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Estimation and Measurement of Alpha Decay Pulses in
Fission Detectors and Their Practical Application for
Verifying Detector Health
Shigehiro KONO, Masao ISHIKAWA, Shuhei SUMIDA, Michinori YAMAUCHI1) , Daijiro ITO2) ,Hiroyuki YAZAWA2) , Masashi KUROSAKI2) , Takashi KOBUCHI3) , Kunihiro OGAWA3,4) ,Mitsutaka ISOBE3,4) and Yoshihiko NUNOYANaka Fusion Institute, National Institutes for Quantum Science and Technology, Naka 311-0193, Japan
1)
NAT Corporation, Tokai 319-1112, Japan
2)
Toshiba Energy Systems & Solutions Corporation, Fuchu 183-8511, Japan
3)
National Institute for Fusion Science, National Institutes of Natural Sciences, Toki 509-5292, Japan
4)
The Graduate University for Advanced Studies, SOKENDAI, Toki 509-5292, Japan(Received 6 November 2023 / Accepted 30 January 2024)
Signals generated by spontaneous alpha decay of uranium in fission chambers are very small and are buried
in background noise. These signals are often simply discriminated by amplitude as unwanted background noise.
However, if these signals can be distinguished from white noise, they could function as a checking source thatis electric-noise-free and is embedded in the detector itself. To confirm our hypothesis, we estimated and measured the pulse height of alpha decay signals in the fission chambers of the Large Helical Device (LHD) at the
National Institute for Fusion Science (NIFS). When background noise was suﬃciently low, the plateau curvesof alpha decay signals could be measured by using the maintenance function of the neutron monitoring system.
When background noise was high, instruments such as an oscilloscope and a multi-channel analyzer (MCA) successfully caught alpha decay signals by using their high-performance trigger functions. Measuring alpha decay
pulses is also an indicator of the health of the detector and its preamplifier because if something were wrong witheither component, the alpha decay signals would prove diﬃcult to measure. This practical application of alphadecay signals to verify neutron instrumentation systems that use the noise-sensitive Campbelling method will bebeneficial for fusion facilities and fission reactors and help meet ALARA policy because this method does not
require an external checking source.
c 2024 The Japan Society of Plasma Science and Nuclear Fusion Research⃝

Keywords: alpha decay, fission chamber, Campbelling, neutron instrumentation, fission reactor, LHD, ITER,
JT-60SA, BWR
DOI: 10.1585/pfr.19.1405015

1. Introduction“Will the preamplifier have any control, monitoring,or health-related functions?” was the question from the
ITER Organization concerning the Micro Fission Chamber (MFC) system for ITER [1]. Neutron sources such as
252
Cf are usually provided with fission chambers to checkthe health of detectors and measurement equipment. However, because the MFC detectors of ITER are installed inside a vacuum vessel as shown in Fig. 1, these checking
sources cannot be used. Although some preamplifiers areequipped with internal test pulse generators for soundness
checking, installing such a signal generator is not ideal because of the electromagnetic noise they would generate.
The Campbelling method, used to measure the signals offission chamber detectors, measures the mean-square voltage (MSV) of small signal fluctuations of a detector [2],and is thus vulnerable to noise. Based on our experience
author’s e-mail: kono.shigehiro@qst.go.jp

Fig. 1 The MFCs will be installed on the upper (#12) and lower(#17) out-boards behind the blanket modules of the ITER
tokamak vacuum vessel.

with neutron monitoring systems for boiling water reactors(BWR), oscillators used for test signals cause noise and affect the soundness of Campbelling measurements. Next,
because alpha decay signals do not generate any electric

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Table 1 Specifications of the fission chamber detectors.

Table 2 Specifications of the preamplifiers.

Fig. 2 System configuration of the neutron monitoring systemof the LHD.

noise, we focused on the spontaneous alpha decay of uranium coated on the electrodes of fission chambers. However, the average energy of an alpha particle is less than
one-tenth the energy of a fission fragment of uranium, andtheir signals can be easily discriminated and removed fromthe neutron signals generated by fission fragments. Thus,
alpha decay signals are not normally utilized. Alpha decay
signals can be measured in good noise conditions, but inhigh-noise environments the signals are unmeasurable because they get buried in the background noise. The neutron
monitoring system that was constructed for the Large Helical Device (LHD) at the National Institute for Fusion Science (NIFS) has good noiseless conditions, therefore we
expected to succeed in measuring the alpha decay signalsof the fission chambers of the LHD [3, 4].

