics Laboratory for lending us the electronic processing system for the large fission chamber, and to the manufacturer, Gamma-Metrics, for helping us to keep it working. It is a pleasure to acknowledge the other members of the JET Neutron Group who gave freely of their time to work
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the night shift during calibrations, and Alan Sykes has provided constant managerial support for the MAST neutron diagnostic. Mark Gilbert provided the MCNP-modelled activation coefficient for the Indium foil diagnostic. References [1] A.C. Darke, et al., Fusion Sci. Technol. (JT-60 Special Issue) 42 (2–3) (2002) 482; B. Lloyd, et al., Fusion 46 (2004) B477. [2] H.W. Hendel, et al., Rev. Sci. Instr. 61 (7) (1990) 1900. [3] N.R. Campbell, Proc. Cambridge Phil. Soc. 15 (1908–1910) 117; R.A. DuBridge, IEEE Trans. Nucl. Sci. NS-5–14 (1967) 241. [4] D.L. Jasby, et al., Rev. Sci. Instr. 70 (1) (1999) 1111. [5] S.S. Medley, D.S. Darrow, A.L. Roquemore, Reconciliation of measured and TRANSP-calculated neutron emission rates in the National Spherical Torus Experiment Princeton Report PPPL-4080, 2005.
[6] M. Isobe, M. Osakabe, M. Sasao, K. Matsuoka, J. Fujita, Rev. Sci. Instr. 66 (1) (1995) 923; E.T. Sugie, et al., Fusion Sci. Technol. (JT-60 Special Issue) 42 (2–3) (2002) 482; C.L. Fiore, R.L. Boivin, Rev. Sci. Instr. 66 (1) (1995) 945. [7] J.D. Strachan, et al., Rev. Sci. Instr. 61 (11) (1990) 3501. [8] W. Heidbrink, P.L. Taylor, J.A. Phillips, Rev. Sci. Instr. 68 (1) (1997) 536. [9] J.F. Briesmeister (Ed.), MCNPTM—A general Monte Carlo N-particle transport code, Los Alamos National Laboratory Report, LA-13709-M (March 2000), version 4C3. [10] J.B. Marion, J.L. Fowler, Fast Neutron Physics, Part. 1, Interscience Publishers, New York, 1960, p. 658; N.J. Roberts, MCNP Calculations of Correction Factors for Radionuclide Neutron Source Emission Rate Measurements Using the Manganese Bath, National Physical Laboratory, Teddington, UK, 2001 (Report CIRM45). [11] P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969 (Section 6.3). [12] O.N. Jarvis, G. Sadler, P. van Belle, T. Elevant, Rev. Sci. Instr. 61 (10) (1990) 3172.
Nuclear Instruments and Methods in Physics Research A 562 (2006) 521–530 www.elsevier.com/locate/nima
Technical note
The calibration of the MAST neutron yield monitors Keith Stammers, M.J. Loughlin EURATOM/UKAEA Fusion Association, Culham Science Centre, Abingdon, Oxfordshire, OX14 3DB, UK Received 27 January 2006; received in revised form 6 March 2006; accepted 7 March 2006 Available online 5 April 2006
Abstract Several neutron detectors have been installed on MAST to monitor the temporal production of neutrons during neutral beam injection. This paper describes the detectors, their calibration and applications of the data. The main neutron diagnostic is a guarded fission chamber, with processing electronics that allow data collection in three modes of operation, and covers the whole range of neutron production rate to be expected from current operations and future upgrades. The scalar mode of operation is calibrated with a 252Cf source inside the vacuum vessel and then MCNP modelling is used to relate this calibration to an extended plasma source. Plasma neutron data are used to extend the calibration to the Campbell and ion-current modes, with final uncertainties of approximately 8% in each case. Corroborative evidence for the accuracy of the calibration, obtained from neutron activation, indicates that the method is satisfactory. The neutron data are used routinely to keep track of the radio-activation of key components of the MAST tokamak. r 2006 Elsevier B.V. All rights reserved. PACS: 52.70.m Keywords: Neutron; Diagnostics; Calibration; MAST; Tokamak
1. Introduction The MAST tokamak is designed for the investigation of the characteristics of low aspect ratio, magnetic confinement devices. The vacuum vessel is in the form of a right cylinder of 4 m diameter and 4.4 m height, and the tokamak is capable of plasma currents in excess of 1 MA. The purpose of MAST is to contribute to the resolution of key issues for ITER and to address outstanding issues specific to the spherical tokamak concept that have a bearing on its viability as a test facility or power plant. A full description can be found in Ref. [1]. A major feature of MAST is the presence of two neutral beam injectors (NBIs), one (originally two) on loan from the Oak Ridge National Laboratory, and one JET-style PINI. This paper includes discharges with up to 3 MW of injected power with a beam energy of about 50 keV. The injection of high-energy neutral particles affects the behaviour of most MAST plasmas in ways that are beneficial to the majority of our experimental programmes, so NBI is a usual requirement. In particular, the current Corresponding author. Tel.: +44 1235 46 6367; fax: +44 1235 46 6379.
