{"id":654,"date":"2020-03-09T01:43:54","date_gmt":"2020-03-09T01:43:12","guid":{"rendered":"http:\/\/www.nuclearphysicslab.com\/npl\/?page_id=654"},"modified":"2020-03-18T15:11:48","modified_gmt":"2020-03-18T19:11:48","slug":"neutron-diffusion-time-measurement","status":"publish","type":"page","link":"http:\/\/www.nuclearphysicslab.com\/npl\/npl-home\/experiments\/neutrons\/neutron-diffusion-time-measurement\/","title":{"rendered":"Neutron Diffusion Time Measurement"},"content":{"rendered":"\n

Author: Luke, Scott & Tim<\/em><\/p>\n\n\n\n

INTRODUCTION<\/strong> The question has been asked if typical 3<\/sup>He detectors\nor other pulse-based electronic radiation measurement systems are appropriate\nor even capable of directly measuring neutron production in pulsed systems\nwhile being insensitive to the transient havoc created by the pulsed system.\nThe answer is yes. Many amateur nuclear fusion experimenters are interested in impulse\ndriven deuterium-deuterium reactions heated by electromagnetic\ncompression.  Characteristic time scales\nof their systems range from 1 to 10us, and typical peak neutron production\nranges from 106<\/sup> to 108<\/sup> neutrons per second in an\nisotropic distribution.  The peak\ncurrents required, described in denominations of kiloamperes, are supplied by\nhigh voltage energy storage capacitor banks initially charges to tens of\nthousands of volts.  These pulsed power\nsystems\u2019 switching initiation, plasma generation and coupling, and subsequent\nring down are all rapid processes fraught with electrical transient noise that\ncan interfere with standard low-level signal processing NIM\ninstrumentation.  Additionally, the\nresponse of the NIM electronics to the detection of a genuine nuclear event\nresults in a deadtime on the order of 10us or longer.  One can then imagine a pulsed experimental\nscenario in which the pulsed-power transient triggered the NIM counting\nelectronics and masks the registration of a true nuclear event within its 10us\nwindow, implying typical NIM-based apparatus is unsuitable for measurement.  Other methods, such as foil activation or\nsuperheated emulsion bubble detectors are seemingly now the only option.  This paper describes a pulsed neutron\nexperiment to demonstrate that a 3<\/sup>He NIM based neutron detection is\nstill a viable method due to the neutron diffusion time in a polyethylene\nmoderator.<\/p>\n\n\n\n

Although the neutron production window may be short (10us or less), the neutron transportation time from the source to the detector is relatively long, with a characteristic time of 100us, which affords the pulsed plasma experimenter the opportunity to \u201clook\u201d for neutrons in the quiescent period after the pulsed event.  <\/p>\n\n\n\n

\"\"<\/a>
Figure 1. Experimental setup<\/figcaption><\/figure>\n\n\n\n

EXPERIMENTAL SETUP:<\/strong> The 12-inch cyclotron was tuned up for D+ ions, with the RF\nsystem tuned to 7.150MHz, for an average magnetic field set to 0.96T (by\nsetting the top coil to 29.007Amps and bottom\ncoil to 29.121 amps) with the AKG270\nspiral poletips.[1]  The ion source used the\nlargest rectangular aperture chimney (hence lowest pressure differential), the\nMass Flow Controller was set to 0.230 scc\/m for an operating pressure of 3E-6\nTorr, the ion source was run with a 10mA\narc discharge current \u2013 all of these parameters balanced for optimal operating\npoint.  Beam tune up was verified with ~8 kV on the internal deflection (Wien filter)\nconfirming successfully acceleration of deuterium.<\/p>\n\n\n\n

The typical 12-inch cyclotron configuration to produce D-D neutrons through beam-on-target operation uses a long-pulsed mode with a duty factor of about 10% for RF thermal considerations given the passive cooling of the RF matching box components.  However, the heretofore “pulsed mode\u201d operation typically used beam-on durations of order 150ms – a lifetime as far as nuclear processes go. For the measurement at hand, the cyclotron was pushed into its shortest pulsed mode operation yet, with only 230 RF cycles at 7.15 MHz (an RF drive pulse of 30 us duration), which resulted in the generation of a 10us beam-on-target pulse after the 20us ring up time.  With such a short pulse duration, the pulse repetition rate could safely be increased up to 200pps (200Hz).  Figure 2 shows the RF pulse structures on an oscilloscope with a time base of 10us\/div: the upper trace is driving RF pulse, lower trace is actual DEE voltage, note the ring-up-time is due to the high Q of the tank circuit.<\/p>\n\n\n\n

