Al (p,n) Si

Background

The shorthand for this experiment indicates the basics of the process that will take place:

²⁷Al (p,n) ²⁷Si

In other words, protons(p) of sufficient energy incident on an ²⁷Al target will fuse with the the nucleus and then emit a neutron(n), leaving ²⁷Si, where in both cases 27 indicates the atomic mass. This will only happen if the protons have sufficient kinetic energy to supply the difference in binding energy between ²⁷Al and ²⁷Si plus the difference between p and n masses. At the neutron threshold energy the neutrons leave with no kinetic energy so a detailed energy balance can be done to measure the difference in binding n or p to a ²⁶Si core.

Goals

At the completion of this experiment students will be able to:

• Describe the principles behind the production of a negative ion beam;
• Operate an inverted sputter negative ion source;
• Describe the principles of high voltage production with a Van de Graaff;
• Operate the FN-8 Tandem Van de Graaff;
• Describe the ion optics principles and devices used in a low-energy heavy ion accelerator;
• Develop and tune an ion beam to target using the accelerator facility;
• Describe the principles behind neutron detection;
• Use a neutron detector (at present BF3), signal processing electronics (NIM based)and DAQ (at present a PCI MCA card and software) to collect data;
• Analyze count rate versus beam energy data to extract the neutron threshold energy;
• Describe the nuclear physics meaning of their results.

Accelerator Operation

In order to acquire sufficiently energetic protons for this experiment, you will have to learn to create and direct a proton ion beam in the tandem accelerator. There are a number of components to the accelerator, which you will need to know the function of and how they will affect the nature of the proton beam.

Components

• The Ion Source: Negative H^- are created here and accelerated at relatively low energies.
• The Tandem Accelerator: Negative H^- ions are accelerated to the center of the tandem, stripped of the electrons to create protons, and then accelerated through the rest of the tandem to a high energy.
• The Target Room: The high energy proton beam is directed onto an aluminum foil target, hopefully creating a nuclear fusion reaction.
• The Detectors: Specialized detectors are used to identify, count and characterize reaction products.

In addition to these main sections of the accelerator, you will need to be able to use the ion optics in the accelerator effectively to steer and focus the beam.

Technique

To find the neutron threshold, we measure the neutron yield as a function of proton energy. Ideally, one would observe a small but gradually (linearly?) increasing background until one reached the threshold for fusion of the proton followed by evaporation of the neutron. This yield will then increase (linearly) with beam energy. One can fit the data to two lines and extract their intercept.

The literature guides us to want to scan the range of 5-7 MeV of proton energy. Step size should be chosen with regard to desired precision, target thickness and, of course, time constraints. When choosing how to scan over the energy range, one should consider beam activation of apertures in the beampipe and other sources of background neutrons. Placing the detector at 0^o in the lab frame is best because the low energy neutrons emitted just at threshold are very forward focussed.

Data Collection

As of September 2010, we will collect data for this experiment by taking timed energy spectra of the output of a [[Detectors | Boron Trifluoride] detector using an Ortec PCI Multichannel Analyzer (MCA) card and the Ortec software package Maestro. One can use Maestro to implement cuts, background subtraction and integration of the spectra or export the data as comma-separated value (CSV) files for use in other software.

Analysis

Ideally, one would observe a small but gradually (linearly?) increasing background of neutron yield until one reaches the threshold for fusion of the proton followed by evaporation of the neutron. This yield will then increase (linearly) with beam energy. One can fit the data to two lines and extract their intercept.

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