E-mail address: [email protected] (K. Stammers). 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.03.012
drive and the effects of increased plasma temperature relax the demands on the power supplies, allowing longer plasma discharge times; up to 0.7 s for this study. The higher plasma temperature available with neutral beam injection allows better access to plasma regimes of reactor relevance. Almost all injection is of deuterium beams into deuterium plasmas, so there is significant production of 2.45 MeV neutrons via the D(d, n)3He reaction and hence a need to monitor the neutron production. Typically the neutron yield is 1013 s1 in discharges of about 300 ms duration. Neutron diagnostics provide a direct measure of fusion performance and, when combined with data from other diagnostics and with modelling, can yield useful information about fundamental plasma behaviour; for example, fast ion losses during saw-tooth crashes. This paper describes the calibration of the neutron counters that provide time-resolved measurement of neutron production. 2. The detectors There are five detectors for monitoring neutrons produced on MAST. They are a 3He proportional counter;
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a small fission counter; a BF3 proportional counter; an activation foil; and a large fission chamber. The 3He proportional counter is a Centronics 15He3/152/ 25 fitted directly with a matching preamplifier. The combination is mounted inside a 50-mm-thick, highmolecular-weight polythene cylinder that is in turn wrapped in 1-mm cadmium sheet and 2-mm of lead shielding. The reaction exploited in this detector is 3
He þ n ! 3 H þ p
ðQ ¼ 0:765 MeVÞ:
This assembly was installed early in the operation of MAST when neutral beams of only hydrogen were being injected. A small quantity of neutralised plasma deuterium would invade the NBI injectors, become ionised and injected, and produce an easily measurable neutron flux. Present operation of MAST NBI, injecting deuterium into deuterium, produces such a copious neutron flux as to saturate this detector during most of the discharge. The small fission chamber, called here FC2, (GE ReuterStokes type RS-P6-0805-134) is part of a Bonner sphere assembly loaned by Princeton Plasma Physics Laboratory (PPPL). Although it was retained in early calibration exercises, its high sensitivity precluded its use during normal operations but data for this detector are reported here to demonstrate the accuracy of the modelling. The BF3 proportional counter (Centronics 13.5EB/20/ 25M) was installed originally in a moderator identical to that of the 3He. The reaction used is 10
B þ n ! 7 Li þ a
ðQ2:5 MeVÞ:
Although purposely less sensitive than the 3He detector, it was still much too sensitive for routine use when deuterium NBI power became significant, even when situated as far as possible from the vessel. To combat this, we designed a moderator/absorber assembly using boric acid to reduce the sensitivity by a factor of 200. This arrangement works well at low neutron production, although saturation begins at about 5 105 counts/s, as does the fission chamber scalar signal. The BF3 calibration factor with the detector attached to a vessel support is 4.17 107 source neutrons per count, so this detector can be used with a plasma source of up to 2 1013 neutrons per second. The large fission chamber, called here FC3, is the primary neutron diagnostic on MAST and is the focus of
this report. It is a cylindrical, guarded chamber (GE Reuter-Stokes type RS-C3-2510-114) with 1.34 g 235U and sits inside a concentric electrostatic shield, this combination being surrounded by a 50-mm-thick, high-molecularweight, polythene moderator. The assembly is positioned as close to the vessel as conveniently possible, 0.7 m from the vessel wall and 0.57 m above the mid-plane. There are two connections (high-voltage and signal) each of which is used in the associated signal-conditioning unit that is on loan from the PPPL and that was used successfully on TFTR [2]. The conditioning unit is a Gamma-Metrics model RCS 100 that provides output signals for three modes of operation:
Pulse (proportional counter) mode - or ‘‘scalar mode’’. Campbell mode. DC (ion current) mode.
