B (p,n) C

Background

In this experiment a proton fuses with ¹¹B nucleus then boils out a neutron to form the short lived isotope ¹¹C. We seek to identify ¹¹C by its half-life and to investigate the angular correlation of gamma-rays which we detect as a result of its decay.

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 gamma ray detection;
• Use a gamma detector (NaI) and signal processing electronics (NIM based)to collect count rates of correlated photons;
• Analyze count rate versus angle to characterize the decay mode of ¹¹C;
• Analyze count rate versus time data to extract the ¹¹C half-life;
• 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 create ¹¹C, we will conduct a fusion evaporation reaction with a proton beam and ¹¹B target. The literature guides us to use a proton energy of about 6 MeV. When designing a target, one should consider the effect of energy straggling and how other materials exposed to the beam (target frames, beam stops) may contribute to contamination of the measurement.

To run the experiment, we expose the ¹¹B target to the ¹H beam for approximately 2 half-lives of ¹¹C. We then remove the4 target to a beam-free room and use 2 sodium iodide detectors and associated electronics (NIM) to identify coincident events.

Data Collection

As of September 2010, we will collect data for this experiment manually! The data are simply a table of counts per unit time versus time.

Analysis

Ideally, one will see a simple exponential decay. The presence of contaminants has the potential to introduce other exponentials and complicate the fit.

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