Time-differentiated study of some effects induced by Cf-252 fission neutrons in the HPGe detector and its environment

A. Dragić, J. Puzović*, D. Joković, R. Banjanac, V. Udovičić, D. Maletić,M. Savić, N. Veselinović and I.V. Aničin

Institute of Physics, University of Belgrade, Belgrade, Serbia*)Faculty of Physics, University of Belgrade, Belgrade, Serbia

Abstract

We measured the time-differentiated spectrum of the Cf-252 source as seen by an HPGe detector triggered by the NE213 liquid scintillator detector. The detectors are off-line coincided from the event-by-event list formed with 10 ns resolution by the quad FADC unit of the CAEN N1728B type. The signatures of the processes induced by fast and slow neutrons, both within the detector itself and in its environment, appear completely separated. The processes induced by fast neutrons are found in the prompt part of the time spectrum and those induced by thermal neutrons in the long tail of delayed coincidences. The half-lives of excited states of the order of hundreds of nanoseconds are well reproduced. We compare the time behavior of spectral structures at 596 and 692 keV, which are the signatures of inelastic neutron scattering on Ge-74 and Ge-72 respectively, and comment on their suitability for determination of the fast neutron flux in general, and in the digital gamma-ray spectroscopy in particular.

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1. Introduction

Neutrons induce a number of nuclear reactions on stable isotopes of germanium, which produce characteristic signatures in the spectra of germanium detectors. These have been studied in great detail many times during the last twenty-odd years, with the aim to either improve on the knowledge of background spectra of germanium detectors, or for determination of neutron fluxes at the position of the detector [1-11]. Some of the studies dealt with environmental neutrons of cosmic-ray origin while some used different neutron sources, mostly with the fission spectrum. The findings were quite similar, irrespective of the origin of the neutrons. Most of these studies were performed with analog spectroscopy systems, while we today witness the increased use of digital spectroscopy systems, which offer the possibility to gain some insight into the dynamics of the processes involved. We thus undertook the time-differentiated study of some of these processes induced by Cf-252 fission neutrons by using one of the contemporary commercial flexible digital spectroscopy systems.

2. The experiment

The detailed description of our laboratory is given elsewhere [0]. Experimental setup used in this study consists of a small volume (2 liters) liquid scintillator (NE213) detector and a 15% efficiency coaxial HPGe detector, positioned as schematically presented in Fig.1.

Fig.1. The layout of the experiment (view from above)

The container of the NE213 detector is made out of 5 mm thick stainless steel, while the cap of the HPGe detector is 1 mm aluminium. The materials in the environment of the detectors are those which are typically found in the experiments involving neutrons and

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gamma-ray spectroscopy: the 5 cm thick lead shield at the back of the HPGe detector, the 4 mm thick rubberized B4C layer in front of this detector, or alternatively a 1 mm thick cadmium sheet, and some 5 cm thick paraffin slabs around the whole setup. The encapsulated Cf-252 source not older than a year, which gives off about 2000 neutrons in 4 sr/s, is positioned in such a way that on their way towards the Ge detector the neutrons traverse the 7 cm thick layer of NE213. The distance between the end-cap of the NE213, which faces the Ge detector, and the closest point of the Ge detector is 10 cm. Fast neutrons with energies higher than 1 MeV, which leave the NE213 detector towards the HPGe detector, thus take less than 10 ns to reach this detector, while the neutrons which start as thermal take about 50 s. Majority of neutrons, which slow down on their way towards the detector, take something in between these two values, but certainly closer to the lower one.

The preamplifier outputs of both detectors are fed to the two out of four identical inputs of the flash ADC unit of the CAEN N1728B type. For this occasion this versatile instrument is set to work in the so-called energy histogram mode, when it performs like a digital spectrometer capable to operate in the list, or the event-by-event mode. For every analyzed event the time of its appearance over the triggering level is recorded with 10 ns resolution, while its amplitude is digitized and recorded in one of the available 32 k channels. The amplitude range is 1.1 V. This enables to off-line coincide the events of given amplitudes with 10 ns resolution, what, as we shall se, appears fast enough for this particular purpose. The software that produces and analyzes the time spectra, which are equivalent to hardware TAC spectra, is entirely homemade.