A fourth signal, log10(count-rate), is not used. The Campbell [3] mode of operation uses the statistical variations in the pulse rate to extend the useful range of the pulse data well beyond the onset of saturation. In practice, the Campbell signal is provided as a voltage proportional to log10(equivalent, linearised Campbell count-rate) with an appropriate offset. The DC mode, in which the signal is a voltage proportional to the ion current, is intended for only the highest neutron fluxes and may not be required on MAST, but is always included for completeness in calibrations. Fig. 1 shows raw data from the three signals for a typical plasma shot. The MAST neutron production rate is too low at present for the detector to enter a truly valid range of operation of the DC mode, with typical signal levels at current neutron rates of 10 mV compared with an available, full-scale range of 10 V. Although the DC data appear unpromising, smoothing to remove 50 Hz pick up is effective at the expense of much reduced time resolution, and in the case of failure of the Campbelling electronics we would at least have some data available, although with larger errors. The HT and signal leads to the fission chamber are both 50 O, super-screened, co-axial cable (type MM15/50LSF) which is very flexible and far easier to install than the rigid cable provided with the signal-conditioning unit. The electrostatic screen around the fission chamber is not
Fig. 1. Raw data signals from the large fission chamber.
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continued around the cables but appears not to be contributing to the small level of 50 Hz pick up on the ion-current signal. 3. Data acquisition A CAMAC crate houses all the data acquisition. The fission chamber pulse signal is taken, along with signals from the proportional counters described earlier, to a LeCroy 8590, 8-channel scalar with 8206A memory unit, and the CAMAC control computer defines the integration time that is set at present to 5 ms. The analogue signals have a potential, full-scale range of 0 to +10 V, matching the range of the Aurora Model 14 transient recorder. The present sampling period is set to 10 ms. 4. Calibration The calibration of the fission chamber falls into four parts: 1. The use of a neutron source inside the tokamak vessel to characterise the response of the pulse mode of detection to a toroidal line-source. 2. Neutron modelling of this data to obtain a calibration factor for a bulk plasma source. 3. The use of suitable, plasma-derived, neutron data to transfer the calibration from the pulse mode to the analogue modes of operation. 4. An independent calibration using indium activation. The need for the third action is a consequence of the unacceptably strong neutron source that would be needed to activate the analogue modes and the calibrations of the Campbell and DC modes are therefore necessarily of secondary and tertiary natures, respectively. Jasby et al. [4] describe this process for TFTR but Medley and Darrow [5], at NSTX dispense with the Campbell mode despite the reduced overlap available between the scalar and current modes during plasma-derived neutrons. 4.1. Calibration of the pulse-mode The preferred source for calibration work is Californium-252 that has a mean neutron energy of about 2.5 MeV, very close to that of d–d neutrons. The procedure for this calibration, as in many other laboratories (see for example Ref. [6]), is to move the 252Cf neutron source around the major axis of the torus, measuring the count rate at discrete points. Despite the popularity of this method, Jarvis et al. [12] ‘‘ y warn against the uncritical use of 252Cf for calibration purposes’’ since in their experience 252Cf neutrons can be ‘‘disproportionately moderated and absorbed, relative to 2.5 MeV neutrons, by massive diagnostic equipment y interposed between the fission chamber and the diagnostic port’’. In their case, however, the source of neutrons was leakage through the
523