\"\"<\/a>
Figure 2. Upper trace is RF drive pulse, lower trace is DEE voltage. 10us\/div<\/figcaption><\/figure>\n\n\n\n

NEUTRON PRODUCTION: <\/strong>It was important that not more than a single neutron would be\nregistered in the detector per cyclotron RF pulse \u2013 placing a limit on the peak\nneutron production. Neutron production rate can be controlled by selecting the\nenergy of the incident deuteron beam by adjusting the radial placement of the\ndeuterated target by means of a linear motion feedthrough.<\/p>\n\n\n\n

The target was position for a nominal 100keV incident deuteron beam\nenergy (r=0.067m). After being positioned, the 3<\/sup>He detector was\ncalibrated by placing a NIST calibrated 252<\/sup>Cf sealed neutron source at\nthe face of the deuterated target, thus giving the ability to quantify peak\nneutron production from the cyclotron during operation. Care was taken not to\ndisturb the 3<\/sup>He detector geometry to maintain the calibration.\nDuring a 5 second CW run of the RF at full operating power, the DEE voltage and\nion source production rate were adjusted for an average neutron production of\n500,000 neutrons per second, which were considered to be isotropic. <\/p>\n\n\n\n

\"\"<\/a>
Figure 3. 252Cf neutron calibration source placement directly in front of the deuterated target.<\/figcaption><\/figure>\n\n\n\n

When operating in this fast cyclotron-pulsed mode with a 10us duration\nof beam-on-target time, an average of 5 D-D fusion neutrons were produced.\nDuring most cyclotron pulses, these neutrons would completely miss the detector\naltogether, with approximately 1 out of 250 cyclotron pulses registering a\nneutron.   The likelihood of more than\none striking the detector per cyclotron pulse was vanishing small.  This was crucial to the measurement.<\/p>\n\n\n\n

NEUTRON DETECTION:<\/strong> The neutron detector consisted of a ~2-foot-long\n3<\/sup>He tube nested within a stack of pure polyethylene blocks. The two\nsides and back were stacked with neutron absorbing borated poly blocks to set\nboundary condition.<\/p>\n\n\n\n

A\npreliminary demonstration of the neutron diffusion effect is given by a digital\noscilloscope set to infinite persistence which recorded the electronic pulses\ngenerated from the detection of neutrons over numerous cyclotron pulse events.\nFigure 4 displays the long times up to which a neutron may take to arrive at\nthe detector.  The upper yellow trace in\nfigure 4 is the rectified reference of the actual RF \u201con time\u201d\npulse (the blip on the upper yellow trace), hence the cyclotron pulse is the\nonly time in which neutron can be produced. The lower blue trace displays the pulses\nfrom the 3<\/sup>He detector NIM electronics, which are seen to continue to\narrive long after the cyclotron RF is off. After 5 minutes of acquisition at 100\npulses per second a total of 250 neutrons events are observed. One can see the\nslowest neutron took 550us after production (RF off) to reach the 3<\/sup>He\ndetector. <\/p>\n\n\n\n

\"\"<\/a>
Figure 4. Infinite persistence oscilloscope image showing neutron registration (blue trace) in excess of 500us after cyclotron pulse (yellow trace) has extinguished<\/figcaption><\/figure>\n\n\n\n

This\nis not a quantitative measurement, since many of the neutron detector pulses\noverlap and blur their individual arrival times.  However, even from this quick glance, one can\nobserve a higher density of detection events immediately after the cyclotron\npulse, and the number of detection events begin to thin out with elapsed time\nfrom the cyclotron-on pulse, after which there is a point that neutrons are no\nlonger detected. This implies an exponential probability with a characteristic\ntime of the diffusion.
<\/p>\n\n\n\n