Table 3 Signal cable lengths of the neutron monitoring system.

2. Specifications of the Instrument
Devices
The neutron monitoring system of the LHD consists ofthree sets of 235 U fission chamber detectors, preamplifiers,and digital signal processing units [5]. The configurationof the system is shown in Fig. 2 [3]. The signals generated by the spontaneous alpha decay of uranium coatedon the electrodes of fission chambers were measured at
the input to the digital signal processing unit; the fission
chambers and preamplifiers were the main contributors formeasuring the alpha signals. The specifications of the fission chambers and preamplifiers are shown in Table 1 andTable 2, respectively. The cable lengths between them areshown in Table 3.

3. Estimation
The fission chamber detectors used in the LHD have
only 235 U as their fissile material. The amount of UO2 isabout 300 mg, so the number of 235 U atoms is 6.76 × 1020 .
The half-life of 235 U is 7.08 × 108 years, so the decay constant of 235 U is 3.12 × 10−17 s−1 . Therefore, the decay rateis 2.11 × 104 s−1 . Because uranium is coated on the inner surface of the outer electrodes of the fission chambers
and alpha particles are emitted toward both the inside andoutside of the electrode, the detection eﬃciency was estimated to be 50%. Therefore, the expected counting rateof the pulses generated by the spontaneous alpha decay of1405015-2

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235

U coated on the electrodes of the fission chambers is
1 × 104 counts per second (hereinafter referred to as cps).
The maximum amplitude of the signal pulses generated in the fission chambers was calculated based on [2]and [7].
Assuming that the specific energy loss of the alphaparticles in argon gas is equivalent to that of air, the rangeof the alpha particles in argon gas was estimated from therange in air, that is R = 0.318E3/2 [8]. Considering thedensity of argon gas, 1.783 kg/m3 , the energy of the alphaparticles at position x cm on the track is shown below.
E = (E1 3/2 − P · x/0.178)2/3 .

(1)Here, P is the argon gas pressure at 0◦ C.
From Eq. (1), the range of the alpha particles, whichhave an average energy of 4.4 MeV, was calculated as
2.1 mm in argon gas pressurized to 787 kPa, which is thefiller gas pressure of the fission chambers.
Equation (2) was used to calculate the energy of fission fragments, assuming that the specific energy loss ofthe fission fragments in argon gas is also equivalent to thatof air [9].
E = (E1 2/3 − P · x/0.0785)3/2 .

(2)

From Eq. (2), the range of light fission fragments having an energy of 93 MeV was calculated as 2.1 mm, andthe range of heavy fission fragments having an energy of
67 MeV was calculated as 1.7 mm in argon gas pressurized
to 787 kPa.
The flight lengths of the alpha particles and the fission
fragments were calculated considering the dual-cylinder
structure of the fission chamber detectors. The inner electrode radius (r1) was 10.5 mm and the outer electrode radius (r2) was 11 mm, thus an electrode gap of 0.5 mm.
When a detector is cut obliquely at an angle φ, the crosssection will be a dual ellipse. When a particle is emitted
from point P on the outer ellipse (the negative electrode) ina direction having angle θ, the particle will reach the inner
ellipse (the positive electrode) at distance X1. The particle will reach another point on the outer ellipse at distance
X2 in the case that θ is larger than the following angle asshown in Fig. 3.

Fig. 3 Flight length calculation in a dual-cylinder electrode detector.

The energy loss in argon gas was divided by a W-valueof 26.3 eV, the average energy lost by the alpha particles
per ion pair formed in argon gas [10], to obtain the electriccharge (Q0 ) created by the ionization of argon gas. Columnar recombination was used to estimate the electric charge(Q) acquired at the electrodes based on Eq. (5) and the values reported in Ref. [11].
Q/Q0 = ln(1 + κ · k/b)/(κ · k/b),
where,
k/b = 2.5 × 10−5 m2 /s when electrode distance is 0.5 mm,
κ = 1.52 × 105 s/m2 for 4 bar gas pressure and 200 V bias,alsoκ = C · X0 (1bar) · d · P2 /(π · (µe + µa )(1bar) · ∆V).
(6)
To calculate Q/Q0 for gas pressure P (bar) and bias V (V),Eq. (7) was used with converted κ’.