diagnostic ports (since the remainder of the vessel was well shielded), whereas for MAST there is a relatively small mass of diagnostic equipment at most sectors and the fission chamber, mounted above the mid-plane (where the diagnostics are) has a good view through the unshielded vessel wall of the plasma or calibration source. We feel, therefore, that our use of a 252Cf source is justified and is supported by MCNP calculations, detailed below, in which detection efficiency for 252Cf and plasma neutron sources are compared. Strachan et al. [7] have proposed a calibration protocol that would facilitate comparison between laboratories, recommending in particular the use of a 252Cf neutron source placed at 60 toroidal locations, the count rate of each neutron detector being recorded. Hendel et al. [2] describe an extension of this procedure performing 930 measurements both toroidally and poloidally, whilst Heidbrink et al. [8] employ a model train that moves constantly along the toroidal axis. This latter method has the twin advantages of simplicity and providing the toroidal integral directly but lacks the value of spatial information gained from discrete measurements. Although we have a device for performing the physical movement with 101-toroidal resolution, its use has been precluded so far due to the short time available for its installation and the calibration work. The compromise is to introduce the source on a string through ports at the top of the vessel. On MAST there is a ring of six ports at a radius of 950 mm on the even-numbered sectors that have a view between the poloidal field coils. The new divertor restricts access through these ports to an 18-mm slot. The poor spatial resolution is far from ideal but the method is programmatically pragmatic. The procedure adopted is to lower the source to the midplane and collect sufficient counts for acceptable statistics; the process being repeated at each of the remaining toroidal positions. In addition a vertical scan is performed at 30 cm intervals in at least one toroidal position. Fig. 2 shows the results of the first calibration in which the effects of the centre column on the mid-plane scan and the internal poloidal coils on the vertical scan are clearly visible. The data from this procedure can be used to obtain a crude estimate of the calibration factor (source neutrons per count), but a better value can be obtained by using the data in an MCNP model. 4.2. MCNP calculation of the pulse-mode calibration factor MCNP is a Monte-Carlo radiation transport code maintained by United States’ Los Alamos National Laboratory [9]. We have used it to calculate the neutron transport through the MAST vacuum vessel to three detectors and to determine the neutron fluxes and spectra at these positions. To calculate the neutron transport using MCNP, one must first define a ‘‘geometry’’. This is done by describing a number of cells using standard surfaces (e.g. planes, cones)
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Mid-plane scan - FC3 0.25
Nett cps
0.20
0.15
0.10
0.05
0.00 0
2
4
6 8 10 Port number
12
14
(a)
Vertical scan - Port 10 - FC3 0.25
Nett cps
0.20
0.15
0.10
0.05
-200
0.00 -100 0 100 Height above mid-plane, cm
200
Fig. 2. Representative calibration data from fission chamber FC3 located at sector 11. MAST has 12 sectors with three ports on the top plate at each sector. Alternate sectors have one of these ports at 950-mm radius, close to the nominal toroidal axis and useful for calibration purposes, see Fig. 4 also.
and/or volumes (spheres, cylinder cubes, etc.). Each cell is filled with a material of a specific isotopic composition and density. The model used in these calculations includes the concrete floor, walls and ceiling of the MAST area but a wooden floor above the pit is not included. The steel vacuum vessel is described as a simple cylinder without port details. The central column, the PF coils, and TF coils that demarcate 12 sectors are described but other internal components of the vessel, such as the coil supports, are not (see Fig. 3). The new divertor was not in place for this work. The responses of the BF3, and the two fission chambers were calculated. Their locations are indicated in Fig. 4. To determine the calibration, the number of reactions in the detector volume per source neutron must be computed. This is done directly in the Monte-Carlo calculation by
(b) Fig. 3. (a) MCNP model of MAST. Model components external to the MAST vessel, including the biological shield. The roof is not displayed, for clarity. The nominal position of the large fission chamber FC3 is shown. (b) MCNP model of MAST. A section of the vessel wall is removed to show modelled components inside the vessel. The positions of the small fission chamber FC2 and a representative calibration port are also shown.
multiplying the neutron flux f(E) by the reaction crosssection sn,x(E) and the number of atoms in the sensitive volume using MCNP’s track length estimator of the particle flux. By further multiplying the reaction rate by the source strength one can directly predict the experimental detector count rate. Two types of neutron source were modelled, a point californium source and a d–d MAST-plasma, fusion neutron source.
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TF coils FC3
12
1
11
BF3
2
10
3
9
4
Ports used for introducing the neutron source.
