A parallel plate 10B neutron detector

Nuclear Instruments and Methods in Physics Research 219 (1984) 575-581 North-Holland, Amsterdam

575

A P A R A L L E L P L A T E t°B N E U T R O N D E T E C T O R * James H. T O D D , Lawrence W. W E S T O N a n d George J. D I X O N ** Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, USA Received 29 August 1983

Parallel plate ionization chambers with vacuum deposited 1°B electrodes have been constructed and tested for l°B plating thicknesses from 24 #g/cm 2 to 61 #g/cm 2. Pulse-height resolutions of the alpha particle and of the 7Li fragment from the l°B(n,a) reaction were measured using low-energy neutrons. The pulse-height resolutions of the chambers were found to be better than a theoretical analysis had indicated.

1. Introduction An increasing need for a better neutron flux detector to monitor the neutron beams at the Oak Ridge Electron Linear Accelerator (ORELA) has led to the development of a parallel plate t°B chamber [1] for use at neutron energies below about 200 keV. The l°B(n,a) reaction was selected after examining three other candidate reactions: H(n,p), 235U(n,f), and 6Li(n,a). Characteristics of importance were good discrimination against background, fast timing, relatively high efficiency, a smooth cross section versus neutron energy, and stability in both timing and gain. Proton-recoil counters based on the H(n,p) reaction are not very suitable below approximately 100 keV in neutron energy due to the fact that the pulse height is small and is dependent upon neutron energy and the pulse-height spectrum extends to zero. The 235U(n,f) reaction extends over the complete neutron energy range of interest but has the disadvantage of a complex resonance structure at the lower neutron energies. Detectors which utilize the 235U(n,f) reaction are generally useful above 200 keV where the cross section is relatively smooth. Glass scintillation detectors loaded with 6Li have been used but these detectors show a high sensitivity to gamma rays. Neutron production at ORELA is by the photonuclear process due to bremsstrahlung from electron impact on a tantalum target. This generates a quite intense "gamma flash" at the beginning of each burst of

neutrons. The slow recovery times of 6Li glass detectors due to the gamma flash prevent their use at many locations at ORELA. The approximately 1 / v response of the l°B(n,a) reaction can be used as a convenient neutron flux monitor from thermal energies up to about 160 keV. The departure [2] from the 1/v response is approximately - 4 % and +2% at neutron energies of 20 and 160 keV, respectively. Two additional general considerations in the chamber development were: 1) The in-beam monitor should not significantly add structure to the neutron beam when experiments are being performed downstream from the flux monitor. 2) Neutron energies are measured by time-of-flight techniques at ORELA so a monitor with a fast recovery from the gamma flash associated with each burst of neutrons was required, as well as "fast" timing of the neutron interactions in the monitor. The t°B(n,a) reaction has previously been in use in parallel plate gas ionization chambers [3] with the t°B in the form of BF3 gas. These chambers have two disadvantages. First, the BF3 gas is chemically active which can cause problems requiring pumping and refilling. The second disadvantage is that the pulse height spectrum extends to zero, causing stability a n d / o r noise problems as well as limiting the accuracy of the measurements.

Table 1 Particle energies for two reactions * Research sponsored by the Division of Reactor Research and Technology, U.S. Department of Energy, under contract W-7405-eng-26with the Union Carbide Corporation. ** 1981 Summer Participant, Oak Ridge Associated Universities. 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Reaction

Tot. particle energy Alpha particle 7Li fragment (MeV) (MeV) (MeV)

l°B(n,a) 2.792 l°B(n, ay) 2.314

1.778 1.474

1.014 0.840

J.H. Todd et a L / Parallel plate "~B neutron detector

576

Table 2 Occurrence percentages relative to neutron energy. Reaction

Thermal

30 keV +10 keV

110 keV +15 keV

160 keV _+15 keV

l°B(n, ct) l°B(n, ay)