(3)When θ > tan−1 (r1 / cos φ × 1/(r2 2 − r1 2 )1/2 )X2 = 2r2 /(tan2 θ cos2 φ + 1) × (tan2 θ + 1)1/2 .

(4)The energy loss of particles in argon gas was calculated bysubtracting the energy at X1 or X2, which was calculated
by Eq. (1) for the alpha particles or by Eq. (2) for the fission
fragments, from the initial energy E1. When X1 or X2 islonger than the range of the particles, the particles will lose
all of their initial energy in argon gas.

Q/Q0 = ln(1 + κ’ · k/b)/(κ’ · k/b),(7)

κ’ = κ · (P/4)2 /(V/200).

(8)where,

X1 = {r2 − [r2 2 − (tan2 θ cos2 φ + 1)(r2 2 − r1 2 )]1/2 }/(tan2 θ cos2 φ + 1) × (tan2 θ + 1)1/2 .

(5)The maximum detector current was calculated based on theassumption that the ionization of argon gas occurs equivalently along the track of the fission fragments and the alphaparticles, and the generated electrons move to the anodewith a constant velocity of We when derived by the electricfield. The following formula was used for the calculation
because the preamplifiers of the LHD are electric-current
pulse amplifiers having a time constant τ0 that was shorter
than the electron collection time Te . A cable terminatedwith a specific impedance r0 has an impedance r0 whenviewed from the sending end. Therefore, unlike charge

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amplifiers, there is no need to consider the cable capacitance for this current pulse preamplifier even if the cables
are lengthened [12].
Imax = Qe /Te [1 − exp(−Te /τ0 )],(9)where,
Qe = Q,
Te = d/Te .
In the case of KSA-01 detectors,
Te : electron collection time; 80 ns
τ0 : time constant; input impedance (r0 )× detector capacitance (Cd)r0 : 75 Ω
Cd: 155 pF
Finally, the pulse voltage was calculated by multiplying
the maximum current by the input impedance (75 Ω), thepreamplifier gain (× 100), and the amplifier gain (× 20).
Figure 4 shows the calculated pulse height distributionof signals generated by the alpha particles and the fission
fragments in a KSA-01 detector biased with 300 V.
When a light fragment loses all its initial energy of
93 MeV in argon gas, it creates a pulse at 736 mV. Whena heavy fragment loses an energy of 67 MeV, it creates apulse at 530 mV. With the exception of these peaks, themode of the pulse height distribution of the fission fragments exists between 250 - 260 mV owing to the narrow
gap between electrodes. The mode of the alpha pulses also
exists between 0 - 10 mV, which is nearly the same level
as that of white noise. The lowest pulse height of the alpha decay signals is 5.7 mV, which corresponds to the casewhere an alpha particle flies the shortest distance between
the electrodes, i.e., 0.5 mm.

4. Measurement
We had a high expectation of success in measuring thealpha decay pulses, even in a currently operational facility
having long signal cables as shown in Table 3, because thecurrent pulse preamplifier is used in the LHD. Two typesof data were acquired to obtain information from alpha decay pulses. One was the equipment maintenance data including the discrimination characteristics and the plateau
characteristics, which were functions installed in the digital signal processing unit. The other was the raw data of theoutput signal from the preamplifier, which was acquired
using an oscilloscope (Tektronix, Inc, model: MSO64).
The signal level of the raw data measured by the oscilloscope was one-twentieth the level of the equipment maintenance data, because the input signal amplifier of the digital signal processing unit multiplied the input signal by 20.
The configuration of the measurement circuit and the measurement setup are shown in Fig. 5 and Fig. 6, respectively.

4.1

Measurement in a low noise channelFirst, the measurements in a low noise case are explained.
Pulse plateau characteristics of fission chamber No. 3(FC3) located at 4-O were measured at a discriminationlevel of −100 mV by using a function of the digital signal
processing unit. Figure 7 shows the pulse plateau characteristics of FC3.

Fig. 5 Configuration of the measurement circuit.