1.0 0.8 0.6 0.4 0.2 0.0 0
2
4
6 8 Sector number
10
12
14
5 7
6 Sector numbers
Fig. 4. Plan view of the of the detector locations relative to MAST. Table 1 Calculations and measurements of detector count-rate using a
2 4 6 8 10 12
1.2
FC2 8
Sector
We modelled the production of neutrons from a deuterium plasma using a specially written MCNP subroutine. This subroutine has been used successfully to model neutron production on other tokamaks including JET and NSTX. The subroutine simulates a neutron emission profile with D-shaped contours of constant emission distributed throughout the vessel. The energy spectrum is Gaussian with a full-width at half-maximum of 320 keV that is appropriate to a beam-heated plasma in which beam-plasma neutron production dominates. Table 2 shows the parameters for the model, and Table 3 the results expressed as the numbers of counts in the detector per source neutron. The calibration factor is defined as R/(counts per plasma neutron) where R is the ratio of the calculated sensitivity to the californium source (counts per neutron) to the measured sensitivity, summed over the measurements in each sector: P C R ¼ Psectors . sectors E
Normalised cps
Six MCNP runs were required to reproduce the calibration experiment. Twenty-five million histories and typically 16 h were required for each run. The results are shown in Table 1. The errors are statistical which arise from the stochastic nature of the Monte-Carlo calculations. The agreement between calculation and experiment is excellent for the BF3 and FC2 detectors, the average countrate being only 20% low for the BF3 and 9% high for FC2. The large disagreement between the calculated and measured counts in FC3 (a factor of 27) is due to the setting of a high threshold in the processing electronics that is used to eliminate pulses from a particles and g-rays. However, the toroidal dependence demonstrated in the experimental data is reproduced in the calculations. This is shown in Fig. 5. The MCNP results have been normalised to the experimental measurements to demonstrate the agreement in toroidal variation. In addition, the BF3 data and the FC3 data in Fig. 5 (both experimental and calculated) have been scaled by factors of 5 and 10, respectively, for convenience of display. This validates the model which we then used to determine the response to an extended plasma neutron source; the ratio of the calculated to experimental responses observed with the californium source being used to provide a correction to the model results.
525
FC2(exp)
FC2 (MCNP)
FC3 (exp)
FC3 (MCNP)
BF3 (exp)
BF3 (MCNP)
Fig. 5. Normalised comparison of experiment with MCNP modelling.
252
Cf source
FC2
BF3
FC3
MCNP BF3 (cps)
MCNP error (%)
Expt. (cps)
MCNP FC2 (cps)
MCNP error (%)
Expt. (cps)
MCNP FC3 (cps)
MCNP error (%)
Expt. (cps)
1.58 102 1.68 102 2.27 102 4.11 102 5.02 102 2.04 102
10.1 9.62 8.33 5.42 6.16 7.45
2.03 102 1.75 102 2.62 102 5.60 102 6.34 102 2.43 102
0.554 1.21 0.553 0.281 0.139 0.298
2.5 1.7 2.5 3.5 4.7 3.4
0.555 1.06 0.551 0.267 0.121 0.259
2.49 1.8 1.82 2.53 4.55 4.62
1.5 1.6 1.6 1.4 1.0 1.1
0.104 0.054 0.055 0.088 0.178 0.180
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4.E+06
Table 2 Parameters describing the plasma neutron source 70.0 50.0 2.5 0.3 320
Table 3 Detector sensitivities and calibration factors Detector
Sensitivity to plasma neutrons (counts/n)
R
Normalised calibration (neutrons/count)
BF3 FC2 FC3
1.928 108 3.3814 107 1.8219 106
0.804 1.087 27.169
4.17 107 3.21 107 1.49 107
The normalisation values R are described in the text.
In this way the detectors are calibrated against the standard neutron source and MCNP has effectively been used to determine the effects of extending the neutron source and differences in the energy distribution of the plasma source neutrons. The values for R can be derived from Table 1 and are listed in Table 3. The resulting calibration factor of the large fission chamber for a volume neutron source is 1.49 107 neutrons per count. 4.3. Calibration of the Campbell and current modes Since only the pulse mode of operation can be calibrated directly, one must rely upon a secondary calibration of the Campbell mode by relating signals from the pulse and Campbell modes during actual plasma neutron production. The calibration of the DC (current) mode is obtained similarly by relating these signals to those of the Campbell mode at higher neutron production rates when the pulse mode has saturated. The DC calibration is therefore of a tertiary nature. Data for this calibration exercise were taken from a range of shots in which the NBI power was scanned across its whole practicable range at the time, ensuring that there would be sufficient information in each operational mode of the fission chamber to allow comparison of signals. Fig. 6 shows the comparison of Campbell and scalar (pulse) modes for a range of NBI shots. The saturation of the scalar mode is clear and allows one to select the upper linear limit of the validity of scalar data. Note that since the raw Campbell voltage represents roughly log10 of the desired, linear signal, it must be linearised and backgroundsubtracted before use. In addition, since the analogue signals are sampled every 10 ms and the pulse counter integrates over 5 ms, the Campbell data (for this calibration exercise) are averaged over the 5 ms periods corresponding to the scalar data. Before taking the slope of the linear
Linearised campbell, a.u.