6.7% 93.3%

7.2% 92.8%

7.7% 92.3%

8.4% 91.6%

The ]°B(n, et) reaction actually consists of a l°B(n, a) and a ]°B(n, aT) reaction with a Q value of 2.792 MeV [4]. The gamma associated with the ]°B(n, aT) has an energy of 478 keV. The total energies of the two particles of the two reactions are therefore 2.792 MeV and 2.314 MeV. Center of mass movement contributes to the energies of the particles but for neutron energies below 1 keV this effect is negligible. A tabulation of the energies of the two particles associated with the reaction induced by thermal neutrons is given in table 1. The relative percentages of occurrences of the reactions, l°B(n,a) and ]°B(n, ay), change from 6.7% and 93.3%, respectively, at thermal neutron energies to 8% and 92%, respectively, at approximately 100 keV [5]. A tabulation of the percentages of occurrence [5] relative to neutron energy is given in table 2. The contribution of the higher energy particles from the l°B(n,a) reaction is negligible when measuring the pulse height resolution at full width at half maximum (fwhm) of a parallel plate gas ionization chamber. The effects of these higher energy particles were ignored in the following caculations of the resolution function.

2. Mechanical details

Fig. 1 is a drawing containing the dimensions and mechanical details of the chambers. The chambers consisted of two sections with a central collector electrode. With this configuration, only one of the two particles can be detected per event and the probabilities of either particle being detected are equal. The l°B depositions are on the two end windows. These two end windows are 0.127 mm thick aluminum. The diameter of the ~°B depositions is 7.62 cm. The diameter of the end windows is 12.7 cm. These dimensions allow for approximately 2½ path lengths for the most energetic particles before these particles could exit from between the parallel plates. The spacings of 1.43 cm between the parallel plates are greater than the 1 cm path length for the most energetic alpha particles generated by thermal neutrons and will stop the most energetic alpha particles generated by neutrons with energies up to 200 keV. After construction, the chambers were evacuated and

CHAMBER CAPACITANCE 5 8 PF

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Fig. 1. Parallel plate l°B chamber.

heated to assist in outgassing. The chambers were then filled to one standard atmosphere with a 90% argon, 10% methane gas. This gas is commonly known as P10 counter gas.

3. Theoretical considerations

The gas mixture of 90% argon, 10% methane was chosen because of the relatively high stopping power of this gas and because of the high drift velocity of electrons. Fig. 2 is a curve showing the drift velocity (w) of electrons in this gas as a function of potential gradient

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and of pressure [6]. The ranges [7] of the alpha particle and of the 7Li fragment, as a function of energy in argon, are shown in fig. 3. The ranges of the particles would be a few percent longer in P10 gas and would not materially affect either the calculations or the measurements, so the ranges in argon were used. The drift velocity of the positive ions is on the order of 1000 times slower than the electrons. All discussions herein will be for a "fast" chamber; the time constants used will exclude all but a fraction of a percent of the contributions of the positive ions. Additionally, the effects of recombination, electron attachment, and diffusion will not be considered so as to simplify calculations. The generation of a signal from a parallel plate chamber can be viewed from a standpoint of work done on moving a charge through a potential difference [8] or from an integral of the induced current caused by a moving charge between two parrallel plates [9]. Both techniques yield' identical results. The pulse height distribution of a parallel plate

chamber, using an idealized situation and thermal neutron induced reactions, is then affected by: (1) the energy of the ionizing particle, (2) the ionization potential of the gas. (3) the angular distribution of the particles. (4) the range of the particles, and (5) the distance between the plates of the chamber. The idealized situation at low neutron energies for the chambers herein is as follows: 1. The initial energy of each particle is constant. 2. The ionization potential (eV per ion pair) is the same for both particles and is constant. 3. The angular distribution is isotropic and the two charged particles are emitted in opposite directions. 4. The ranges of the particles are constant. 5. The distance between the plates is constant. Fig. 4 is a representation of the tracks for an alpha particle and of a 7Li fragment in a parallel plate chamber. The movement across the chamber of the electrons generated along the track of the ionizing particles can be represented by the movement across the chamber of the total charge concentrated at the centroid of each track. The centroid of the alpha particle and of the 7Li fragment depends upon the range-energy relationships of the particles. The ionization density as a function of remaining energy can be obtained from the range-energy relationships. Fig. 5 is the relative ionization density [10] of a 1.474 MeV alpha particle in P10 gas at STP. Fig. 6 is the same for an 0.840 MeV 7Li fragment [7]. The centroid of the alpha particle track is located at 0.6R,~. The

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centroid of the 7Li fragment track is located at ( 1 / 3 ) R Li. These are represented in fig. 4. The theoretical pulse height distribution for the integrated current per pulse in the parallel plate chamber with an isotropic angular distribution under these conditions will be constant between an upper and lower limit [9]. These limits are Q~ and Q ~ ( 1 - 3R,J5d), where R a is the alpha track length and d is the distance separating the two plates. The 7Li limits are QL~ and QLi (1 - RLi/3d), where RLi is the 7Li track length and again d is the distance separating the two plates.