Fig. 4 Histogram of the pulse height distribution of a KSA-01
fission chamber at P = 787 kPa and V = 300 V. Alphacount rate (red) = 1 × 104 cps, neutron (fission fragments)
count rate (blue) = 1 × 104 that is only for simulation.
(Note: the actual count rate depends on the strength of
the neutron source.)

Fig. 6 Photo of the measurement setup. A plain stitch wire forthe earthing of the signal earth was used.

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Fig. 7 Pulse plateau characteristic curve of FC3. The range ofthe bias voltage for FC3 was 0 - 400 V. The measurement
time at each bias voltage was 60 s. The interval of the bias
voltage was 50 V. The discrimination voltage was set to−100 mV. (hereinafter these parameters are expressed as:
range = 0/400 V, interval = 60 s, step = 50 V, dscrm =−100 mV)As can be seen in Fig. 7, the plateau curve of FC3 had
an ideal plateau curve: the counting rate starts from 0 cps
at a bias voltage of 0 V and it has a saturation curve over
50 V. This indicates good signal/noise (S/N) conditions inwhich the noise level is low and the alpha pulse height ishigh enough to distinguish the alpha pulses from the noise.
The discrimination characteristics of FC3 were measured with detector biases of 0 V or 300 V by using a function of the digital signal processing unit as shown in Fig. 8.
A swelling on the discrimination line was observed in thediscrimination characteristics of FC3 biased by 300 V. The
swelling was created by the alpha pulses, which were expected before measurements were performed.
The discrimination curves show the integrated valuesof the number of pulses higher than the discriminationlevel, therefore, the derivative of the discrimination curve
provides the pulse height distribution. The derivative ofFig. 8 (b) was subtracted from that of Fig. 8 (a) to obtain thepulse height distribution of the alpha decay signals only, removing noise data. Figure 9 shows the calculation results.
The zone hatched in gray was not considered because theerror of the count rate was large due to background noise.
The peak of the pulse height distribution of the alpha decay signals was read out as −50 mV from Fig. 9. This value
was 8.8 times larger than the expected value of −5.7 mV,
which was calculated in Sec. 3 and considered the preamplifier gain (100 times) and the input gain of the digital
signal processing unit (20 times).

4.2 Measurement in a high noise channel
Next, the measurements in the high noise case are explained. The discrimination characteristics were also measured in fission chamber No. 2 (FC2) located at 10-O. In
this case, the discrimination characteristics of FC2 did not

Fig. 8 Discrimination characteristic curves of FC3. (range =0/−200 mV, interval = 60 s, step = 10 mV, bias voltage(HV) = (a) 300 V/(b) 0 V)

Fig. 9 Diﬀerence between the derivatives of Fig. 8 (a) and (b),which indicates the pulse height distribution of the alphadecay signals except for the zone hatched in gray where
the count rate error was large due to background noise.

show any diﬀerence between the detector biases of 0 V or
300 V as shown in Fig. 10.
When the pulse plateau characteristics were measured, the plateau curve was initially a flat line, as shownin Fig. 11.

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Fig. 12 Pulse plateau characteristic curve of FC2 after connector mating/unmating. (range = 0/400 V, interval = 60 s,
step = 50 V, dscrm = −100 mV)

Fig. 10 Discrimination characteristic curves of FC2. (range =0/−200 mV, interval = 60 s, step = 10 mV, HV = (a)300 V/(b) 0 V)

Fig. 13 Alpha pulse shape triggered with −5 mV when FC2 wasbiased by 300 V [Horizontal Scale: 100 ns/div, Vertical
Scale: 5 mV/div, hereinafter, the oscilloscope’s scales
are the same except for Fig. 15].

of the digital signal processing unit was repeatedly matedand unmated to decrease contact noise, after which, theplateau curve improved slightly, as shown in Fig. 12.
In this case, it was shown that the alpha signals were
small and easily buried in the background noise, therefore,
noise mitigation was crucial in measuring the alpha signals.