Shots 7070 - 7080
Major radius (cm) Minor radius (cm) Elongation Triangularity Spectrum width (FWHM) (keV)
3.E+06
2.E+06
1.E+06
0.E+00 0.E+00
1.E+06 Scalar, cps
2.E+06
Fig. 6. Showing the onset of non-linearity in the relative responses of fission chamber’s Campbell and scalar modes. Each point represents average count-rate over a scalar integration period (5 ms) and Campbell signal averaged over the same period. The reason for outlying points is not known.
the the the the
region, Campbell data below its region of validity must be removed. For this data, a lower Campbell limit of 2 104 is taken (most easily found from a log–log plot) and the linear plot of restricted data (Fig. 7) yields the final calibration relationship. Although the signal-conditioning electronics could be adjusted to make a zero intercept and provide a simple constant of proportionality between the two signals, this would be very time consuming and unnecessary, so we use a two-parameter calibration factor. The relationship between the Campbell and the DC (current mode) signals is derived from high-power NBI shots in a similar manner, as shown in Fig. 8, and the equations relating the three signals are Neutrons ¼ 1:49 107 P
ðby source calibrationÞ
4
CB ¼ 5:59 10 þ 0:863 P DC ¼ 7:80 105 þ 1:29 108 CB
ð1Þ
where P is the scalar pulse rate, CB is the linearised Campbell signal and DC is the current-mode signal. Since the scalar calibration uses nett counts, the intercept is zero. With a little algebra, the final calibration equations for each operation mode are n=s ¼ 1:49 107 P 11
Scalar mode 7
n=s ¼ 9:66 10 þ 1:73 10 CB n=s ¼ 8:62 1011 þ 1:34 1015 DC
Campbell mode DC mode: (2)
Conservative thresholds for the valid ranges of each mode are used as guides in the on-line analysis programme to construct a complete temporal profile of the neutron rate. The thresholds used in practice, in equivalent neutrons per
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1.E+06
Linearised Campbell, a.u.
y = 0.8632x - 55942 8.E+05
6.E+05
4.E+05
2.E+05
0.E+00 0.0E+00
4.0E+05 8.0E+05 Scalar, cps
1.2E+06
Fig. 7. Data selected from the linear, overlapping ranges of the Campbell and scalar modes of operation. The fitting parameters are used in the generation of the final calibration equations. 0.05
Fig. 9. Showing the analysed data from all three modes combined into a single, temporal record. Data from each mode are separated by horizontal lines, for clarity and to demonstrate matching at the thresholds. In this figure, the lower limit to the heavily smoothed DC data has been set arbitrarily to 2 1013 n/s just for this demonstration, although this is below the acceptable range of routine operation for the DC mode.
y = 1.29E-08x + 7.80E-05
DC, Volts
0.04
R2
= 9.93E-01
0.03
0.02
0.01
0 0.E+00
1.E+06
2.E+06
3.E+06
4.E+06
Linearised campbell, a.u. Fig. 8. Data selected from the linear, overlapping ranges of the Campbell and DC modes of operation. The fitting parameters are used in the generation of the final calibration equations
second, are:
operational mode are combined to provide a temporal profile of neutron rate. The selected data ranges displayed in Figs. 7 and 8 show considerable scatter. The analogue threshold for the pulse counter is set very high to be sure of not counting the 235U alpha decays and gamma rays (at the expense of neutron sensitivity) so the pulse data is unlikely to be the source of scatter. The cause of this behaviour is not known at present and the noise signal, when there is no neutral beam injection, is negligible in all signal modes. 5. Treatment of errors There are several potential contributions to the uncertainty in the calculation of plasma neutron rate:
Lower limit for Campbell mode : 5 1012 n=s Lower limit for DC mode : 2 1014 n=s: The lower DC limit has been set to a value that will not be exceeded at present since the onset of non-linearity in the Campbell signal has not been observed. Indeed, the maximum neutron rate seen on MAST in these experiments is 8 1013 n/s and the DC mode data are not used. The DC threshold may be revised when the neutral beam system is upgraded to 75 kV at which the predicted neutron rate of 3 1014 n/s may be reached. In Fig. 9 is shown an example of how the data from the valid range of each
calibration of the source, Monte-Carlo calculation of the detector scalar response coefficient, statistics of the detector signals, transfer of the scalar calibration to the other modes of detector operation.
