Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2001-08-23
2004-05-04
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S078000, C702S126000, C702S189000, C702S198000, C250S264000, C250S269200, C250S266000, C378S119000, C378S124000, C330S041000, C330S308000, C257S297000
Reexamination Certificate
active
06732059
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to systems for detecting and counting nuclear particles, and more particularly to counting such particles while reducing the background counting rate in gas-filled particle counters. The specific embodiments described relate to reducing background counting rates in both multi-wire counters and ionization chambers used to detect and count alpha particles, but the same techniques could be applied to counting other charged particles as well.
1. The Need for Low Background Alpha Counting
Low background alpha particle counting is important in various fields where very low concentrations of activity must be detected. Two important examples are in the testing of environmental samples and the specification of materials for the electronics industry. Data in the former case are used for such purposes as tracing radioactive emissions in the environment and estimating long term dosages to humans. In the latter case, materials which will be in intimate contact with silicon digital processing and/or storage chips must have low alpha particle emissions since these emissions can create charges within the chips that can change the values of digital numbers stored there and so introduce errors in computed or stored values.
A particular example of this is the need for low alpha lead. In presently used high density packaging technologies, silicon chips are often directly soldered to a mounting substrate using ball grid arrays or related soldering technologies. In this case the lead in the solder is in intimate contact with the silicon chip and so must have low alpha emissions for the chips to function reliably. For the next generation of high density circuits, it has been stated that:
“Measurement techniques and standards for alpha radiation effects are not adequate to support the increased alpha sensitivity anticipated for advanced technology processes.” [ITRS-1999, Assembly & Packaging, pg 235]
2. Current State of the Art
There are two major techniques presently used to measure alpha particle emission: gas-filled counters and silicon spectrometers. At this point, the two have similar background counting rates, but for different reasons.
Gas-Filled Counters
To set the context of the present invention, we briefly review the operation of, and distinction between, gas-filled ionization and proportional counters, as understood by those skilled in the art. A more comprehensive presentation can be found in Knoll. [KNOLL-1989, Chapters 5 & 6]. Ionization chambers are simply gas-filled volumes fitted with electrodes so that an electric field can be applied to the volume and any charges generated therein collected. When an alpha particle traverses the gas and loses energy, it produces an ionization track, composed of gas ions and the electrons knocked off them. The more massive ions drift slowly toward the negative cathode, while the lighter electrons drift toward the positive anode about 1000 times more quickly. [KNOLL-1989, pp. 131-138]. In simple ion chambers only the total collected current is measured, which is proportional to the average rate of ion formation within the chamber. Ion chambers can also be operated as counters in pulse mode, where the currents induced in the anode by the drifting electrons are amplified and integrated so that each ionization track produces a single output pulse and is counted individually. [KNOLL-1989, pp. 149-157] However, since the induced currents flow for the full electron drift time, the amount of integrated charge produced by a track varies, depending upon its starting location within the counter. Frisch grids, whose operation is beyond the scope of this discussion, can be used to minimize this effect. In general, since detector capacitances are large and the total amounts of ionization charge produced are low, signal-to-noise is poor when ionization chambers are operated in pulse detection mode.
Proportional counters seek to increase signal-to-noise, compared to ionization chambers, by using gas avalanche gain to increase the number of charges produced. Avalanching occurs when the average amount of energy a drifting electron acquires between successive collisions with gas molecules is larger than their ionization energy. Then, on average, each collision produces a second electron and the number of electrons increases exponentially with distance. Provided the total avalanche distance is strictly limited, the final number of electrons will be strictly proportional to the starting number, but many times larger. Very large electric fields are required for avalanche multiplication to occur, of order 1 to 10×10
6
V/m, which are usually produced by applying a voltage of order 1 to 2 KV to a wire whose diameter is typically 0.02 to 0.08 mm in radius (0.001″ to 0.003″). Since the electric field falls of inversely proportionally to the distance from the wire's center, avalanching can occur only within about 100 microns of the wire's surface which, in turn, provides the limitation required to assure gain proportionality. [KNOLL-1989, pp. 160-165] Further, because essentially all the avalanche charge is produced close to the wire, there are no drifting electron induced charge effects in proportional counters, so that output pulse amplitude and charge are proportional to the initial charge in the ionization track, independent of its original location within the counter. Proportional counters are commonly operated in single pulse counting mode. [KNOLL-1989, pp. 180-185] Because the avalanche process is very fast, it lasts only as long as the ionization track arrives at the anode wire. In a well designed counter, this time is short compared to the time it takes the ions formed in the avalanche to drift away from the anode wire, typically a few microseconds. As it is this latter process that induces the detector's output signal current in the anode, all output pulses in such well designed detectors have approximately the same shape.
The current state of the art in low background alpha counting uses a multi-wire gas-filled proportional counter with an ultra-thin entrance window. These counters can achieve sensitivities of about 0.05 &agr;/cm
2
/hr. [IICO-1999] They are typically constructed as shown in FIG.
1
. The detector
1
includes a conducting chamber
3
sealed on one side with an ultra-thin window
4
. A grid of anode wires
5
is suspended next to the chamber wall opposite the entrance window. The entire volume is filled with a counting gas
6
. The anode is biased via a large value resistor
7
connected to a voltage source
8
and also connected via a capacitor
10
to a charge sensitive preamplifier
11
. The preamplifier output connects to a shaping amplifier
13
and then to a discriminator
15
and counter
16
. The sample
20
is placed close to the entrance window
4
and emits alpha particles into the chamber. The window
4
thus defines a sample region, namely a region of the chamber volume at or near which a sample is to be located. In other chambers, the sample may be located within the chamber, in which case the chamber structure that supports the sample would help define the sample region.
A specific alpha particle
22
is shown. This particle creates an ionization track
23
in counting gas
6
. These charges drift toward the anode
5
, where they are amplified by the high electric field in the vicinity of the wires and then collected. [KNOLL-1989, pp. 160-165] The resultant charge signal is integrated by the preamplifier
11
, resulting in a pulse being output from the shaping amplifier
13
. When discriminator
15
senses this pulse crossing a preset threshold T, it emits a short output pulse which is then counted by the counter
16
.
However, in addition to ionization tracks generated by alpha particles such as alpha particle
22
emitted from the sample
20
, ionization tracks
25
,
26
, and
27
also are generated by alpha particles emitted from the chamber backwall,
Momayezi Michael
Wahl John
Warburton William K.
Desta Elias
Hoff Marc S.
Townsend and Townsend / and Crew LLP
Warburton William K.
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