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4. Calculations

of chamber

parameters

The chamber was operated at a + 500 V. This gave an electron drift velocity of 6 x 10 +6 c m / s . The maxim u m drift time across the chamber is given by

T= d/w,

where the range, R, of the 7Li fragment is 0.58 cm. A plot of the resulting idealized pulse height spectrum is shown in fig. 7. Note that the expected pulseheight resolution from an ionization chamber such as the one under consideration is very broad as compared to some other forms of neutron detectors.

where d = distance between plates, w = electron drift velocity, T = 1.43 e r a / 6 × 106 c m / s = 238 x 10 -9 s. [As stated earlier, all calculations will be for the l°B(n, aT) reaction with a total particle energy of 2.314 MeV.] The maximum and minimum charges generated by the alpha particle are:

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= 9.07 X 10-15C,

Fig. 8. Block diagram of electronics.

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5. Measurements All measurements were made using the Oak Ridge Electron Linear Accelerator (ORELA) as a neutron source. Neutrons with energies ranging from thermal to 1 keV were used. Fig. 8 is a diagram of the electronic system used to make the pusle height distribution measurements. A chamber with 28 # g / c m 2 l°B layer was used in the following measurements. The preamplifier used was a charge-sensitive unit with a 50-#s decay time. The amplifier was a doubly differentiating type with equal differentiating and integrating times. Pulse height spectra were accumulated using amplifier time constants of 0.25, 0.5, 1.0, and 2.0 us. The spectra for 0.25, 0.5, and

1.0 #s are shown in figs. 9 and 10. A pulser was used for gain equalization and electronic noise calculations in all measurements. Table 3 gives the resolution of the system, noise contribution, and the measured resolution with measured noise subtracted. The resolution calculated with the discussed theoretical considerations is also given in table 3. The measured pulse height resolution of the 7Li fragment, approximately 16%, is close to the calculated value of 14%; however, a later measurement with a thinner layer of l°B(10 # g / c m 2 vs 28 # g / c m ) in a different chamber gave a resolution of 7.5% fwhm. This later measurement indicated that the thickness of the layer was controlling the resolution for the 7Li fragment for this particular chamber. The pulse height resolutions

Table 3 Pulse height resolution of the l°B chamber. Total

Electronic noise

Resolution

Calculated

(~)

(~)

(~)

(~)

~a 7Li

0.25 #s 20.78 21.2

7.5 12.8

19.4 16.9

53 14

~, a 7Li

0.5 #s 21.25 17.99

5.0 8.5

20.7 15.9

53 14

"r a 7Li

1.0 #s 22.09 17.8

3.7 6.3

21.8 16.7

53 14

~" a 7Li

2.0 #s 20.73 16.49

3.0 5.2

20.5 15.7

53 14

580

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of the alpha particle have an average of approximately 20.6%. This compares with a calculated value of 53%. Detailed investigation and testing have not explained this discrepancy. Fig. 11 is a comparison of the theoretial pulse height distributions and the measured pulse height distributions of the chamber. The electron drift time across the chamber from plate to plate was calculated to be 238 ns. The minimum drift time of the centroid of an alpha pulse normal to the plates would be 138 ns. It would be expected that shorter time constants in the amplifier would improve the resolutions and would lower the gain of the system. This is due to the fact that the initial peak current of the signal is proportional only to the energy of the particle. This effect has been previously exploited [11] to achieve better energy resolution from ionization fission chambers. For the present system a change in pulse height resolution for different amplifier time constants was not detectable, probably because the increase in the electronic noise at shorter time constants obscured this improvement. The change in gain was measured and, as expected, the gain decreased with shorter time con2ooo

