Advanced Materials Research Vol. 1084 (2015) pp 162-167 Submitted: 22.08.2014 (2015) Trans Tech Publications, Switzerland Revised: 13.10.2014 doi:10.4028/www.scientific.net/amr.1084.162 Accepted: 22.10.2014 Portable Gamma-Ray Spectrometer for High Intensity Beam Measuring VUKOLOV Artem, GOGOLEV Alexey a, CHEREPENNIKOV Yury, OGREBO Andrey, EGIOYA Alexander National Research Tomsk Polytechnic University, 30, Lenina Avenue, Tomsk, 634050, Russia a gogolev@tpu.ru Keywords: X-ray, gamma radiation, spectrometer, counter. Abstract. The paper presents the development of the Laboratory "X-ray Optics" of the Institute of Applied Physics and Technology of Tomsk Polytechnic University, a portable gamma-ray spectrometer, which dimensions are 50 20 30 mm. The device is able to count and analyze gamma quanta with energies from hundreds of kev up to units of MeV with loading up to 10 9 pulses/min. The problem has been solved by improving the known scintillation counters, using modern silicon photomultiplier tubes, selecting the optimal scintillators and developing original electronic scaler. Introduction Several types of gamma counters are currently known [1]. The widely known counters are gasdischarge counters typically made in the form of a metal cylinder filled with gas (cathode) with a thin wire (anode), stretched along its axis. Temporal resolution of gas-discharge counters is 10-3 -10-7 s. For gas-discharge counters the detection efficiency is about 100% for charged particles and about 5 % for gamma quanta. Gas-discharge counter disadvantages are: high voltage sources of more than 300 V; large dimensions; weak mechanical strength; short life of unit 10 9 pulses; small count rates of unit 10 3 pulses/s. These disadvantages make such a counter completely inapplicable for measuring the count rate and energy of gamma quanta with energies from hundreds of kev up to units of MeV with loading up to 10 8 pulses/sec. There are scintillation detectors of nuclear particles, which basic elements are a scintillator and a vacuum photomultiplier tube (PMT) allowing to convert weak flashes of light into electric pulses detected by electronic equipment [2]. The crystals of some inorganic (ZnS for heavy particles; NaI(Tl), CsI(Tl) - for the light particles and quanta) or organic substances (anthracene, plastics - for photons) are usually used as a scintillator. Scintillation detectors have high time resolution determined by the type of detected particles, scintillator and resolution time used in electronic equipment. To focus and accelerate electrons onto the anode and dynodes the high voltage is applied (600-3000 V). Antimony-cesium bialkali (CsSb) photocathode allows PMT to work in the spectral range of 300-680 nm with a quantum efficiency reaching 25% at the maximum over the line of 420 nm. When a scintillation detector is in operation it is possible to achieve a minimum pulse width of ~10-9 in the pulsed mode at the photomultiplier tube output. Maximum loading of the spectrometer can be 10 9 of gamma quanta per second. The main disadvantages of these counters are: large dimensions; high-voltage power sources up to 1000 V; strong sensitivity to electromagnetic fields. There are semiconductor particle detectors, which main element is a semiconductor diode [3]. The maximum input statistical loading is 5 10 4 pulses/sec. Semiconductor detectors are highly reliable and can operate in magnetic fields. But for the operation they require cryogenic cooling, high-voltage power source up to 1000 V and in the operating position they have large sizes and weight. There are semiconductor PIN detectors. [4] However, the small thickness of the workspace (about a hundred of micrometers) does not allow to use them for measuring high-energy particles of more than 150 kev. In the late 90s the spectrometers based on avalanche photodiodes got a rapid development [5]. Micro-pixel avalanche photodiodes (MCP APD) with the metal-resistor-semiconductor structure All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 109.123.176.159-19/01/15,05:27:41)
Advanced Materials Research Vol. 1084 163 have a working area of 1 to 3 mm 2 and consist of pixels (pixel size is 45 45 µm), each of which is a silicon photodiode, made of a low-resistance substrate of p-type. In addition to the magnetic field immunity, MCP APD have the following advantages: compactness (the diameter of the sensitive area is ~1 mm); gain coefficient (10 5 10 6 ) not lower than vacuum PMT; low bias voltage (25-80 V); moderate cost. This device can have the following disadvantages: a high intensity of noise pulses; a temperature dependence of most parameters of MCI APD; a high quantum efficiency in the green region of the spectrum (10-20%) different from the well-developed vacuum PMT; and the maximum loading of the spectrometer is 10 6 pulses/sec. New spectrometer Laboratory "X-ray Optics" of the Institute of Applied Physics and