3. The results

The singles spectrum of NE213, which is mainly due to the Compton scattered gamma rays and elastically scattered neutrons, is nearly exponential and featureless, and serves practically only as a trigger for the HPGe spectrum. It stretches up to somewhat beyond 10 MeV. The singles spectrum of the in this case lightly shielded germanium detector consists of the correspondingly rich environmental background spectrum, of the gamma rays given-off by the californium source, and of the different features induced by fission neutrons of californium in the Ge detector and in the surrounding materials. It stretches up to some 8 MeV. Direct spectra similar to this one have been thoroughly studied on many occasions and we shall here deal only with the interesting portions of this spectrum that are coincident with different delay times with the NE213 spectrum.

3.1. Testing the system

We first comment on general features of the time spectrum. In this simplest of cases the time spectrum consists of time intervals between the nearest events in time of given amplitudes in the two detectors. Either of the detectors can serve as a start channel. The time tag of every event is determined by the moment of crossing the set triggering level. In order to minimize the amplitude walk there is a possibility to choose between different types of trigger, termed here as simple, digital and CFD. We stick to the digital trigger which is found to work reliably, and if necessary off-line correct for amplitude walk [12].

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To start with, we present the time spectrum without amplitude cuts on both detectors, i.e. the whole NE213 spectrum vs. the whole HPGe spectrum, Fig.2. The measurement time was 18 days. The width of the nicely shaped “prompt” distribution is 90 ns. The tail of delayed coincidences starts discernibly at some 100 ns after the prompt peak.

Fig.2. The time spectrum of the whole NE213 spectrum vs. the whole HPGe spectrum

The same time spectrum as in Fig.2, but off-line corrected for amplitude walk, according to procedure described in [12], is presented in Fig.3. The FWHM is now 15 ns, and delayed coincidences perhaps start already at about 20 ns after the prompt peak.

Fig. 3. The same time spectrum as in Fig.2 but off-line time corrected for amplitude walk.

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Now, to test the workings of the system there is in the HPGe spectrum a lifetime which is conveniently found within the reach of our time resolution. The level in question is the fission fragment Te-134m, with the half-life of 164 ns, the production of which was studied in the fission of californium by Gautherin et. al. [13]. This state depopulates by the three cascading gamma rays, of 115, 297 and 1279 keV, which should die out in our delayed spectra with this half-life. The portion of the singles, prompt, delayed up to 1 s, and random coincidence spectra containing the 1279 keV line are presented in Fig.4.a. When we put the narrow software gate on the 1279 keV line in the HPGe spectrum the resulting time spectrum looks like the one in Fig.4.b. It is triggered either by prompt fission neutrons or by the gamma rays that populate this isomer. The fit through the well defined exponential tail of delayed coincidences at the 68% CL yields the half-life of 160(9) ns (the line of 297 keV exhibits a very similar behavior). Though the statistics is poor and the error correspondingly large, this still justifies our further investigations of time relations contained in these spectra. To further demonstrate the general properties of the time spectra we now proceed to the discussion of the effects which neutrons induce in the materials that surround the detectors.

Fig.4. The portion of singles, prompt, delayed and random coincidence spectra containing the 1279 keV line from the decay of Te-134m (a), and the time spectrum of the 1279 keV line, which yields the half-life of 160(9) ns (b).

3.2. Effects in the environment

The materials which typically surround the detectors in experiments that involve neutrons and gamma rays include lead, paraffin (hydrogen), boron, cadmium, and, in our case, iron. Time structure of the signatures of the effects which neutrons induce in these materials in our HPGe spectra is particularly simple and illustrative. This structure follows the general pattern which is determined by the relation of the time of flight of the neutrons to the time resolution of our system: all the effects induced by fast neutrons which we see (mostly the inelastic scattering) tend to fall into the prompt peak of our time spectrum, while the effects induced by slow neutrons (mostly capture reactions) fall into the (very long) tail of delayed coincidences. The completeness of this separation is perhaps best illustrated by the cases of boron and iron. Fig.5a presents the portion of the