4.3
Fig. 11 Pulse plateau characteristic curve of FC2. (range =0/400 V, interval = 60 s, step = 50 V, dscrm = −100 mV)

The input signal cable was removed from the digital
signal processing unit so that the pulse plateau characteristics could be measured without the input signals, after
which, the counting rate at each bias voltage was 0.
Because a loose input signal connection might be thecause of noise, the input signal connector on the rear panel

Direct measurement of the alpha decay
signals

After the measurement shown in Fig. 12, to confirm
the noise conditions in FC2, the output signals from thepreamplifier for FC2 were directly observed by an oscilloscope having a storage mode with a trigger level of 5 mV.
Even in high-noise conditions such as FC2, the oscilloscope was able to catch the alpha decay signals. Figure 13
shows the waveform of the pulses generated by the spontaneous alpha decay of 235 U coated on the electrodes of FC2
when the detector was biased at 300 V.
The measured average alpha pulse height was 5.7 mV,

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Fig. 14 Pulse shapes of the alpha decay signals captured by the oscilloscope when FC3 was biased by 0 V (a) to 400 V (e).

and the average noise amplitude was approximately 4 mV,
which explains why the discrimination characteristics cannot distinguish the alpha signals from the noise. The oscilloscope screen indicated that the trigger frequency was
451 Hz (i.e., cps), which corresponded to the pulse counting rate at −103 mV in the discrimination voltage shown in
Fig. 10 (a).
Under good noise conditions, the waveforms of FC3
captured in the same manner as FC2 were depicted moreclearly than those of FC2. The results are shown in
Fig. 14 (a) through (e). The pulse heights of the alpha signals are higher than the noise level. The alpha pulse heights
vary depending on the detector bias voltage. The alphapulses were obtained at detector bias voltages of 0 V, 100 V,
200 V, 300 V, and 400 V, and are summarized in Table 4.
Note that 0 V bias indicates the noise level. The noise levels of FC3 were approximately half the noise levels of FC2
when the detectors were biased, even though these noiselevels were the same when the detectors were not biased.

The signal level of the alpha decay pulses measured by theoscilloscope was also higher than those of Sec. 3.

5. Discussion
5.1

Measurement of the neutron pulse signal

The pulse signals generated by the alpha decay could
be monitored in the LHD, a currently operational facility,
which begs the question of why the pulse heights of thealpha decay signals were larger than those of the estimation. Firstly, it was suspected that the preamplifier gains
between the calculation and the actual equipment were different. To confirm this, further data were acquired by usinga 252 Cf checking source to obtain the neutron pulse signals.
Figure 15 shows a screen capture of the oscilloscopethat displayed the pulse waveforms of KSA-01 detectors
exposed to 252 Cf and biased at 300 V.
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Fig. 15 Screen capture of the oscilloscope when FC3 was
exposed to 252 Cf and biased at 300 V [Horizontal
Scale:100 ns/div, Vertical Scale:10 mV/div].

Table 4 Pulse height voltages of the decay alphas and the fission
fragments of KSA-01 detectors measured in the preamplifier output and signal levels multiplied by 20.

Fig. 16 Pulse height distribution of a KSA-01 detector exposed
to a 252 Cf neutron source. These data were measured by
an MCA having a pulse shape selection function.

fragments in channels 3000 to 8000 (corresponding to 375- 1000 mV) was large. However, the pulse height rangeof the fission fragments almost equals the value measured
by the oscilloscope at the 300 V bias, as shown in Table 4.
Therefore, the preamplifier gain used in the calculation waschanged from 100 to 150 based on the values measured by
the oscilloscope and the MCA so as to equalize the neutron
pulse height distribution of the calculation with that of themeasurements.

5.3

gain of the input circuit of the digital signal processing
unit. The multiplied values correspond to the signal levelsof the discrimination and the plateau characteristics. In Table 4, the alpha decay pulses are distributed between 100 140 mV and the fission fragments pulses are distributed between 380 - 1040 mV, when the detector is biased at 300 V.