Our 252Cf neutron source was calibrated last about twenty years ago using the manganese bath technique (see for example Ref. [10]). This method is known to be accurate to about 2% for the original, nominal source
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strength so we use the published half-life to correct for decay and assume 2% uncertainty. The Monte-Carlo code MCNP is used, with the scalar calibration data as described earlier, to generate a response coefficient for volume, plasma neutron sources. The accuracy of the calculation depends upon the detail of the model and the time available for running the code. The error calculated by MCNP for the scalar response is 8% and dominates the error for the scalar calibration with the neutron source. While the error on the scalar signal is taken to be ON, the noise on the Campbell and (heavily smoothed) DC signals are negligible at typical neutron rates. In addition, the three signal modes are not independent, so we cannot use conventional error analysis to derive the uncertainties on the calibration equations. The scatter on the points in Figs. 7 and 8 is very much larger than indicated by counting statistics and noise. Without attempting at this stage to explain this feature, we treat the errors as instrumental in origin and analyse them to determine the errors on the calibration equation parameters. Referring to Fig. 7: we note that the deviations of the points from the fitted line appear to be independent of the signal magnitude. Since these mode signals are not independent, one can take either signal as the independent variable (in this case the scalar signal) and examine the distribution of deviations from the fitted line—Fig. 10. Even though the distribution may not be normal, we fit a Gaussian that yields a value for a standard deviation and use this value as the instrumental uncertainty on each point, following the treatment of Bevington [11]. For the DC vs. Campbell data: analysis of the deviations from the fitted line (Fig. 8) shows them to be independent of signal magnitude (the larger apparent spread at higher data values is due to the higher density of points). We therefore treat the data in the same way as the Campbell vs.
scalar data and Eqs. (1) now become Neutrons ¼ 1:49 107 ð0:087Þ P ðby source calibrationÞ CB ¼ 5:59 104 ð0:116Þ þ 0:863ð0:0121Þ P DC ¼ 7:80 105 ð1:49Þ þ 1:29 108 ð4:98 103 Þ CB
where the errors are expressed here as fractional errors. Rearranging these equations we arrive at Eq. (2a): Scalar mode n=s ¼ 1:49 107 ð1:30 106 Þ P Campbell mode n=s ¼ ð9:66 1011 1:37 1011 Þ þ ð1:73 107 1:52 106 Þ CB DC mode n=s ¼ ð8:62 1011 1:29 1012 Þ þ ð1:34 1015 1:18 1014 Þ DC: ð2aÞ 6. Applications of the data Since these neutron diagnostics are dependent upon MCNP for ultimate calibration, the results from each of them are not truly independent, but a comparison is useful for confidence in the application of the modelling. Shown in Fig. 11 are data from the fission chamber and the neutron activation diagnostics. The agreement shown here is very good and gives us confidence in the modelling and calibration methods. An indium foil of 13 g is positioned at the end of a re-entrant port close to the mid-plane, the induced activity being measured with a calibrated germanium detector. The calibration of the activation diagnostic will be reported elsewhere. Measurements of the time-resolved neutron emission can be correlated with the instabilities in the plasma. Fig. 12
0.14
2.0E+14
0.12 1.6E+14
Indium Foil, neutrons
Normalised deviation frequency.
ð1aÞ
0.1 0.08 0.06 0.04
y = 1.01E+00x - 2.78E+12 R2 = 9.93E-01
1.2E+14
8.0E+13
4.0E+13
0.02 0.0E+00 0.0E+00
0 0.E+00
5.E+04
1.E+05
5.0E+13 1.0E+14 1.5E+14 Fission Chamber, neutrons
2.0E+14
2.E+05
Campbell signal bin value Fig. 10. Distribution of deviations from the fitted line of Fig. 7. The standard deviation for the fitted Gaussian is 2.42 105.
Fig. 11. Comparison of fission chamber and indium foil activation data. Each point represents the integral of a whole day’s shots. The point close to the origin represents just two NBI shots in the morning, with the foil being analysed in the evening.