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HPGe spectrum that contains the 478 keV Doppler–widened line from the (n,) reaction on 10B, while Fig.5b depicts the part of the spectrum with the 847 keV line from (n,n’) reaction on 56Fe (found mostly in the NE213 casing). The first reaction has the high cross section for thermal neutrons, while the second is induced by neutron energies higher than about 1 MeV. The figures represent the corresponding part of the spectrum gated with the prompt peak of the time spectrum (top spectrum) and the same portion of the spectrum gated with the tail of delayed coincidences from the region adjacent to the prompt peak and stretching up to the delays of 1µs (middle spectrum). The spectrum of random coincidences in the time window of the same width is also presented (bottom spectrum). The difference between the two cases is striking; the 478 keV structure is present only in the spectrum of delayed coincidences and on its place in the prompt spectrum the weak lines from the decay of californium appear, while the strong line of 847 keV exists only in the prompt spectrum and is completely absent in the delayed spectrum. It should be noted that due to high TOFs thermal neutrons are present in even greater number at much higher delay times, and that the integrated intensity of the structure at 478 keV (as indeed of all the lines induced by thermal neutrons) is correspondingly much higher when all delayed events are included.

Fig. 5. The prompt, delayed (up to 1 µs) and random coincidence spectra of the 478 keV line from the reaction on boron (a), and of the 847 keV line from the reaction on iron (b). The line at 868 keV is from neutron capture on Ge-73. Note that the first scale is linear while the second one is logarithmic.

Now the case of cadmium becomes instructive. Fig.6 is same as Fig.5, but for the portion of the spectrum containing the most prominent line of 558 keV from the assumed thermal neutron capture on Cd-113.

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Fig.6. Same as Fig.5, but for the line of 558 keV induced by neutrons on the isotopes of cadmium

It is perhaps surprising to find out that the prompt line is much more intense than the delayed one, implying that for the somewhat harder neutron spectrum, which we have in this case as compared to the one in the case when the boron absorber is present, the (n,n’) reaction on Cd-114 contributes much more to the intensity of the 558 keV line, than the commonly assumed neutron capture by Cd-113. Of course, when the higher delay times are included, the intensity of the line due to neutron capture significantly increases. The ratio of the prompt and delayed intensities could thus in principle serve as a simple estimate of the relative hardness of the neutron spectrum. We shall not further pursue this matter here, but now proceed to the discussion of the two most interesting features induced by neutrons in the HPGe detector itself.

3.3. The 596 keV structure

We first discuss the peculiarities of the spectral distribution at 596 keV, which is attributed to the inelastic neutron scattering exciting the first state of Ge-74 (isotopic abundance 36%). Comprehensive studies of this distribution were performed in refs. 6, 8, 9,10. Two properties of this process are relevant here: first, the lifetime of the 596 keV state is short, of the order of 10 ps, and second, the depopulating radiation is mostly a gamma ray, meaning that detection efficiency is significantly smaller than 100% and that it depends on the position of the interaction point of the neutron within the detector. This results in many possible sums of the recoil and 596 keV gamma ray contributions, covering the whole spectrum from the lowest energies, where only the recoils would contribute, over the recoil sums with the single Compton scattered gamma ray, up to the sums of the fully absorbed 596 keV radiation with the recoil spectrum. This last possibility results in the well known quasi-triangular spectral structure starting at 596 keV and extending up to additional highest possible recoil energies for the given neutron spectrum, reduced by the so-called pulse-height defect (equivalent to quenching in scintillators) [8]. Since the distribution of summing events over the entire spectrum strongly depends on the size of the detector, the shape and intensity of the triangular distribution is likely to vary strongly from one experimental situation to another. This is

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why this spectral distribution, unlike the 692 keV distribution that we discuss in the next section, is ill suited for determination of the fast neutron flux. Time wise, we find that the triangular summing distribution is fully contained within the peak of prompt coincidences (Fig.7.a), while the delayed coincidences now show up the normal 596 keV line from neutron capture by Ge-73 (Fig.7.b). The 609 keV line is from the same capture reaction, and it turns out that, if the neutron flux at the detector is high, at least some of the intensity of the ubiquitous background line of this energy, which is usually attributed to 214Bi, is due to this process. This may refer especially to heavily shielded detectors used in low-background work, where the gamma-ray background is subdued but neutrons produced by muons abound. If there were any delayed signals due to the delayed collection of charges produced by the recoiling ions, they would be found in the low-energy part of the spectrum, together with the more frequent recoils from elastic neutron scattering – which would all be found around and below our rather highly set triggering level, of some 25 keV. As will become evident in what follows, this relatively high triggering level may be considered sort of advantage in this particular case.