5.2 Measurement of pulse height distribution by MCA
Figure 16 shows the pulse height distribution of KSA01 detectors measured by a multi-channel analyzer (TechnoAP, APV8104-14LW) having a waveform selector. The
pulse heigh is calculated based on the integrated area ofthe pulse. In Fig. 16, channel 8000 corresponds to 1 V.
The highest peak around channel 1300 (corresponding to
160 mV) in Fig. 16 indicates the peak of the alpha decay
signals. This value (160 mV) was slightly higher than thatmeasured by the oscilloscope and multiplied by the input
circuit gain shown in Table 4. At the time the measurements were performed by the MCA, background noise wasincluded in the integrated area to calculate the pulse height.
Because the 252 Cf used was not strong, the statistical error of the measurement of the pulses created by the fission

Calculation of the stopping power of alpha particles in Ar gas

Even after changing the preamplifier gains in the calculation, the pulse heights of the alpha decay signals werestill lower than those of the measurements. Therefore, thestopping power of argon gas for 4.4 MeV alpha particles,
which is the average energy of the decay alphas of 235 U,
was calculated by using the Bethe-Bloch formula [13] tore-evaluate the pulse heights of the alpha decay signals.
Figure 17 shows the calculated specific energy loss of thealpha particles in argon gas.
The calculated curve was overlayed with the functionof 11.724/E1.1 . Consequently, Eq. (10), which shows theenergy of the alpha particles as a function of position x,
was obtained assuming that E1 was the initial energy of
the alpha particles.
E = (E1 2.1 − P · x/0.02278)1/2.1 ,

(10)

where
P is gas pressure converted to standard gas
conditions at 0◦ C.
From Eq. (10), the range of 4.4 MeV alpha particles in argon gas at 787 kPa was calculated as 0.66 mm.
In Fig. 18, which was re-calculated using this range(0.66 mm), the mode of the pulse height distribution of thealpha decay signals fell between 50 - 60 mV, which was

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Fig. 19 Histogram of pulse height distribution reconsidered forthe recombination rate.

Fig. 17 Specific energy loss of the alpha particles in Ar gas calculated by the Bethe-Bloch formula.

trodes, the light fission fragments lose 27.8 MeV, the heavy
fission fragments lose 31.5 MeV, and the alpha decay particles lose 2.2 MeV. When the recombination rate of the
decay alphas is calculated as the rate of energy loss, i.e.,
the energy loss of the alphas divided by the average of theenergy loss of the fission fragments, the pulse height distribution is as shown in Fig. 19. The highest pulse heightof the alpha decay signals becomes 73.7 mV, which is halfof the measured pulse height of the alpha decay signals.

5.5

Fig. 18 Histogram of the pulse height distribution of a KSA-01
fission chamber at P = 787 kPa and V = 300 V, using
the re-calculated range of 4.4 MeV alpha particles in Ar
gas. The preamplifier gain was set to 150 for this andsubsequent calculations.

slightly closer to the measurement. Even though this value
met the rough calculation in Fig. 9, the pulse height voltages of the alpha decay signals of Table 4 were still two to
three times those of the values of Fig. 18.

Because the two peaks indicated at the right side of
the pulse height distribution in Fig. 19 were not observedby the MCA as shown in Fig. 16, self-shielding by the uranium layer coated on the negative electrodes of the fission
chambers was examined.
When fission fragments or alpha particles are emitted
from the inside of the uranium layer coated on the negativeelectrodes, these particles lose their energy inside the uranium layer. The energy losses in the uranium layer were
calculated in the same way as those in argon gas, i.e., byusing stopping powers calculated by the Bethe-Bloch formula (assuming that this formula can be applied to the uranium layer). Equation (11) provides the energy of the particles at distance x from the point of emission.

5.4 Re-consideration of the columnar recombination rate of the alpha particles
The pulse height voltage of 50 - 60 mV in Fig. 18 corresponds to the case in which the alpha particles completely lose their energy of 4.4 MeV in argon gas. If
the columnar recombination rate of the alpha particles issmaller than that of the fission fragments, the pulse heightsof the alpha decay signals would be slightly higher and still
be closer to those of the fission fragments. As describedin [2], the recombination rate is proportional to the product of the concentrations of the two species involved. In
fission chambers, ionization is produced along the track of
the fission fragments or the alpha particles, and the densityof the ionization products is proportional to the energy lossof the particles. In the shortest distance between the elec-

Consideration of self-shielding by uranium coated on the negative electrodes

E = (E0 2.05 − 2.05 · a · x)1/2.05 ,

(11)

where
“E0 ” is the initial energy, 93 MeV for light fission
fragments, 67 MeV for heavy fission fragments, and
4.4 MeV for decay alphas.
Coeﬃcient “a” is 1.89×109 for light fission fragments,
5.19 × 109 for heavy fission fragments, and 50000 for
decay alphas.
When E=0 in Eq. (11), x provides the range of an emitted particle in the uranium layer. The range of the lightfission fragments was calculated as 2.8 µm and that of the
decay alphas was 203 µm. These ranges are longer than the
1-µm-thick uranium layer, so these particles could fly out