Technology of Tomsk Polytechnic University has developed a miniature device with sizes of 50 20 30 mm, capable to count the gamma quanta of high intensity and analyzing the detected energies. In the presented scintillation spectrometer of ionizing radiation we used a silicon photomultiplier tube characterized by a high gain factor k = 10 6 and the quantum efficiency of 15 to 23 %. Silicon photo multipliers represent detectors of the 3rd generation with the improved parameters, the so-called "M-type", whose feature is the quick output allowing to get a signal with pulse front rise time of about 100 ps and the recovery time of less than 1 ns required for high speeds of reading. Also, this type of PMT has additional advantages such as: compact dimensions with different size of the active region: 0.25 0.25 mm 2, 1 1 mm 2 3 3 6 mm 2, and 6 mm 2 with micro cell dimensions of 20 µm, 35 µm and 50 µm, magnetic field immunity, low operation voltage is 30 V, mechanical strength and immunity to ambient light. In this device, two types of scintillators are used: bismuth orthogermanate Bi 4 Ge 3 O 12 (BGO) with a radiation length of 1.13 cm and a well-known CsI (Tl). BGO scintillator allows to detect radiation of gamma quanta with energies from hundreds of kev to several MeV and intensity up to 10 9 pulses/min. A luminescence spectrum of BGO scintillator is slightly rightward in the area of 480 nm, than the sensitivity peak of bialkali photocathode of the photomultiplier tube. BGO luminescence time at room temperature is 300 ns, i.e. not much worse than that of Nal (Tl) and CsI (Tl). However, unlike the scintillator NaI (Tl) and CsI (Tl) the afterglow of BGO in the millisecond range is very little - 0.005%. Therefore, in general a BGO scintillator is more fastacting than NaI (Tl) and CsI (Tl). BGO scintillator advantage is its good moisture resistance. It has been revealed that the scintillation detector based on BGO crystal of relatively small sizes of 75 25 mm 2 is comparable by efficiency of detection of gamma quanta with energy of 4-17 MeV with a scintillator NaI (Tl) having sizes of 150 100 mm 2. In the mode of fast-acting counter a counterspectrometer is equipped with a BGO scintillator. In the mode of spectrum analyzer of gamma quanta a spectrometer is equipped with scintillator CsI (Tl), for measuring energy larger than 1 MeV a BGO scintillator is used. Scheme of the device Fig. 1 shows a flow diagram of the device. The counter consists of scintillator 1 and is glued using silicone sealant 2 with silicon PMT 3 with active area of 6 6 mm 2. The power supply of silicon PMT with voltage of about 30 V is carried out by module 4. To use all capabilities of PMT a charge-sensitive fast-acting preamplifier-discriminator 5 is installed, which generates an output pulse provided that the charge pulse at the input exceeds a certain threshold corresponding to the detected light photons. At the output PMT produces negative pulses; the duration of each pulse is 10-20 ns. Largeamplitude pulses correspond to the detected photons (light quanta), but there are also many small pulses occurring due to the noise in the crystal of silicon PMT. These pulses must be cut off by the discriminator.
164 Radiation and Nuclear Techniques in Material Science Figure 1. Flow diagram of a scintillation counter spectrometer: S - scintillator; SS - silicone sealant; PMT photomultiplier tube; AD - amplifier discriminator; FD - frequency divider; MC - microcontroller; PC - a personal computer Pulses are delivered in three directions, directly in the analog-to-digital converter of the microcontroller (MC) 7 for the analysis of pulses and further the digitized information about pulses is delivered to a personal computer (PC) 8. As MC various controllers can be used. Most of the available budget controllers are capable of counting loadings to 500,000 pulses/sec. To increase the device performance in the counting mode in the case of exceeding the threshold of 500,000 counts/s the second passage through the logical counter-divider by 100 has been applied. Pulses are delivered to the two-decade frequency divider 9, assembled on the two decimal counters-chips (HEF4016BT1). When reaching the calculated pulses multiple of 100 the TTL pulse of 20 ns duration is delivered to MC. The frequency divider control is provided by MC. Thus, the productivity increases by 100 times and reaches about 50 MHz with an error less than 2%. Replacing the chips of a decade divider by the elements with greater transmission capacity in terms of frequency, one can achieve the counting rate up to 10 9 pulses/sec. Figure 2. The principal diagram of a power supply Fig. 2 shows the principle diagram of a power supply. It has been assembled on the generatortimer chip 9 (NE555), which serves as a master oscillator of rectangular pulses on operational amplifier LM311 included in the diagram as a voltage comparator. The generator has been assembled using the classical scheme. The output pulse repetition rate of the generator is given by