Fig.7. The complex spectrum of prompt coincidences containing the structure at 596 keV from inelastic neutron scattering on Ge-74. The dashed line in the base of the structure is merely to guide the eye (a) and the same portion of the spectrum of delayed coincidences, which is seen to contain only the lines from neutron capture by Ge-73 (b).

3.4. The 692 keV structure

The quasi-triangular structure starting at 692 keV, similar in shape to that at 596 keV, which is due to inelastic neutron scattering on the first excited state of Ge-72 (27.7%), has been studied many times and in greatest detail [3-6]. It differs in two important respects from the 596 keV case. Firstly, the 692 keV state is an isomer state, with the half-life of 444 ns, and secondly, the depopulating radiation is pure E0, meaning that detection efficiency for the 692 keV radiation is practically always 100%. This transition energy sums with the recoil energy from neutron scattering, which depends on the incident neutron energy and the scattering angle, and is reduced by the pulse-height defect. The shape of the resulting spectral distribution has been studied in detail, mostly because the intensity of this distribution has been frequently used to estimate the fast

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neutron flux at the position of the detector [2,3,4,11]. These studies were performed with analog spectroscopy systems, where the integration constants are long enough so that the summing of recoils with the 692 keV radiation is practically always complete [6]. This justifies the use of this distribution for the determination of the fast neutron flux. It is somewhat unexpected that, despite the significant differences between this case and the case of the structure at 596 keV, the two structures are in direct spectra of very similar, and sometimes of practically identical shape (see for instance [3, 6, 10, 14, 15]).

In the case of digital spectroscopy, however, suitability of the structure at 692 keV for fast neutron flux estimation remains to be verified. The time structure of the 692 keV distribution, as might be expected, turns out exactly opposite to that of the 596 keV distribution. This is best apprehended by comparing Fig.8a, where the spectra of prompt, delayed and random coincidences containing this structure are presented, with Fig.8b, which shows the same spectra in the region of the structure at 596 keV (which is produced by merging Figs.7 a and b). It is seen that in the prompt spectrum, where the spectral background is quite high and the time interval narrow, and the intensity of the 692 keV radiation is consecutively low, the structure is barely discernible. On the other hand, in the delayed spectrum (here up to 1 s), where the spectral background is virtually absent, the structure has the pronounced and typical triangular form.

Fig.8. The spectrum of prompt, delayed, and random coincidences (practically negligible) containing the structure of 692 keV, from inelastic neutron scattering on Ge-72 (a). Compare this with the same spectra for the structure at 596 keV (b).

Now, if we take the 30 keV wide software gate, so as to fully embrace this structure (Fig.9), and find the corresponding time spectrum of coincidences with the entire NE213 spectrum, we obtain the time spectrum presented in Fig.10a. The fit through the nicely exponential tail of delayed coincidences (Fig.10b) yields at the 68% CL the half-life of 459(11) ns, which compares satisfactorily with the established value of 444 ns. This indirectly confirms that at all delay times all the emitted 692 keV radiations are contained within the triangular distribution, and that they are correctly timed. Other lines that are superimposed on this structure are mostly in the prompt spectrum, and there are practically none in the delayed spectrum, as can be seen by comparing Figs. 8.a and 9.