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Fig. 20 Histogram of pulse height distribution taking into consideration self-shielding by uranium.

from the uranium layer even though they lost some of theirenergy in the uranium layer. The range of the heavy fission fragments was calculated as 0.52 µm, which is shorter
than the thickness of the uranium layer. Therefore, half ofthe heavy fission fragments could not fly out from the uranium layer. Even if the heavy fission fragments could fly
out from the uranium layer, the particles would have lost
significant energy in the uranium layer. The heavy fission
fragments emitted from the middle of the uranium layer
may fly out into argon gas having small energies and create ionized pulses similar to those of the decay alphas. We
considered this to be the cause of the diﬀerence between
the results measured by oscilloscope (Table 4) and by
MCA (Fig. 16). The fission chambers were not irradiatedby a 252 Cf checking source in the measurements by oscilloscope, however, the fission chambers were irradiated in themeasurements by MCA. A slight shift in the pulse height
distribution of the alpha pulses to a higher voltage was also
observed by the MCA when the fission chambers were irradiated by the 252 Cf checking source.
The pulse height distributions were calculated bychanging the depth of the emission points from 0 µm (i.e.,on the surface of the uranium layer) to 1 µm (i.e., on thebottom of the uranium layer) every 0.05 µm step. Figure 20
shows a summation of the 21 results of these pulse heightcalculations. The peaks at the right side of the pulse height
distribution became smaller in Fig. 20, with the highest
pulse height of the alpha decay signals being 72.2 mV.

6. Application of the Alpha Decay Signals
As confirmed in Sec. 4, the signals generated by the
spontaneous alpha decay of the uranium coated on the electrodes of the fission chambers were observable in a system
in a currently operational facility having long cables between the detectors and preamplifiers. In contrast, in a laboratory setting, short cables are usually used to mitigate
the influence of noise because charge-sensitive preamplifiers are used to measure the alpha decay signals. It can besaid that the use of an electric-current pulse preamplifier
made the measurement of the alpha decay signals possible in the LHD, therefore, facilities that use the same typeof preamplifier for their fission chambers can measure thealpha decay signals.
Alpha decay pulses can be an indicator of measurement system health, i.e., verify that the components of asystem are functioning properly. If something were wrong
with any component in the system, the alpha decay signals
could not be observed because they would be small anddiﬃcult to measure.
A discrimination circuit is used to distinguish the neutron signals from the other signals. For this purpose, thediscrimination level is set at the x-intercept of the extension line of the slope of the noise portion in a discrimination curve, which is around −150 mV in Fig. 10. When thenoise level is mitigated lower than the pulse height of thealpha decay signals, the discrimination level can be set atthe highest noise level to distinguish the alpha decay signals from the noise. The discrimination level was evaluatedat −100 mV comparing Figs. 8 (a) and (b) in the case of
the LHD. The plateau characteristics measurements, usually provided by neutron measurement equipment as a basic maintenance function, can be used to easily verify thehealth of the detectors and the preamplifiers. The plateau
characteristics should show an ideal curve, as shown in
Fig. 7, and the pulse count rate should be stable under thesame noise conditions. Changes to the shape of the plateau
curve or the pulse count rate of the alpha decay signals
from the initial conditions may indicate that the detectors
or the preamplifiers are not functioning properly. Thus,
when such changes are detected, these components shouldbe thoroughly checked.
However, when the noise level is higher than the alpha decay pulses, the plateau characteristics will indicate asemi-flat line, as shown in Fig. 11. Even in such noise conditions, an oscilloscope equipped with a high-performancetrigger function can catch the alpha pulses provided thatthe neutrons can be measured. The oscilloscope can pickup the alpha pulses directly in the signals from the preamplifier by using several trigger settings, such as the fallingedge trigger with an adequate trigger level (e.g., 5 mV).
An MCA equipped with a waveform-selection function,such as the pulse rise time (e.g., 80 ns) and the pulse width(e.g., 200 ns), can also catch alpha pulses. The pulse height
and the pulse height distribution of the alpha decay signals
initially recorded under good conditions will be comparedwith subsequent measurements, and if any changes are detected, a more detailed investigation will be performed.
The same is true in the case of the plateau curve. If thenoise conditions are insuﬃcient to measure the neutron
signals, the noise conditions should be improved first.
General methods for verifying the soundness of detectors and preamplifiers in detail are as follows. External
equipment such as a pulse generator or a function generator can be used to verify the gain of the preamplifier, the1405015-10