Advanced Materials Research Vol. 1084 165 the elements C1, R1, R2, and is 48-50 khz. Generation output is regulated through the input of chip NE555. When the input voltage is low (less than 0.7) the output is reset to a low-level state regardless of what mode the timer is at the moment and what it is doing. The rectangular pulses from input NE555 through a limiting resistor 10 are delivered to the base of a transistor key (in our device 2N5551), whose loading is a throttle 12 with inductance of 6800 microhenry. When this transistor is sharply locked a large EMF of self-induction is induced in the throttle 12. Obtained in such a way high-voltage pulses are delivered to a rectifier constructed on the diode 13 (DL4148) and capacitor 14. Output voltage can be regulated by means of a comparator constructed on the chip 19 (LM311). Via the voltage divider 15 and 18, the output voltage is supplied to the inverting input of a comparator and compared with a reference voltage delivered to the positive input. Changing the reference voltage by variable resistor 20 one can regulate the output of comparator associated with a reset input of chip NE555. At excess of the output rectified voltage of the threshold value set by variable resistor 20 a low-level supply to input NE555 occurs and the generation stops. The rectified voltage decreases, the comparator changes its state to a logical unit and allows the generation. Such a scheme allows to establish the accuracy of the supply voltage to a few tens of millivolts. Through limiting currents of resistor 16 the supply voltage is delivered to the silicon PMT. When a particle is passing through the scintillator glued to PMT the light flashes of duration less than 300 ns occur in the area of the spectral range of 480 nm. PMT converts flashes to voltage pulses. Capacitor 17 cuts off constant voltage. Through matching resistor the signal passes to amplification circuit. As an amplifier of discriminator the classical scheme of the transistor amplifier is used. Test of the device Fig. 3-5 show the spectra of isotopes Cs137, Na22 and Co60, respectively. The crystal CsI (Tl) with sizes of 15 5 5mm 2 with light-reflecting coating was used as a scintillator. Resolution of the spectrometer for line Cs137 was 9%, the line width was Δ62keV. In the spectra of Na22 and Co60 in the energy range of more than 1 MeV one can observe the decrease in the detection of spectral lines intensity. Reducing of the amplitude is associated with the small size of scintillator CsI (Tl) used in the spectrometer. This is the main drawback of the solidstate PEM; a small sensitive area up to 6 mm 2 does not allow to use scintillators of large volumes. Figure 3. Spectrum of isotope Cs137 Figure 4. Spectrum of isotope 22Na
166 Radiation and Nuclear Techniques in Material Science Figure 5. Spectrum of isotope Co60 Fig. 6-7 show the spectra of isotopes Na22 and Co60 respectively. The cylindrical crystal bismuth orthogermanate (BGO) with dimensions of 10 mm in height and 10 mm in diameter with a light-reflecting coating was used as a scintillator. In the spectrum of Co60 in the energy range of more than 1 MeV an amplitude peak is significantly greater than that observed from the scintillator CsI (Tl) of similar size. Unfortunately, the existing analyzing equipment has not allowed to efficiently handling pulses with duration of less than 0.5 ms, so the spectral resolution is 30% with a spectral linewidth of Δ389 kev. Figure 6. Spectrum of isotope Na22 (BGO scintillator) Figure 7. Spectrum of isotope Co60 (BGO scintillator) Conclusions From the analysis of the given spectra it is clearly seen that the resolution for BGO scintillator is 3 times worse than for CsI(Tl) one. Bismuth orthogermanate is advisably to use for creating fastacting spectral counters that count the number of gamma quanta with energies from hundreds of kev to several MeV and intensity up to 10 9 pulses/min. Spectral capabilities of BGO allow to count particles of the selected energy range using amplitude discriminators in the measuring equipment. Thus, the use of solid state PMT has allowed creating a miniature spectrometer of gamma quanta with sizes of 50 20 30 mm. Acknowledgement This work was particularly supported by the grant of the Ministry of Education and Science of the Russian Federation within Program "Nauka" number 2456.
Advanced Materials Research Vol. 1084 167 References [1] C. Gruppen, Particle detectors, second ed., Cambridge University Pres., New York, 2008. [2] Information on http://www.doza.ru/catalog/spectrometers/3500/ (in Russian) [3] Information on http://aspect.dubna.ru (in Russian) [4] Information on http://www.apmtek.com (in Russian) [5] Y. Kudenko, The Near neutrino detector for the T2K experiment - T2K, Nucl. Instrum. Meth. A598 (2009) 289-295