\"\"<\/a>
Figure 5. Schematic of time-to-amplitude-converter (TAC) installation.<\/figcaption><\/figure>\n\n\n\n

To quantify\nthe thermalization and diffusion time, an Ortec model 567 time-to-amplitude-\nconverter (TAC) was employed as outlined in figure 5.  The TAC converts a time period, initiated\n(start) and subsequently terminated (stop) by electronic pulses within a\npreselected time window.  In the present\ncase, the full-scale time window was set 1ms. \nThe TAC then delivered a proportional output pulse, ranging from 0 to\n10V, corresponding to a time period of 0 to 1ms.  Thus, if the period between the start and\nstop pulse was 500us, then the TAC would then output a 5V pulse.  The TAC output was then binned by a\nmulti-channel analyzer (MCA) to generate the temporal profile. The MCA used was\nan Ortec EZMca set to a conversion gain of 512 channels. If the time between\nstart and stop pulses exceeded 1ms, the TAC simply reset without triggering an\noutput pulse, and awaited a new start pulse, thus ignoring cyclotron pulses\nthat did not result in a detected neutron. \nA ten-point calibration of MCA channel-to-time interval calibration of\nthe TAC-MCA system was performed with 70, 100, 200, \u2026, 900uS time intervals generated\nfrom a Tektronix arbitrary waveform generator. To perform the measurement of\nneutron thermalization and diffusion time, a TTL pulse synchronized with the\nbeginning of the RF pulse started the TAC clock, and the NIM pulse arising from\nthe 3<\/sup>He detector registering a neutron provided the stop pulse.  These two signals form the basis of the\nmeasurement.  The RF pulse consisted of a\n20us ring-up time, followed by a 10us flat top, during which an average of 5\nneutrons were produced.  The neutron\npropagation from the target, through the chamber wall, through approximately 8\ninches of air, and then finally diffusing through the polyethylene before\nentering the 3<\/sup>He is the measured quantity.  That process, presumably dominated by the\nduration spent in the polyethylene is long compared to the 10us RF pulse (the\ntime window in which a neutron could be produced). The multichannel analyzer\u2019s\nbinning created a histogram of cyclotron pulse-neutron detection time intervals\nover a 1-hour period of acquisition, or 720,000 cyclotron pulses. Figure 6\nshows a fit to the data, yielding a measured diffusion time of approximately\n94us.
<\/p>\n\n\n\n

\"\"<\/a><\/figure>\n\n\n\n

To understand the background, we performed an acquisition run of five minutes of neutron events were collected with all cyclotron systems operational, including the pulsed RF, except the ion source discharge power supply was shut off and no neutrons were detected during that time.<\/p>\n\n\n\n

REFERENCES<\/strong><\/p>\n\n\n\n

[1] Koeth, T.W., Undergraduate Education With the\nRutgers 12-Inch Cyclotron<\/em>, Physics\nProcedia, Volume 66, 2015, Pages 622\u2013631 <\/p>\n\n\n\n

[2]\nTimothy W. Koeth, Neutron Production with\na 12-Inch Cyclotron<\/em>, November 18, 2017 <\/p>\n\n\n\n

[3]\nK. Ruisard, G. Hine, T.Koeth, A. Rosenberg, \u201cThe Rutgers Cyclotron: Placing Student\u2019s Careers on Target<\/em>,\u201d <\/p>\n\n\n\n

[3] J. G. Beckerley, \u201cNeutron Physics \u2013 A Revision of I. Halpern\u2019s Notes on E. Fermi\u2019s\nLectures in 1945<\/em>,\u201d AECD- 2664, Oct 16, 1951 <\/p>\n\n\n\n

[5] V.A.Livanov, A.A. Bukhanova, and B.A. Kolachev, \u201cHydrogen in Titanium<\/em>\u201d Israel Program for\nScientific Translations, Jerusalem 1965. Part 3, Chapter 1<\/p>\n","protected":false},"excerpt":{"rendered":"

Author: Luke, Scott & Tim INTRODUCTION The question has been asked if typical 3He detectors or other pulse-based electronic radiation measurement systems are appropriate or even capable of directly measuring neutron production in pulsed systems while being insensitive to the transient havoc created by the pulsed system. The answer is yes. Many amateur nuclear…<\/p>\n

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