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Volume 19, 1405015 (2024)

Fig. 21 Histogram of the pulse height distribution of the micro
fission chamber for ITER.

linearity of the gain, and the frequency characteristics. For
detectors installed in inaccessible areas, the methods fordetailed verification are limited. A decrease in the pulse
height indicates either a leak of detector gas or an increasein the conductivity of the cables or the connectors. Increased unexpected pulses may occur when the insulation
resistance of the detector or the cables deteriorates. In suchcases, measuring the insulation resistance will provide information about the defect. Connector tapping may also be
useful to check for a bad contact in the connector.
A method to check the health of the fission chamber detectors and the preamplifiers without having to use
an external checking source is convenient especially when
the detectors are set in a place that people cannot access.
Even if the detectors are located in an accessible area, thismethod helps meet the ALARA policy of avoiding exposure to radiation as low as reasonably achievable.
In the case of the Micro Fission Chamber (MFC) system of ITER, detectors are installed inside the vacuum vessel of the tokamak where people cannot access the detectors because the materials will become radioactive after DD or DT plasma operations. Even in such cases, thehealth of the detectors and the preamplifiers can be verified
when the ionizing reaction in the detectors and the signal
amplification by the preamplifiers are confirmed by plateau
measurements and by observing the alpha pulses via oscilloscope or MCA. Figure 21 shows the calculated pulse
height distribution of the micro fission chamber (200 mm ×
14 mm ϕ) for ITER. The pulse heights of the fission fragments are calculated to be lower than those of the KSA01 detectors because the electrode gap of the micro fission
chamber for ITER is narrower than that of KSA-01. However, the largest pulse height of the alpha decay signals is
estimated to be 70.1 mV, which is almost the same as KSA01. Hence, the measurement of the alpha decay signals is
expected to be possible in ITER, although the cable lengths
between the micro fission chambers and the preamplifiers
is approximately 30 m and between the preamplifiers andthe signal processing units is at maximum 150 m.
This method was used to test a neutron monitoring

Fig. 22 Pulse shape of the alpha decay signals measured in JT60SA.

system having fission chambers in JT-60SA at QST in
Japan, which facilitated the management of operations by
not having to use an external neutron source. The typesof fission chambers and preamplifiers of JT-60SA are thesame as those of the LHD. In JT-60SA, the cable length
between the fission chambers and the preamplifiers is 5 m
and between the preamplifiers and the signal processing
units is 50 m. This method contributed to resolving noise
issues, and the alpha decay signals were measurable by oscilloscope. Figure 22 shows the pulse shape of the alphadecay signals measured in JT-60SA, even though the noiselevel was high, the results are approximate to those of FC2
in the LHD.
Furthermore, if this method can be applied to the verification of the Startup Range Neutron Monitor (SRNM)
system of BWRs currently shut down in Japan, it will bringgreat benefits because the system uses fission chambers installed inside the reactor vessel.

7. Conclusion
Until now, alpha decay pulses were not used for practical applications because the pulse heights were either too
small to observe, or they were thought to be mixed in withwhite noise. However, we were able to observe the pulse
signals generated by the spontaneous alpha decay of uranium coated on the electrodes of fission chambers in the
LHD of NIFS. These alpha decay pulses can be applied to
new and improved investigation methods for fission detectors and preamplifiers in ITER and JT-60SA. Using alphadecay signals is also helpful in meeting the ALARA policyin terms of not having to use radiation sources and can bebeneficial for verifying the health of systems having fission
chambers, in both fusion facilities and fission reactors.

Acknowledgments
This work was performed with the support and under the auspices of the QST Collaboration Research
Program and the NIFS Collaboration Research Program(NIFS22KIEH003).

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Plasma and Fusion Research: Regular Articles

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