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This justifies the use of the intensity of this distribution for fast neutron flux estimation in the case of digital spectroscopy as well. The treatment of the below-the-triggering-level pulses in a given DSP system is essential in this respect. The baseline correction (BLC) will never fully suppress the summing of below-the-treshold recoil pulse and the over-the-threshold 692 keV pulse, but will only reduce the amplitude of the recoil pulse in the amount which depends upon the moment of the 692 keV decay. The exact shape of the distribution might depend upon the type of the BLC, but the number of events should be conserved, as long as the trigger is high enough. If the trigger is lowered, and the recoil pulse is shifted to over-the-triggering-level pulse, the event would shift from summing to pile-up, and would be rejected. The condition under which the intensity of the 692 keV structure can in DSP systems be used for estimation of the fast neutron flux is therefore that the threshold is high enough, to let all the recoil pulses sum with the 692 keV pulses.

Fig. 9. The “triangular” structure at 692 keV in the singles HPGe spectrum, from the inelastic neutron scattering on Ge-72. The position of the 30 keV wide software gate, which is used to produce the time spectrum shown in Fig.10, is also shown.

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Fig.10. Time spectrum of delayed coincidences of the 692 keV structure within the gate shown in Fig.9 with the entire NE213 spectrum (a), which yields the half-life of the 692 keV state of 459(11) ns (b).

Finally, to corroborate this conclusion, we took and analyzed the direct HPGe Cf spectrum in the so-called oscillogram mode, when the pulses are sampled at 10 ns intervals, starting at the given number of points before the defined triggering level. To illustrate, we present in Fig. 11 the pulse with the amplitude that corresponds to 692 keV, preceded by its corresponding recoil pulse. Firstly, we produced the distribution of time intervals between the appearance of the recoil and 692 keV pulses. Depending on the smallest amplitude of recoil pulses included, which significantly exceed the noise in the baseline, we obtain for the half-life of the 692 keV state the values in the interval from

Fig. 11. The joint pulses due to the recoil of the excited Ge-72 and the deexciting 692 keV radiation, separated on the average for the mean life of the 692 keV state. The distribution of these time intervals yields 444 ns for the half-life of this state, in excellent agreement with the accepted value.

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442 to 446 ns. Next, we produce the spectrum in the 692 keV region, by the algorithm that defines the baseline after subtracting the corresponding recoil pulse. This, for the first time, produces the normal line at 692 keV, instead of the common summing structure (Fig.12, to be compared with Fig.9). The integral of the line equals the integral of the structure, though the line appears somewhat wider than it should, due to the increased noise in the baseline, caused by subtraction of recoil pulses. This additional analysis validates our earlier conclusions concerning the suitability of the 692 keV structure for the determination of the fast neutron flux at the position of the detector.

Fig.12. The spectral line at 692 keV, from the deexcitation of the first excited state of Ge-72, obtained by subtraction of the adjoined recoil pulses. This compares with the structure in Fig.9, which results from the standard digital spectroscopy algorithm for baseline correction. In both cases the integral of the distributions contains all the events induced by fast neutrons on Ge-72.

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4. Conclusion

We measured the time-differentiated spectrum of the Cf-252 source as seen by an HPGe detector triggered by the NE213 liquid scintillator detector. The detectors were off-line coincided from the event-by-event list formed with 10 ns resolution by the quad FADC unit of the CAEN N1728B type. We tested the workings of the system on a number of effects generated by neutrons and found this time resolution satisfactory for the purpose. The signatures of the processes induced by fast and slow neutrons, both within the detector itself and in its environment, appear completely separated. The processes induced by fast neutrons are found in the prompt part of the time spectrum and those induced by thermal neutrons in the long tail of delayed coincidences. The half-lives of excited states of the order of hundreds of nanoseconds are well reproduced. We also compared the time behavior of spectral structures at 596 and 692 keV, which are the signatures of inelastic neutron scattering on Ge-74 and Ge-72 respectively. We find the intensity of the structure at 596 keV unsuitable for determination of the fast neutron flux, while the intensity of the 692 keV structure remains suitable for this purpose even in digital spectroscopy, under the condition that the threshold is higher than some 25 to 30 keV, and the germanium recoil pulses are let to sum with the 692 keV pulses.

Acknowledgments

The authors are grateful to Prof. I. Bikit for kindly lending the californium source, and to the Nuclear Physics laboratory of the Faculty of Physics, Belgrade, for lending the HPGe detector. The work is supported by the Ministry of Science of the Republic of Serbia under the Project No. 141002.

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