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stants, indicating that the system was "clipping" the trailing edge of the pulse from the chamber. Fig. 12 shows the decrease in gain as a function of time constants. All measurements previously discussed were from a chamber with a ]°B deposited thickness of 28/~g/cm 2. Four other chambers with deposition thicknesses from 10 # g / c m 2 to 61 g g / c m 2 were available. Measurements on the puulse height resolution as a function of thickness were made over this range of thicknesses. As expected, because of its longer range, the resolution of the alpha particle did not exhibit a detectable change. The pulse height resolution of the more massive 7Li fragment did change from a resolution of 7.5% fwhm for the 10 F g / c m 2 deposition to 36.1% fwhm for the 61 ptg/cm 2 deposition. This change is shown in fig. 13. Measurements were made on both the time resolution of the chamber and the noise of the electronic system. The time resolution of the chamber, with 0.5 ~s time constants, was measured at 36 ns. This resolution was unfolded from a narrow resonance in a filter located

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upstream in the neutron beam. Noise calculations for the preamplifier and amplifier used in the system gave a noise contribution to the pulse height resolution of approximately 9% for the alpha particle and approximately 16% for the 7Li fragment. This is compared with measured values of 8% and 14%. These calculations were made using 0.5/~s time constants in the amplifier.

6. Summary Plated l°B parallel plate chambers can be used as excellent neutron flux detectors up to neutron energies of about 200 keV. The high cross section for 2200 m / s neutrons (3837 + 9 b) [2] and the approximately 1/v response both contribute to its usefulness. The pulse height spectrum, with amplifier time constants equal to or greater than 0.5 /~s, show a clean separation from noise. This separation is achievable at least up to layer thicknesses of (51/xg/cm2. Higher neutron energies cause changes in the energies of the particles due to centerof-mass motion. These changes can cause the particles to have both higher and lower energies. All signals generated by the lower energies are cleanly above noise for neutron energies up to several MeV. This monitor therefore would not be subject to increasing count rate, as is a parallel plate 1°BF3 monitor, as a function of

581

center-of-mass motion caused by increasing neutron energies. This is true up to at least 5 MeV. The sensitivity to gamma radiation is low. The "gamma flash" generated by ORELA is detectable only a small percentage of the time. Recovery is within one time constant of the amplifier used. The pulse height resolution of the ]°B parallel plate chambers ( - 20%) for the experimental conditions used is much better than predicted by theory presented in this paper and in the literature [9,10,12]. This discrepancy is not understood by the authors. The authors are indebted to the operations staff of ORELA. The ionization chamber construction was by F.E. Gillespie and others of the Instrumentation and Controls Division of Oak Rige National Laboratory. The preparation and deposition of the l°B plates was by the group of E.H. Kobisk of the Solid State Division.

References [1] R.D. Lowde, Rev. Sci. Instr., 21 (1950) 10. [2] G.M. Hale, ENDF-300 (Brookhaven National Laboratory, 1979) p. IV.1. [3] L.W. Weston et al., Nucl. Sci. Eng. 34 (1968) 1. [4] F. Ajzenberg-Selove,Nucl. Phys. A320 (1979) A320. [5] R.L. Macklin and J.H. Gibbons, Phys. Rev. 140 (1965) B324. [6] T.E. Bortner, G.S. Hurst and W.G. Stone, Rev. Sci. Instr. 28 (1957) 103. [7] L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables 7 (1970). [8] D.H. Wilkerson, Ionization chambers and counters (University Press, Cambridge, 1950) p. 59. [9] W. Franzen and L.W. Cochran, in Nuclear instruments and their uses, ed., A.H. Snell (Wiley, New York, 1962) p. 3. [10] R.W. Lamphere, in Fast neutron physics, part 1, eds., J.B. Marion and J.L. Fowler (Interscience, New York, 1960) p. 449. [11] H,H. Knitter and C. Budtz-Jorgensen, in Proc. Conf. on Nuclear cross sections for technology, Knoxville, Tenn., NBS-594 (1980) p. 947. [12] B.B. Rossi and H.H. Staub, Ionization chambers and counters (McGraw-Hill, New York, 1949) p. 125.