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Fig. 12. Example plasma shot data. The sudden decreases of the neutron signal correlate well with the sharp rises in Da emission due to saw-teeth.
shows the neutron emission as a function of time during shot 11835 that was a 550 kA, beam-heated discharge. The drops in the global neutron emission of nearly 40% are correlated with saw-tooth crashes, indicated by the rise in the Da signal. Although the neutron production on MAST is insufficient at present to cause health physics concerns, it is prudent to consider at this stage the consequences of upgrading the neutral beam injection facilities. To this end, we have a suite of computer codes that accumulates integrated neutron production from each shot and calculates the current radio-activity of the vessel steel and the Inconel centre column, and the decay time at which a specified, total activity in Bq/kg will be reached. The activities of the main isotopic constituents are provided individually. When neutron activation becomes a problem and the movement of activated components must be controlled, the codes can estimate the current activity of stainless-steel components near the vessel wall and of the Inconel centre column. In particular, the activity of items that have been exposed for a known period or range of shots can be estimated, again with a prediction of decay time to some prescribed level. 7. Summary and conclusions The main neutron diagnostic on MAST is a guarded fission chamber with processing electronics that allow data
collection in three modes of operation: scalar (pulse), Campbell and ion current. The scalar mode of operation is calibrated with a 252Cf source inside the vacuum vessel and then MCNP modelling used to relate this calibration to an extended plasma source. Neutral beam-heated plasmas are then used to extend the calibration to the Campbell and ion-current modes. Corroborative evidence for the accuracy of the calibration, obtained from neutron activation, indicates that the calibration method is satisfactory. The neutron data are used routinely to keep track of the radio-activation of key components of the MAST tokamak. Acknowledgements This work was funded jointly by the United Kingdom Engineering and Physical Sciences Research Council and by the European Communities under the Contract of Association between EURATOM and UKAEA. The views and opinions expressed herein do not necessarily reflect those of the European Commission. We are indebted to Lane Roquemore of the Princeton Plasma Physics Laboratory for lending us the electronic processing system for the large fission chamber, and to the manufacturer, Gamma-Metrics, for helping us to keep it working. It is a pleasure to acknowledge the other members of the JET Neutron Group who gave freely of their time to work
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the night shift during calibrations, and Alan Sykes has provided constant managerial support for the MAST neutron diagnostic. Mark Gilbert provided the MCNP-modelled activation coefficient for the Indium foil diagnostic. References [1] A.C. Darke, et al., Fusion Sci. Technol. (JT-60 Special Issue) 42 (2–3) (2002) 482; B. Lloyd, et al., Fusion 46 (2004) B477. [2] H.W. Hendel, et al., Rev. Sci. Instr. 61 (7) (1990) 1900. [3] N.R. Campbell, Proc. Cambridge Phil. Soc. 15 (1908–1910) 117; R.A. DuBridge, IEEE Trans. Nucl. Sci. NS-5–14 (1967) 241. [4] D.L. Jasby, et al., Rev. Sci. Instr. 70 (1) (1999) 1111. [5] S.S. Medley, D.S. Darrow, A.L. Roquemore, Reconciliation of measured and TRANSP-calculated neutron emission rates in the National Spherical Torus Experiment Princeton Report PPPL-4080, 2005.
[6] M. Isobe, M. Osakabe, M. Sasao, K. Matsuoka, J. Fujita, Rev. Sci. Instr. 66 (1) (1995) 923; E.T. Sugie, et al., Fusion Sci. Technol. (JT-60 Special Issue) 42 (2–3) (2002) 482; C.L. Fiore, R.L. Boivin, Rev. Sci. Instr. 66 (1) (1995) 945. [7] J.D. Strachan, et al., Rev. Sci. Instr. 61 (11) (1990) 3501. [8] W. Heidbrink, P.L. Taylor, J.A. Phillips, Rev. Sci. Instr. 68 (1) (1997) 536. [9] J.F. Briesmeister (Ed.), MCNPTM—A general Monte Carlo N-particle transport code, Los Alamos National Laboratory Report, LA-13709-M (March 2000), version 4C3. [10] J.B. Marion, J.L. Fowler, Fast Neutron Physics, Part. 1, Interscience Publishers, New York, 1960, p. 658; N.J. Roberts, MCNP Calculations of Correction Factors for Radionuclide Neutron Source Emission Rate Measurements Using the Manganese Bath, National Physical Laboratory, Teddington, UK, 2001 (Report CIRM45). [11] P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969 (Section 6.3). [12] O.N. Jarvis, G. Sadler, P. van Belle, T. Elevant, Rev. Sci. Instr. 61 (10) (1990) 3172.